Tutorial on the AS-i Technology

 

1. Introduction
2. AS-i Associations
3. Benefits
4. Versions and Specifications
5. Characteristics
6. Connectivity
7. The Actuator Sensor Interface system
8. Safety at Work
9. AS-i Limitations
10. Standards and Regulations
11. References

 

1. Introduction



In 1990, in Germany, a consortium of successful companies created a bus system for networking sensors and actuators called Actuator Sensor Interface (AS-Interface or in short AS-i).

This system was meant to meet some requirements based on the experience of their own founding members and to supply the market whose hierarchy is bit-oriented. Hence, the AS-i network was conceived to complement the other existing systems and to simplify and speed up the connection between sensors and actuators and their controllers.

 

Technological Scenario – Source: ATAÍDE, F.H. (2004)


Figure 1.1: Technological Scenario – Source: ATAÍDE, F.H. (2004)

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2. AS-i Associations

The AS-International Association was founded in 1991 as a user group for manufacturers and users of the AS-Interface® system, whose objective is turn the AS-i into a world standard for the bit-oriented field of industrial automation pertaining to the Sensor Bus category. The AS-Interface UK Expert Alliance supports and promotes the technology in UK.

The group provides to its members the latest market and technology information, including support to technical information, product certification, activities, courses, exhibitions and other events. Further details on http://www.as-interface.net or http://www.as-interface.com.

In 1999 the AS-i network was regulated by the EN 50295/IEC 62026-2 standard. The associations AS-International Association or AS-Interface UK Expert Alliance are open to new members interested in developing certified products.

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3. Benefits



An industrial system formed by AS-i networks is considered to be the most economical and ideal for communication between actuators and sensors. The benefits for using an AS-i network range from hardware savings to the commissioning of an AS-i network itself.

Figure 3.1 illustrates some factors that should be considered when choosing an industrial network and each particular benefit when using AS-i networks.

Criteria for choosing the industrial network
Figure 3.1: Criteria for choosing the industrial network
Source: AS-International Association (2008)

 

Under this approach, the benefits may be summarized as follows:

Simplicity
An AS-i network is very simple and needs only one cable to connect the input and output modules from any manufacturer. AS-i users do not need deep knowledge of industrial systems or communication protocols. Unlike other digital networks, the AS-i network does not need terminators or equipment description files. Simplicity is its strong point.

 

Performance
AS-i systems are efficient and very fast, making them able to replace large and high-costs systems. There are AS-i masters specially designed to communicate with legacy control systems and provide a smooth integration of existing technologies. Best of all is that this is accomplished in a simple and reliable way.

 

Flexibility
Expansibility is very easy to get – just connect a module, address it and then connect the network cable. Check if the power supply LED is connected and the connection to the next module is enabled. The AS-i network supports any cabling technology: star, bus, tree, ring or other configuration up to 100 m of cable. Or else, by adding repeaters it is possible to expand the system up to 300 m. The AS-i network is easy to install, since it needs no terminators at the ends.

 

Cost
AS-i networks typically reduce cabling and installation costs by 50% in comparison to other conventional networks (Figure 3.2). The use of a single cable for connection to discrete devices reduces the need for cabinets, conduits and trays. The savings obtained in the network are really significant, since using few cables brings down installation and commissioning costs and engineering time.




Figure 3.2: a) Conventional systems; b) AS-i network.
Source: Stonel Corporation

 

Cost savings with hardware and the AS-i network viability for especial applications are shown in Figure 3.3.



Figure 3.3: AS-i system economic viability
Source: AS-international Association (2008)


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4. Versions and Specifications

Original Specification (1994, Version 2.04)
In the early networks, the slave modules interlinking the final elements enabled the connection of four digital inputs and four digital outputs, resulting in a total of 124 inputs and 124 outputs on a single network (AS-i 2.0 or AS-i 1 specs). However, that architecture had only a maximum 31 slaves.

Its main features were related to the automatic substitution of a network module and the update time was easily calculated by multiplying the number of I/O nodes with the deterministic update time for each node (approximately 150 microseconds).

This simplified calculation does not include the Management Phase which is negligible for typical installations.

 

Enhancements (1998, Version 2.11)
Following its introduction the users quickly adopted the technology, driving the demand for additional functionalities and features. As a consequence, these demands were addressed with certain specification enhancements and the specification for the AS-i 2.1 (or AS-i 2) was released. The new functionalities added are:

  • Increase the number of possible binary devices from 31 to 62 at one master. The maximum bus capacity increased to 248 + 186 I/O, but the cycle time changed to 10 ms.

 

  • An additional bit on the status record is used to signal peripheral errors. The indication of the slave performance was standardized and expanded.

 

  • The number of slave profiles increased from 15 to 225 with the addition of new ID codes.

 

  • Better analog signals treatment, thereby broadening the spectrum of action of AS-i networks.

 

 

The chips for the AS-i network version 2.1 are made by two consortiums: Siemens and Festo, the joint developers of the SAP4.1 chip, compatible pin by pin with the SAP4, and the group of eight other members (Bosch, Hirschmann, ifm electronic, Leuze, Lumberg, Klockner Moeller, Pepperl+Fuchs, and Schneider Electric), which developed the A2SI chip. Both chips provide all functionalities of 2.1 version.

Additional Capabilities (2005/2007, Version 3.0)
Up to 2005, the AS-i network success worldwide, with approximately 10 million nodes in operation, promoted the introduction of new requirements for the network. Furthermore, the increased usage of Ethernet based industrial protocols called for a low-level solution that overcome the inherent shortcomings of Ethernet (e.g. restricted topology, large data frame, costly usage of switches etc). This specification addressed the users’ requirements by defining new profiles for binary and analog data plus the introduction of a serial data transmission profile (3.0 or AS-i 3 specs). The following is an incomplete list of the new capabilities

  • Binary I/O nodes supporting extended addressing mode (A/B) with 4 inputs and 4 outputs;
  • Binary I/O nodes supporting extended addressing mode (A/B) with 8 inputs and 8 outputs;
  • Configurable (8, 12 or 16 bits) fast analog channel;
  • Full Duplex bit serial data channel.

With these new capabilities, AS-i becomes the ideal partner network for of the currently available Ethernet based industrial protocols. Gateways to Ethernet//IPTM, PROFINET, Modbus/TCP and others are available.
Some controls experts have voiced the opinion that within the next 10 years networking solutions positioned between AS-Interface and Ethernet will not be used in any new installation.

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5. Characteristics



The name Actuator Sensor Interface is a simple and elegant solution to integrate discrete sensors and actuators on process control systems. This network has a series of features, as follows:

  • Compatibility: Sensors and Actuators from different manufacturers may be connected to a standard digital serial interface;
  • Access procedure: Cyclic polling, single-master system;
  • Addressing: Slaves receive a permanent address from the master or hand-held type;
  • Topology: Without restrictions (linear, ring, star or tree structure);
  • Medium: Two unshielded, non-twisted cables (2 x 1,5 mm²) for data and electrical power (usually  24 Vdc), typically up to 200 mA per slave, up to 8A per bus;
  • Fast installation: electromechanical interface with piercing technology;
  • Cable length: range 100 m, scaleable by repeater up to 300m;
  • Signals: Data and electrical power via the same line, max. 8 A possible;
  • Number of slaves: Up to 62 slaves per network (version 2.1);
  • Data: 4 inputs and 4 outputs for each slave; for more than 31 slaves, only 3 outputs (maximum of 248 binary inputs/outputs per network).
  • Useful load: 4 bits/slave/message transmitted. All slaves are requested sequentially by the master and receive 4-bit data. Each slave responds immediately with 4-bit data;
  • Cycle time:  Max. 5 ms and 10 ms according to 2.0 and 2.1 spefications, respectively;
  • Error detection: Effective error detection and retransmission of incorrect telegrams;
  • AS-Interface chip: 4 I/O configurable for data, 4 output parameters and 2 control outputs;
  • Master functions: Cyclic slave scanning, data transmission for slaves and for the control unit (PLC or PC). Network initialization, slave identification, transferred slave and data diagnostic. Also, reports errors to the controller and addresses the replaced slaves;
  • Valves: Installed directly on the application, reducing piping and increasing the actuator response speed;
  • Low cost: Low connection cost per slave and eliminates PLC input and output modules;
  • Reliability: Highly reliable operational level in aggressive industrial environments;
  • Open standard: Developed by renowned industries affiliated to the AS-i International Association, whose transmission protocol is standardized;
  • Optional: output power supply cable and stop control.


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6. Connectivity



The AS-i network can be connected to main control level by two ways. The first one is the direct connection (Figure 6.1, left). In this case, the master is part of a PLC or PC being executed in the time cycle determined by these devices. An AS-i master can be built by any manufacturer, as it is an open standard.

The second way is connecting it with a gateway between a higher-level network and the AS-i network (Figure 6.1, right). There are other couplers for other field networks, such as Profibus, Interbus, FIP, DeviceNet, CAN, etc.
.



Figure 6.1 – Interconnection with other digital networks.
Source:
AS-International Association (2008)

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7. The Actuator Sensor Interface system



The name Actuator Sensor Interface represents its own concept. Although technically meaning a bus, the term interfaceshows that it provides a simple program for accessing field sensors and actuators.
The AS-i industrial networks were designed to be applied on automated environments to replace the traditional actuator and sensor switch (on/off) connections for a single bus. In addition it can also be connected to sensor and actuator buses that perform an analog conversion or vice versa.

Traditionally, these connections are made of twisted pairs that connect each actuator and sensor, one by one, to the corresponding controller, typically a PLC – Programmable Logic Controller.
The AS-i system is configured and controlled by a master that programs the interface between a controller and the AS-i system. The master continually exchanges information with all sensors and actuators linked to the AS-i bus in a pre-determined and cyclic way.
Figure 7.1 illustrates the entire AS-i system enhancing its main components: cable, AS-i power supply with its decoupling circuit, the AS-i master and slave.

  • Interface 1: between the slave and the sensors and actuators;
  • Interface 2: between the devices (power supply, master and slave) and the transmission medium;
  • Interface 3: between the master and the host, in other words, any entity that accesses the AS-i network from an upper level.

 



Figure 7.1: Components and interface
Source: SANCHES, L.B. (2004)

 

7.1 Transmission Medium

AS-Interface is a standard and open network system (EN 50295) that connects actuators and sensors in a very simple way. A single cable connects the actuators and sensors with the upper control levels.

The connection of the elements can be done in tree structure, star, line, or a combination of both. Since there are no conventional connections and with the reduction of terminal block and connector links, costs and mounting time, as well as errors, decrease.

In the simple connection technique using parallel cables, each device’s contact is connected separately to the ends and terminal blocks of sensors and actuators. The AS-i network substitutes the traditional arrangement of multiple cables, passage boxes, conduits, trails, and cable ducts for a single cable specially developed for the AS-i network.

The AS-i network features a single pair of wires that transmits the data and electrical power to the sensors and actuators (usually 24Vdc) at the same time. The maximum network configuration includes 62 slaves that are accessed cyclically by a master on the upper control level. The response time is short for every connected slave: 10 ms.

Formerly, sensors and actuators were connected to the controller via the terminals, connectors and terminal blocks. AS-i enables the reduction of installation and maintenance costs, with a standard twisted cable that allows the exchange of data and electrical power between devices. Slaves are connected directly to the bus, without additional wiring. A flexible two-way cable was designed as standard for the AS-i network. There is also a round shape cable for use only under the manufacturer specification.

 

7.1.1 Standard Flexible Cable
This H05VV-F 2X1.5 high voltage flexible cable complies with the CENELEC or DIN VDE 0281 standards, it is inexpensive and easy to get.


Figure 7.2: Typical AS-i cables
Source: AS-International Association (2008) e Turck Networks.

 

The unshielded, non-twisted AS-i cable has two parallel conductors that convey data and power to the slaves. Its external jacket is yellow and has a characteristic geometric shape that was designed to avoid fixation with reversed polarity (Figure 7.3).


Figure 7.3: Cross section of AS-i cable.
Source: LIAN, S.C.P. (2003)

The cable does not need cutting or stripping to be connected. This practice generally causes undesirable voltage drops and is a constant bad contact source. On the other hand, it has an interesting way to be installed, which favors costs savings in its implementation.

This is a simple principle: the contact with the internal conductors is done by conductive blades that penetrate in the plastic insulation to reach the internal copper wires.

The internal shielding has a “healing” property that closes it when the blades are disconnected, without being seen when being cut lengthwise. Evidently the shielding remains perforated, but without the risk of a short circuit. Figures 7.4a and 7.4b illustrate the concept.

 



Figure 7.4: a) Module and bus coupling; b) Perforation pins
Source:  SILVA, W.A.C.M (2008) and AS-International Association (2008).

 

In addition to the power supply available to slaves through the yellow cable – that became a sort of registered trademark for the AS-i system – and serves almost all purposes, some slaves may need a supplementary power supply, especially the most powerful actuators. An additional black cable with the same properties is used only for supplying power. It also uses the same previous penetration technique and supplies up to 24 Vdc.

When selecting an adequate transmission media, two relevant electrical considerations should be done: the DC resistance on the power supply and the transmission features on the frequency band used on the communication. At least 2A of current must be available for transmission on the slave power supply. Within these requirements, other cables can be used on specific cases, like for conducting larger currents or the need for movable cables. Besides these two types of cables there is also a red version for until 230 Vac.

 

7.1.2. Round Cable
This cable was designed specifically for the AS-i, with almost similar electrical features, but with a specific type of installation. This cable can be shielded or unshielded, but preferably the unshielded are used, with the following characteristics (at a frequency of 167 kHz):

  • R´: < 90 mΩ/m
  • C´: < 80 pF/m
  • Z : 70 Ω to 140 Ω
  • G´:  ≤ 5 µS/m

It is also recommended a cable with a transversal cross-section of 2 x 1.5 mm2.



Figure 7.5: Unshielded round cables
Source: AS-Interface Association

 

7.1.3 Connections of the AS-i line
Any connection to the AS-i line shall meet the following requirements, whether a conventional technology or an insulation piercing technology is used:

  • Contact resistance maximum of 6 mΩ;
  • Minimum allowable current of 1.5 Inom (minimum of 3A for a general AS-i line);
  • Contact voltage range of 10 V to 70 Vdc;
  • Shock and vibrations in compliance with item 7.4 of IEC 60947-5-2;
  • Strain relief in compliance with annex E of IEC 60947-5-2;

If a clamp or a screw terminal connector is used for connections, its capability shall be at least a 2 x 1.5 mm2. If plug connectors are used, the D.2 type according to annex D of IEC 60947-5-2 is preferred.

 

7.1.4 Cable Length
The maximum length for an AS-i network cable shall not exceed 100m without the use of repeaters. A maximum of two repeaters to extend the length of the line to 300 m is permitted. Four repeaters may be used to extend the line to 500 m if the master is centrally positioned on the first line segment.

The length of AS-i line is calculated by adding the line length to two times (2x) the length of the connection accessories.
Example: 50 m of yellow cable and 5 tap-offs with 2m of cable gives a network length of  50 + 2 x (5 x 2) = 70 m network.

Figures 7.6 and 7.7 show solutions for extender and repeater connections to extend the AS-i line.

 



Figure 7.6: Solution with one extender and one repeater 
Source: AS-Interface Association

 



Figure 7.7: Solution with two repeaters
Source: AS-International Association (2008).

 

 

7.2 Power Supply



The AS-i power supply has four functions, as follows:

7.2.1 Power Supply
The power supply unit works with a voltage of 26.5 V to 31.6 Vdc, and supplies a current of 0 A to 8 A (typical current per slave is 200 mA) under normal operating conditions. The power supplied to the slaves and partially to the master through two wires is the same used to transmit AS-i data, and can be connected to any point on the network. On long lines, the voltage drops must be taken into account but should not exceed 3 V on a whole 100 m cable. The unit has an internal overload protection circuit with current limit.

7.2.2 Balancing
The power supply unit also balances the AS-i network, which is operated as a non-grounded symmetrical system. For noise immunity the AS-i cable must installed as symmetrical as possible through the balancing circuit shown in Figure 7.8. The shielding connection must be at an adequate point on the machine or the system. Only at this point it can be connected to the system ground (GND).

 



Figure 7.8: AS-i power supply simplified diagram
Source: SANCHES, L.B. (2004)

 

7.2.3 Data Decoupling
The third function of the power supply unit is decoupling data, which is normally done by the decoupling network on the power supply module. This network consists of two inductors of 50 µH each (L1 and L2) and two resistors of 39Ω each in parallel. The inductors perform a differentiation job on the voltage pulses to convert the current pulses generated by the transmitters connected to the network. At the same time, they prevent short circuits on the cable. The coupling between the inductors shall be as close as possible to 1, meaning that the mutual inductance must tend to 200 µH.

7.2.4 Safety
The fourth function is related to safety. The AS-i system was designed as a system for low voltages with safe separation (Protective Extra Low Voltage). In other words, according to the relevant IEC standards, safe separation between the power supply and the AS-i network is required.

 

 

7.3 Redundancy

This situation is not very common on AS-i network, since it is a system where prevail discrete communication and also robustness, determinism and simplicity. The redundancy can work at the master and the power supply levels. So far there is no redundancy at the cables and slaves levels.

A network may have a redundant master. It will stay in monitoring mode and will take over control when noticing a failure or lack of communication on the part of the active master. In regard to the power supply, this is also possible with a power extender, when two power supplies are connected in a redundant mode.

 

 

7.4 Interface 1: Sensors and Actuators

7.4.1 The AS-i Slave
The AS-i slave, as shown in Figure 7.1, represents the link between the AS-i transmission system and Interface 1, to which the sensors and actuators are connected. The slave powers the sensors/actuators and handles communication between them and the master. When the AS-i specification was developed, it was clear that the slave needed to be small and compact as well extremely inexpensive to be able to be integrated directly to the sensors and actuators. This can be achieved only through the use of a highly integrated circuit, which originated the famous AS-i chip.

The AS-i slave chip allowed for sensors and actuators to be integrated to the AS-i bus as a slave device that recognizes the master output command and sends data in response. A great number of sensors and actuators used recently in automation allow low cost per connection in AS-i slave chips. In the case of analog devices, the data exceed 4 bits of useful information per cycle: the data are divided and sent in several cycles.
An AS-i chip can be used in two possible ways:

  • Embedded in sensors and actuators, with the elements integrated to the AS-i (Figure 7.9) and every data bit and parameter available to the device (sensor or actuator).

 



Figure 7.9: Sensor or actuator with “integrated AS-i”.
Source: AS-International Association (2008).

 

  • Another way is using the AS-i slave chip embedded in modules where conventional sensors and actuators may be connected. Figure 7.10 shows a module with two inputs for sensors and two inputs for binary actuators.

 



Figure 7.10: Module 2I/2O for conventional sensors/actuators
Source: AS-Interface Association.

 

7.4.1.1 Structure of the AS-i Slave

Regardless of whether the slave is implemented using the integrated circuit or some other way, it will have the structure shown in figure 7.11, whereby the connections for interface 1 may be physical or logical, and those for interface 2 must be physically present.

In the supply voltage the data path is separated from the power path by an inductance.
In order to implement it in a integrated circuit (IC), this inductance is realized electronically and guarantees that the slave sustains a highly-enough resistance on the communication data frequency band. The supply voltage is furnished on the Vout terminal.



Figure 7.11: AS-i slave architecture
Source: SANCHES, L.B. (2004).

 

In the receiver the voltage pulses detected on the AS-i cable are filtered, digitized and written to the receive register. At the same time the received signal is subjected to various plausibility checks to ensure that no noise pulses have corrupted the master request.

In the sender the information from the send register is encoded and sent out over the AS-i cable as a current pulse sequence according to the APM modulation.

The sequence controller finally decodes the master requests, checks them for errors, carries out the commands encoded in them and if appropriate causes a reply to be sent. The sequence controller also has a memory which is used for recording the slave address and which can store it for an unlimited time without power (non-volatile).

The slave has the following registers and flags:

  • Address register: This 5 bit-wide register contains the current slave address. If the address sent in a master request agrees with the address contained in this register, then this slave will reply. After a RESET the register is loaded with the address contained in the non-volatile memory. Its contents can be changed by the master using the commands “Delete Address or “Address Assignment.
  • Identification registers: These 4 bit-wide registers contain the I/O configuration and the ID Codes of the slaves. They are permanently stored and loaded from the non-volatile memory after a RESET. They are (with the exception of the ID Code 1) fixed at the time the slave is manufactured and cannot be modified.
  • Data output register: The “Data Output” register is 4-bits wide and contains the data from the last “Data Request”, which was received without any error by the slave. Those bits that are allocated to an output in accordance with the I/O configuration are output on the respective data port, and the information from the other bits is ignored. After a RESET the register is loaded with the default value FHex.
  • Parameter output register: This 4 bit-wide register contains the parameters from the last “write parameter” request from the master that was received without error by the slave. The bits are output on the corresponding parameter ports.
  • Receive register: This 12 bit-wide receive register contains the last message sent by the master for further processing in the on the sequence control.
  • Send register: This 5 bit-wide send register has the slave response to be sent.
  • Status register: This 3 bit-wide register indicates certain slave status conditions or errors:
    • Flag S0: is set during address storage if the new address has not yet been permanently stored.
    • Flag S1: is set if the input FID reports a peripheral error.
    • Flag S3: is set if an error has occurred while reading the address from the permanent memory.
  • Synchronization flag: Ifthe slave has correctly received a master request, decoded it and, if appropriate, acknowledged with a reply, the “Synchronization” flag is set. In the synchronized state the master pause is monitored after the master request for one bit time, and the slave response can start after a two bit times.
  • Data exchange blocked flag: This flag is set by a RESET, and it is reset by errorless receipt of the first parameter request to its own slave address. This prevents data requests from being accepted as long as the parameter ports have not been loaded with the nominal parameters. This behavior is necessary for preventing “misunderstandings” between the master and slave. It could happen, for example, that due to poor electrical contact on the AS-i cable a slave – without the master knowing it – receives no supply voltage for a brief time and performs a RESET. Then the parameters ports are reset, and any associated functions of the slave are set to the default state. As a consequence, the slave could respond differently than the master expects.

 

7.4.2 Interface 1
As shown in Figure 7.11, the interface 1 has four data ports which, depending on the selected I/O Configuration, can be either as inputs ports, as outputs ports or bi-directional communication. A data strobe output is also provided, which signals when output data are present and when input data are expected.

For actuator slaves it is recommended that the time-out monitor, also called a watchdog and which is integrated in the slave IC, be activated. If, while a timing member is running, no new correctly received “Data Request” arrives at the address of the slave, the actuator can use the watchdog to place the system in a safe state. Such time-out monitoring allows a variety of error possibilities to be covered, such as hardware faults in the master, interference on transmission cable or loss of the address in the slave, and makes the AS-i system safer. Normally, a time-out period is specified from 40 to 100 ms.

In addition to the data ports, which are provided for cyclical data exchange with the master, additional ports are available which are used for (acyclic) parameter output. Here again an additional strobe ouput indicates when a new parameter message has arrived.

The FID input is used to signal peripheral errors. If the slave electronics detect an error (such as overloaded supply voltage caused by an external short), this input can be used to display the event locally, through LEDs and to report it to the master using the status register. The master makes an entry in the list of peripheral errors and sets a collective flag.

Finally, a DC voltage is provided to the connected sensor or actuator on the Vout port which is generally within the tolerance range of 24V +10/-15%.

 

 

7.5 Interface 2: Transmission system

Interface 2 includes the specifications necessary for an effective data exchange between the elements connected to the AS-i bus. It defines the way to accessing the physical medium, the data exchange on the electrical level and how to deal with some communication errors, as well as the time requests on the transactions.

 

7.5.1 Network structures
The topology for AS-i networks is left to the user’s discretion to simplify the project. The restriction is not observed if the maximum limit for the cable length is 100m. If longer lengths are required, repeaters are used to extend the network range, provided the limit of 62 slaves and one master is observed. No terminal impedance is needed, simplifying the installation. Structures in tree, linear, star or ring are possible (Figure 7.12).


Figure 7.12: Physical network topologies
Source: AS-International Association (2008)

 

7.5.2 Modulation
The selection of an adequate modulation for the AS-i should be taken into consideration the following requirements which led to the creation of a new modulation procedure known as Alternating Pulse Modulation (APM). The most important is:

  • The message signal superimposed on the supply voltage for the sensors and actuators must be direct current free;
  • The slave’s sender (and where possible the master’s sender) must be able to generate the signal in a simple, i.e. cost-effective and space-saving manner. 
  • Since the AS-i cable has impedance which increases greatly over the frequency, the message signal must be relatively narrow-banded.
  • High levels of noise radiation are also unacceptable.

 

The result was Alternating Pulse Modulation (APM), a procedure for serial transmission in the base band and is shown in Figure 7.13. The bit sequence is first encoded into a bit-sequence that performs a phase change whenever the signal changes (Manchester coding). The result is a send current that in conjunction with the single inductor in the system uses differentiation to generate the desired signal voltage level on the AS-i cable.

Each rise in the send current thus results in a negative voltage pulse and each drop to a positive voltage pulse. In this way it is quite simple to generate signals in the slaves that have a higher voltage than their actual supply voltage. This eliminates inductors from the slaves and keeps the integrated electronics small and inexpensive. On the receiver side these voltage signals are detected on the line and converted back into the send bit- sequence. The receiver synchronizes itself with the first detected negative pulse, which it interprets as a start bit of message.

If the voltage pulses approach sin2 pulses, the requirements for low limit frequency and low noise emission are met at the same time. This is done by means of suitable shaping of the send current pulses, which are generated like the integral of a sin2 pulse. Using this modulation procedure and the available topologies, bit times of 6µs are attainable, which allows a gross transmission rate of 167 kbit/s.

Since the cables do not have terminators, the message pulses have a large amplitude variation. The AS-i represents an extremely robust system able to deal with the problem caused by cable end reflections that reach the higher frequencies.

 


Figure 7.13: AS-I network modulation signal
Source: SANCHES, L.B. (2004)
.


7.5.3 Access procedures
Since the AS-i was designed to replace star type 2-point connections (traditional cable tree), a bus access procedure was selected which reproduces this topology and is able to ensure a defined response time (the master-slave access with cyclical polling). The master sends a telegram that is received at a particular slave address, and the slave contacted at this address replies within the provided time. This operation is described like a transaction. The transmission system only enables the connection to a bus with one master and a maximum of 62 slaves.
The procedure chosen for AS-i allows the construction of very simple and thereby cost-effective slaves while providing at the same time the greatest possible flexibility and integrity. In case of a brief disturbance on the line, for example, the master can repeat telegrams to the address which received either no reply or an invalid reply. This means it is not necessary to repeat the entire cycle over again.

 

7.5.4 AS-i messages
There are two types of AS-i messages: those sent by the master and the slave responses. Figure 7.14 illustrates a transaction and the periods of time involved. The AS-i message consists of a master request, a master pause, a slave response and a slave pause. All master requests are exactly 14 bit times in length, and all slave responses have a length of 7 bit times. A bit time corresponds to a uniform 6µs.


Figure 7.14: Structure of an  AS-i. message
Source: SICK Industrial Sensors  (2010).

 

The master pause is allowed to be at least 2 and maximum of 10 bit times in length. If the slave is synchronized, it can begin to send its response after as soon as 2 bit times. If it is not synchronized, it requires 2 bit times longer, since is monitoring the master pause during this time for any additional information before it can accept the poll as valid. If the master, however, has not received the start bit for the slave response after 10 bit times, it can assume that there is no response forthcoming and it can begin with the next request. The pause (slave pause) between the end of a slave response and the next master request should be no more than 1.5 to 2 bit times in length.

A master poll of a standard slave consists of:

  • Start Bit (ST). Identifies the beginning of the master request. Its value is always “0”.
  • Control Bit (SB). Identifies the type of request: “0” for data, parameter request or address assignment; and “1” for command request.
  • Address (A4..A0). Address of the contacted slave address requested (5 bit).
  • Information (0, D3…D0). These 5 bits contain in order to the type of request, the information to be transferred to the slave.
  • Parity Bit (PB). The number of all "1" in the master call has to be even.
  • End Bit (EB). Identifies the end of the master request. Always has value “1”.

The slave response consists of:

  • Start Bit (ST). Identifies the beginning of the slave response. Its value is always “0”.
  • Information (D3..D0). These 4 bits represent the properly information sent to the master.
  • Parity Bit (PB). The number of all “1” in the slave response has to be even.
  • End Bit (EB). Always with value “1”, it signals the end of the slave response.

AS-i Specification Version 2.1 (1998) created the possibility of connecting 62 slaves to an AS-i network instead of previous 31. To make this possible, an information field bit is used for select bit, as it is known. Hence, the slaves connected to the bus were divided in two groups of a maximum 31 slaves: group A and group B. Therefore, a slave, besides having an address, received a distinguishing type: A or B. This change was introduced in a way to avoid losing the compatibility of old version slaves with new version masters.
The former slaves can be addressed normally, but occupy two addresses each. They do not distinguish slave “A” from slave “B” and do not recognize a selection bit as such, but as normal information bit. The masters adapted to the new version have ways to identify the slave type and send requests properly. This will be seen further on.

 

7.5.5 Data integrity and error response
Reliable error recognition is of great importance for a faultless communication over the AS-i cable, which is generally unshielded. Since the AS-i telegrams in the transactions are quite short, the error detection is different from the one normally applied to other field networks. The master request contains 11 bits of data to be checked, and the slave response has 4 bits. The addition of bits for checking message errors would drastically reduce the achievable network transmission rate.

So instead AS-i performs greater checking on the bit transmission itself. This makes use of the knowledge of redundancies in the code and the fixed lengths of the telegrams. As a result the following errors can be distinguished:

  • Start bit error;
  • Alternating error;
  • Pause error;
  • Information error.
  • Parity error;
  • End bit error; and
  • Telegram length error.

Each master request and each slave response is subjected to these checks. If one of the errors named is detected, the request is considered faulty or the response is invalid.

 

7.5.6 Analog Signals
AS-i supports the transmission of analog signals and the digitalized analog signal value is separated in multiple parts and transmitted in several cycles. An analog input signal of 12-bit data requires 6 cycles, forming the total transmission time of 30 ms (in the version 2.1 of the AS-i specification).

An A/D conversion circuit must be integral part of the slave device with analog I/O signals. This circuit will execute not only A/D conversion, but also freeze the converted value until all bits are totally transmitted, and only then will be ready for a new analog signal sample.

The AS-International defined a standard for analog signal transmissions (defined on the profile S-17 of the AS-Interface specification). To facilitate its use, some PLCs already offer functional blocks to be applied with analog signal values.

To ensure data consistency between master and slave, a handshake bit was defined in the profile, which is inverted by the slave and returned. Thus the master may check if the slave has responded and the slave may check if the master wants the last request to be repeated or if it wants the next data. On the other hand, this reduces the useful load on each frame to 3 bits per cycle, while ensuring the right transmission of data even with disturbances. For analog inputs, the master requests and the slave responds; for analog outputs, the slave requests and the master responds.


Figure 7.15: Analog value transmission sequence.
Source: ATAIDE, F.H. (2004).

 


7.6 Interface 3: The AS-i Master

The AS-i master represents the bridge to the user’s controller or to a host fieldbus system. It organizes data traffic on the AS-I cable independently, so that at its interfaces to the controller and to the connected sensors and actuators the system behaves like a traditional cable tree, while allowing additional user functions such as the transmission parameter settings or monitoring and diagnostics information.

The AS-i specification divides the master into three layers, which describe the master from the AS-i cable to the host interface (Figure 7.16).


Figure 7.16: AS-i Master Structure in layers 
Source: SANCHES, L.B. (2004)

 

The physical transmission of the requests from the master is specified by the interface 1 and has basically the same characteristics of the slave.

The lower logic layer is the transmission layer, which is responsible for the transmission and reception of individual telegrams. The automatic telegram repetition when the slave response fails is possible and ensures the upper layers integrity.

The sequence control or execution control is just above the transmission layer and passes requests for data transmission to the latter. The sequence control controls the sequence telegrams. In addition to the actual sequence control, the sequence control layer processes functions that are requested by the host through the master layer. In addition to the sequence control, the sequence control layer processes functions that are requested by the host through the master layer.

The highest layer is the master layer, in which the AS-I functions are adapted to the respective host system. This layer is where profiles are formed which allow a restriction of the master functions usable by the host.

 

7.6.1 Master Requests
The AS-i master connects the interfaces 2 and 3 through messages sent to the slaves, one by one. In the following section are shown some possible few requests that the master can perform to a given slave through the interface 2, and the behavior expected from the slave to cope with these requests or, in reality, its responses.

Figure 7.17 shows all possible requests a master can make to a slave, per AS-i specification 2.1, which accepts the extended addressing. One can notice the presence of a select bit on the requests. This bit replaces the bit previously used on the exchange of common data. The addition of this new bit made possible addressing twice as much the initial number of slaves: 62. The slaves then, besides having an address between 0 and 31 gained an A or B type, that is defined by the select bit.

 



Figure 7.17: Master requests based on specification 2.1
Source: BECKER et al. (2002

 

These requests are analyzed one by one, as follows.


Read I/O configuration: With this request the master can read the set I/O configuration of a slave.  This is sent in the slave response to this request and is used together with the slave “Read ID-Code” requests for unambiguous identification of a slave. The I/O configuration refers to the data ports on Interface 1 of the slave and is defined as shown in Figure 7.18. In this definition IN means a process input, OUT a process output, I/O a bi-directional behavior of the port, and TRI indicates high-impedance outputs with no funcion. The latter state is assumed when during the reset a read error of the slave´s data memory means that no unambiguous I/O configuration could be determined.

Wherever an output OUT (and no bi-directional behavior) is defined, this means that the corresponding information bit in the slave response is undefined. Likewise, the information bit from master request remains without meaning wherever an input (IN) is defined, even though the bit is sent.

 


Figure 7.18: I/O configuration.
Source: BECKER et al. (2002)

 

This I/O configuration allows unneeded information to be hidden. At the same time the I/O configuration is used together with the ID Codes for identification of the slaves. This reference is the slave profile. The I/O configuration is 4 bits long, is fixed by the manufacturer and is stored in the slave so that it cannot be changed.

Read ID Code: The slave ID code compliant with the version 2.1 of the specification has two other codes, besides the original ID code read by the master on the “Read ID Code” request: "Extended ID Code 1" and "Extended ID Code 2". Together they identify different slaves. The slaves compliant with the new specification have the ID code “A” in hexadecimal, while an ID code “B” indicates a “safety at work” slave. All of the slaves with an ID code equal to “A” also have two other ID codes.

Read Extended ID Code 1: This command is similar to the previous one and reads the slave extended code 1. The user can change this part of the ID Code.

Read Extended ID Code 2: This code extends the slave configuration possibilities and, as the original ID code, cannot be modified by the user, being explicitly defined by the manufacturer.
A slave profile issued by the combination of the ID codes and the I/O configuration contains a parameter behavior definition, data ports and other additional slave characteristics.

Data Request: This command is most commonly used on the AS-i to attribute slave output values on the interface 1 and get input values in response. As said before, the ports behavior is defined by the I/O configuration. The data ports can be used in various ways, not only as binary inputs and outputs of process variables. They also are used as additional configurations and as digitalized representation of analog process variables.

Parameter Request: This command is used by the master to send the bit pattern for the parameter outputs on Interface 1 that control certain functions in the slave. The last sent parameter value is stored in the slave until it is overwritten by a new one or reset.

For a standard slave 4 bits of parameter data are available, and 3 bits for a slave in extended addressing mode, since one of them is used as selection bit. A parameter request to address 00HEX is not possible, since the slave would interpret this as an addressing request.

Address Assignment: This command allows the master to permanently set the slave address with the previous address 00HEX to a new value.

The slave sends a recognition response and starts the recording of the non-volatile memory, which cannot last longer than 500 ms. During the process the slave begins responding to the requests on the new address. This request allows replacing damaged slaves without restarting the network.

Reset Slave: This command can be used to set a slave to its base state. It has the same effect as the reset after applying supply voltage or the reset on the reset input of Interface 1. It does not last longer than 2 ms.

Delete Operating Address: The command “Delete Address” is used to temporarily delete the operating address of a slave and is needed in conjunction with “Address Assignment” because the “Address Request” can only be performed by a slave having an operating address 00HEX. For example, to change a slave address the “Delete Address” request is used first and next the “Address Assignment”. The slave acknowledges error-free receipt of “Delete Address” with the reply 00HEX and can be reached from this point on under the new address. Deleting the operating address in this manner is not permanent. To restore the original address stored in non-volatile memory after executing this command, use the command “Reset_AS-I Slave”.

Read Status: This request is used to read out the status register of the corresponding slave. Its content is sent in the slave response to this request. The status register of a slave contains three flags whose meanings are as follows:

  • S0: "Address Volatile". This flag is set when the slave-internal routine for permanently storing the slave address is running.
  • S1: "Peripheral Error". This flag is set when the slave has detected a high input on the interface 1 FID port, which indicates an external failure on the equipment.
  • S3: "Read error non-volatile memory". This flag is set when a read error occurs during a reset while reading the non-volatile memory.

The bit S2 is not used yet and is reserved for future enhancements. The master can use the information from status register for diagnostic purposes. Slaves according to Specification 2.0 do not support the flag “Peripheral error”. When the master is communicating with a slave having extended ID Code 2 equal to FHEX, it ignores flag S1.

Broadcast: command requests containing 15HEX are defined as “broadcast” commands. These are characterized by the fact that they do not need to be replied to by the slaves. For this reason, they are atypical of normal AS-i data communication and until now only the broadcast command “Reset” is defined.

 

7.6.2 Transmission Layer
This layer, also known as transmission control, exchanges individual telegrams with the slaves. It receives a remittance request from the sequence control, together with the data that will be sent via the communication channel with the addition of the start bit, the parity bit and the end bit, which generate the master frame (telegram). This telegram is sent according to the time requirements of the transmission system, as described on 7.5 section.

The function for data transmission offers the sequence control, two transmission methods: one-time transmission or repeatable transmission.

In the first case, if there is no response from the slave after the waiting time limit, or, if the response is not a valid one, the transmission control reports the error immediately to the upper layer, without resending the telegram.

In the second one, the error is only reported after a second unsuccessful try. On the other hand, if the transaction is successful, the transmission control provides the sequence control with the data sent by the slave without the additional start, end and parity bits. The transmission control also reports an AS-i Power Fail (APF) signal to indicate that the supply voltage on the line is too low.

Figure 7.19 illustrates the state machine that models the transmission control behavior. The MT initials come from multiple transmission and means that the transmission control resends a telegram in case of error on the first one.

Alternately, ST means single transmission, and the error is reported after the first failure. The status transitions occur in function of logical operations represented by the operators in italic and and or. The inputs and outputs are separated by a slash (“/”)



Figure 7.19: Transmission control
Source: SANCHES, L.B. (2004)

 

7.6.3 Data fields and lists on the sequence control
The execution control or sequence control generates AS-i telegrams in the correct sequence, using the individual frame sending service executed by the lower layer. To this effect, it has a set of data and lists that keeps the image of the AS-i network and its slaves constantly updated, which is used by the upper layer to implement the interface with the host.

These data fields are presented as follow:

Input Data Image (IDI): This field contains the copies of the most recent slave data received by the data request. Each slave occupies 4-bit memory.
Output Data Image (ODI): This field contains the most recent data written by the host to be sent to the slaves by a data request. Here also are required 4 bits to each one of the 62 slaves.
Analog Input Data Image (AIDI): This field contains the most recent data received from the slave analog inputs according to the 7.3 and 7.4 profiles, like the IDI data.
Analog Output Data Image (AODI): This field, like the previous one, contains the most recent data to be sent to the analog outputs.
Configuration Data Image (CDI): This field contains the I/O and ID codes for each slave. Therefore, 2 bytes are required for each slave.
Permanent Configuration Data (PCD): This field is similar to the previous one, although in a non-volatile area. The ID codes or I/O configuration of a slave absent from the network are filled with the F value (hexadecimal).
Parameter Image (PI): This area is reserved for each slave 4-bit parameter. Therefore, two slaves occupy one byte.
Permanent Parameter: This field keeps the parameters configured for each slave and is in a non-volatile area, like the PCD.
List of Detected Slaves (LDS): Each slave corresponds to one bit in this list and is activated when the slave is detected correctly.
List of Activated Slaves (LAS): In this list the bit corresponding to the slave is activated when the slave is activated correctly.
List of Projected Slaves (LPS): This list is on the non-volatile memory and represents the slaves that are supposed to be connected to the AS-i network when it is turned on.
List of Peripheral Fault (LPF): In this list the bit corresponding to the slave is activated when a high signal is detected on the slave FID pin (section 7.4).

 

In addition to these data fields, the execution control reports the master conditions to the host through the flags. The flags are the following:

  • Config_OK: This flag is set when nominal and actual configurations are in agreement. This Config_OK flag enables simple monitoring of the configuration;
  • LDS.0: Indicates the presence of a slave with address “0”, which is not allowed on the normal operation;
  • Auto_Address_Enable: Indicates that the automatic addressing is enabled;
  • Auto_Address_Available: Indicates that there are conditions to execute the automatic addressing*;
  • Mode: Indicates if the master is on “Configuration” mode (1) or “Protected” mode (0).
  • Normal_Operation: Indicates that the master is transiting cyclically between the normal operation stages;
  • AS-i Power Fail (APF): Indicates voltage on the bus below the lower limit;
  • Offline_Ready: Activates when the offline phase is complete;
  • Periphery_OK: No slave is reporting a periphery error;
  • Offline: When activated by the user, switches the master from a sequence control to offline phase;
  • Data Exchange Active: Enables data exchange between the master and the slaves.

 

*For the sake of terminology, a distinction is made between this flag and the previous one. The first one is user-defined and permits the auto addressing, provided certain conditions are met, which is indicated by the “Auto Address Available” flag.

It’s noting that four of these flags are user enabled - host - and affect the master behavior: the two last ones in the list, “Auto Address Available” and “Mode”. All the others flags cannot be altered by the user and are controlled by the master.

The master behavior is divided in several stages, or phases, which are executed by the execution control.

Figure 7.19 illustrates the status device that shapes the execution control behavior. The dotted area indicates the normal operation mode, i.e., when the master performs a cyclic data exchange with the configured slaves and keeps the "Input Data Image" up to date and the data ports according to the "Output Data Image" area. Besides the exchange of input and output data carried out during the “Data Exchange” stage, information is exchanged in the normal operation cycle of the network management in the other two stages.

The operational detailing on each stage will permit to grasp the execution of the entire control process behavior, and, consequently, most part of the master behavior.

 



Figure 7.20:
Execution control stages
Source: SANCHES, L.B. (2004)

 

7.6.4 Transmission Phases
After the supply voltage has been turned on, the master cycles through various transmission phases. First initialization takes place in the offline phase, followed by the detection stage, when the slaves connected to the bus are identified. After being detected, the slaves are activated on the next stage and are ready to enter the normal operation cycle (dotted line in Figure 7.20), which is formed by a data exchange phase, a management phase and finally an inclusion phase. As well as the cycle of data exchange, management and inclusion phases is identified as a normal operation, the offline phase composes the master initialization, whereas the detection and activation phases compose the master start-up.

Initialization
Initialization during the offline phase places the master in base state. The data in the "Input Data Image" for all slaves are set to zero (inactive inputs), and the "Output Data Image" to one (inactive outputs). This ensures that the state of the slave outputs does not change when the master is turned on.

The sequence control can be brought to any other state to the offline phase by setting the offline flag. The offline flag thus has the function of resetting the complete network and the master.

Startup
In startup operation the sequence control detects all the connected slaves and activates them.

In the detection phase the master sends requests to read the slave I/O Configuration and ID codes, one by one. Slaves that respond to all requests are entered in the List of Detected Slaves (LDS). Their I/O Configuration and their ID codes are stored in the Configuration Data Image (CDI).

In the activation phase the master operation modes are considered as follows:

    • Configuration mode (also called as “project” mode);
    • Protected mode.

In configuration mode all detected slaves (with exception of the zero address) are activated by a "Parameter Request", where all slave output parameters are written, and a “Data Request” written on the corresponding slave ports. If the slave responds correctly to these two requests it is activated and is included on the LAS. In protected mode only the detected slaves which are also listed in the List of Projected Slaves (LPS) and whose I/O configuration and ID Codes agree with the projected configuration are activated. The master thus exchanges data only with the pre-configured slaves. At the end of the activation phase a check is made to see whether the nominal (detected) and actual configuration (projected) agree and then Config OK flag is set. Figures 7.21 and 7.22 show flow sheets that simulate the master behavior on the detection and activation phases.



Figure 7.21: Detection phase flow sheet
Source: SANCHES, L.B. (2004)

 



Figure 7.22: Activation phase flow sheet 

 

Normal Operation
The normal operation is where the actual data with the connected sensors and actuators takes places. A cycle consists of data exchange, management and inclusion phase.

 

7.6.5 Function sequence in the master

In normal operation after the detection and activation phase is concluded, there is cyclical communication between the master and all connected slaves. Such a cycle consists of the data exchange phase, the (optional) management phase and the inclusion phase. On each cycle, the management phase is attributed an AS-i transaction, and the same happens in the inclusion phase. This mechanism makes possible to keep a high scanning speed of all slaves, updating their output data and reading the input data in a same transaction, without harming the management operation on the network that occurs in the two other phases, which can be completed in multiple cycles.

Figure 7.23 illustrates the behavior of the master during the Data Exchange phase. Note that the communication with a certain slave must fail along the 3 cycles so that it is removed from the lists of detected slaves (LDS) and activated slaves (LAS). This guarantees that the network will operate adequately even in situations subject to noises that cause failures. In this last case, the Config OK flag is deactivated.

 


Figure 7.23: Simplified flow sheet of the data exchange phase.

 

After the Data Exchange phase is completed, the management phase may begin. In the management phase “acyclical” telegrams are sent to the slaves. Unlike the Data Exchange phase, when all activated slaves are accessed before the next phase, only one telegram is sent and the functions requiring more than one telegram to be completed are executed through several cycles.

During the management phase the master uses as many requests as possible to execute the tasks required by the host. If there is no function to be executed, the master may send status-reading telegrams, “dumb” telegrams or even send no telegrams in this phase.

After the management phase, comes the inclusion phase, when all new slaves are searched at the end of each cycle. During each cycle, a slave is requested. This request is carried out by the transmission control without repeat in case of error, because the error is not critical. If an activated slave responds, or there is no response, the next slave is requested at the following activation phase. If a non-activated slave responds, its ID codes are requested on the next phases and the LDS is then updated.

On the next phase, depending on the master operation mode, the slave is activated and moves into the LAS. The activation is carried out by sending a request parameter to the slave for update according to the Parameter Image (PI) field. Finally, on the last stage the master sends a data request by sending input data according to the "Input Data Image".

Hence, a slave inclusion occurs the same way as for the start-up procedures. If a slave with “0” address is detected during the inclusion phase, even if a projected slave is not present, the automatic addressing is blocked by the deactivation of the "Auto_Address_Available" flag. The same happens when a located slave is not projected. The Config OK flag is set at the end of each inclusion phase. Immediately next, the Data Exchange phase of a new cycle begins.

It is important noticing that each phase of the normal operation cycle alternates between the type A and the type B slave groups. Before the creation of the extended addressing, this did not happen, because the two types of slave did not exist. After the version 2.1, however, the cycle occurs alternately, as shown in Figure 7.24. It is worth mentioning that the slaves compliant with the old specification are accessed in every cycle, since they do not have A/B differentiation. Therefore, the data scanning cycle on the extended address is twice as large as the previous one, but the old-specification slaves continue to perform the exchange of data in the same time as the previous cycle.


Figure 7.24: Normal operation of the execution control with extended addressing
Source: SANCHES, L.B. (2004)


7.6.4 The master layer and the Interface 3
The master interface with the host (interface 3) is only defined in a logical way, while the implementation is up to the manufacturer to carry out. It is specified by the standards only as functions that should be executed by the master. The master layer is the one that adapts these functions to the specific host. As explained earlier, the host is normally a fieldbus system with an upper hierarchy, like a PLC or a PC. So, accessing the master and thus an AS-i network is done in many ways. If the host is a PC, for example, the master may be on a board connected to the main board and is accessed through drives that implement the function defined by the interface 3 and are adapted for this system on the master layer. The possibilities are virtually innumerable.

If the host is a PLC, the master is nearly always an external module, regarded as an ordinary I/O module, which is accessed by adequately mapped memory areas. The interface is implemented in a different way. The same reasoning applies to the AS-i gateways. On the market, there are AS-i gateways for Profibus DP, DeviceNet, Modbus, and others.

There are few functions that make possible the effective data exchange between the master and the slave. Most of them access the master data fields that maintain an updated image of the network to collect the information they need. Those carrying the data exchanged are executed on the management phase, which may send a telegram on each cycle.

The functions:

  • "Read Input Data",
  • "Write Output Data",

They access the "Input Data Image" and “Output Data Image" fields to return or write data adequately.

  • "Write Parameter"

It writes a set of parameters on ”Parameter Image” and also executes a request of parameter script on the management phase. Thus, when a request…

  • Read Parameter

…is performed by the host, the slave must be accessed directly, but only on the parameter image, which is always updated with the slave real parameters.

In addition to these functions there are:

  • "Get LDS",
  • "Get LAS",
  • "Get LPS",

They return the corresponding lists and also access the adequate data fields on the execution control.

The permanent data are accessed by:

  • "Get LPS";
  • "Get Permanent Parameter";
  • "Get Permanent Configuration";

And it is recorded by the corresponding

  • "Set LPS",
  • "Set Permanent Parameter",
  • "Set Permanent Configuration".

Note that the functions that recording data permanently bring the master to the offline phase and reinitiate its behavior.

It is also possible to read the configuration fields through the functions:

  • "Read Actual Configuration Data",
  • "Read Parameter Image".

The functions:

  • "Project Actual Configuration Data",
  • "Project Actual Parameters"

They implement one of the AS-i features that most contribute to simplify its use: to be able to store a configuration detected as permanent, in order to make the AS-i network configuration even simpler.

Finally, it is possible to collect information about the master behavior through:

  • "Get Flags"

and configure it by:

  • "Set Configuration Mode",
  • "Set Offline Mode",
  • "Set Data Exchange Active",

which activate the corresponding flags.

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8. Safety at Work

The AS-i Safety Work Systemallows safety components such as emergency stops, light barriers, safety limit switches, contactor control, etc directly to the AS-i network. The objective is to add functionalities related to personal and equipment safety. There is no need for changes or expansions on the existing system, because the safety components and the monitor work with the AS-i standard protocol.

Only are needed adding safety components to the system. The safety monitoring is not performed by the network master. For this purpose, a safety monitor is the component required by a safe network.

The monitor does not interfere with the network traffic and just monitors it to determine the safety condition. Hence, equipments and plants can be adjusted on a safe status through the safety monitor outputs. See an example for a network with safe components in Figure 8.1.


Figure 8.1: Safety-related and standard components in an AS-Interface network.
Source: AS-International Association (2008).

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9. AS-i Limitations

The AS-i was intentionally conceived and optimized for use below the fieldbus systems described earlier (section 6). Some features of the higher fieldbuses are either not realizable in conjunction with AS-i or can be implemented only with limitations. Here is what the user should know:

  • The amount of data transmitted using AS-i is limited to 4 bits per slave and can be communicated per cycle. Longer messages can be sent by dividing them over multiple cycles and this may be used on processes with slowly changing parameters such as temperature, level or pressure (analog values).
  • AS-i is strictly master-slave, with cyclical slave scanning. This avoids asynchronous transmissions by sensors and actuators. The slaves must wait 10 ms (in the case of network with 62 slaves) until being requested again.
  • The data transmission from slave to slave is only possible through a master.
  • The AS-i network can be up to 100m without the use of repeaters. This is a physical limit and was a compromise settled on during development as a result of required cable structure (unshielded 2-conductor cable), topology flexibility, the targeted cycle time and the number of stations, as well as the elimination of terminating resistors at the cable ends.

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10. Standards and Regulations

AS-i components and installations shall comply with all statutory regulations and the latest edition of the following:

  • Statutory Obligations (including the Electricity Ordinance, Chapter 406);
  • IEC 364: Electrical installations of buildings;
  • IEC 60947-1: Low-voltage switchgear and controlgear – Part 1: General rules;
  • IEC 529: Degrees of protection provided by enclosures (IP Code);
  • IEC 439: Low-voltage switchgear and control gear assemblies;
  • IEC 62026-1: Low-voltage switchgear and controlgear – Controller-device interfaces (CDIs);
  • IEC 62026-2: Actuator sensor interface (AS-i);
  • IEC 61131-2: Programmable controllers – Part 2: Equipment requirements and tests;
  • IEC 61076-2-101: Connectors for electronic equipment: Part 2-101 Circular connectors (M8 & M12);
  • EN 954-1: Safety of machinery. Safety related parts of control systems. General principles for design;
  • CISPR 11: Industrial, scientific and medical (ISM) radio-frequency equipment – Electromagnetic disturbance characteristics – Limits and methods of measurement.

In addition to the local regulations for installations, inspections and tests of equipments shall be taken into consideration.

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11. References

11.1 Bibliography

  • ATAIDE, F.H. (2004). Estudo Técnico EST-DE-0007-04 - AS-Interface, SMAR Equipamentos Industriais Ltda, fevereiro.
  • BECKER, R.; MÜLLER, B.; SCHIFF, A.; SCHINKE, T.; and WALKER, H. (2002). AS-Interface - The Automation Solution, AS-International Association, Germany.
  • ERCEGOVAC, M.; LANG, T.; and MORENO, J.H. (2000). Introdução aos Sistemas Digitais, Bookman.
  • LIAN, S.C.P. (2003). AS-i Master Bus Controller. Thesis submitted to the School of Information Technology and Electrical Engineering, The University of Queensland, June.
  • MADELUNG, O.W. and KRIESEL, W.R. (1999). AS-Interface - The Automation Solution, Hanser.
  • SANCHES, L.B. (2004). Estudo Técnico EST-DE-0087-04 – Protótipo AS-i, SMAR Equipamentos Industriais Ltda, agosto.
  • SICK Industrial Sensors (2010). AS-Interface Technologies. Available on: http://www.sick.com/us/en-us/home/Pages/Homepage1.aspx. Last access: July 16
  • SILVA, W.A.C.M. (2008). Notas de Aula do Curso Superior de Tecnologia em Automação, CEFET-RN, available on: http://www.cefetrn.br/~walmy/RI_A3.pdf. Last access: September 16.
  • STANDARDS AUSTRALIA. (2000). Low-voltage switchgear and controlgear - controller-device interfaces (cdis), Part 2: Actuator sensor interface (AS-i).
  • SYSTEM ENGINEERS (2008). General Technical Specification, Actuator Sensor Interface (AS-i). Available on: http://www.myelectrical.com/pages/tools/downloads/Acutuator Sensor Interface (AS-I) Specification.pdf. Last access: September 16.
  • Virtual Academy of AS-International Association. Available on: http://www.as-interface.net/academy/content/sys/start/start.en.html. Last access: November, 17 2008.
  • WIKI AS-INTERFACE (2008). AS-Interface. Available on: http://en.wikipedia.org/wiki/AS-Interface. Last access: September 13.

 

11.2 Additional Information

Further details on the AS-i technology also visit the portals:

AS-i International: http://www.as-interface.net/
AS-Interface UK Expert Alliance: http://www.as-interface.com/

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