What is PROFIBUS?
PROFIBUS is an open field, supplier-independent network standard, whose interface permits a vast application in processes, manufacture and building automation. This standard complies with EN 50170 e EN 50254 standards. Since January, 2000, PROFIBUS is firmly established with IEC 61158, alongside seven other fieldbus systems. IEC 61158 is divided in seven parts, named 61158-1 a 61158-6, which contain the OSI model specifications. This version was expanded to include the DPV-2. Worldwide, users can now use as a reference an international standard protocol, whose development aims at reducing costs, gaining flexibility, trust, orientation to the future, suit the most varied applications, interoperability and multiple suppliers.
Today, more than 23 million of Profibus nodes are installed and more than 1000 plants with PROFIBUS PA technology are running. They are 24 regional organizations (RPAs) and 33 PROFIBUS Competence Centers (PCCs) located strategically worldwide, as to offer support to their users, including Brazil. The São Carlos Engineering School (University of São Paulo) has the only PCC in Latin America.
- Over 1300 members al over the world;
- More than 23 million nodes installed successfully;
- Over 2800 products and more than 2000 suppliers serve the widest possible application needs;
- An extensive catalog of products may be obtained on the site www.profibus.com.
Next we will see a few key points of this technology. More details on the Technical Description available on the site www.profibus.org.br.
Figure 1 – PROFIBUS Industrial Communication
The information technology played a decisive role in the development of the automation technology and changed the hierarchies and structures of offices. It now arrives to the industrial environment and its several sectors, from process and manufacture industries to buildings and logistic systems. The possibility of communication between devices and the use of standardized, open and transparent mechanisms are essential components of today’s automation concept. The communication expands rapidly in the horizontal direction at the field level, as well as in the vertical direction integrating all of the hierarchy levels of a system. According to the characteristics of the application and the maximum cost to reach, a gradual combination of different communication systems like Ethernet, PROFIBUS and AS-Interface are the ideal open networks conditions for industrial processes.
With respect to actuators/sensors the AS-Interface is the perfect data communication system, as the binary data signals are transmitted through an extremely simple and low-cost data bus, together with the 24Vdc power supply required to feed those sensors and actuators. Another important feature is that the data are transmitted in cycles, in a very efficient and fast way.
At field level, the peripherals distributed, like I/O modules, transducers, drives, valves and operation panels’ work in automation systems, through an efficient, real-time communication system, the PROFIBUS DP or PA. The process data transmission is made in cycles, while alarms, parameters and diagnostics are transmitted only when necessary, in a non-cyclic way..
As to cells, the programmable controllers like the PLCs and the PCs communicate between themselves and so require that large data package be transferred in several and powerful communication functions. Furthermore, the efficient integration to the existing corporate communication systems, such as Intranet, Internet and Ethernet is absolutely mandatory. This need is met by PROFINet protocol.
The industrial communication revolution in the technology plays a vital role in the optimization of the process systems and has made a valuable contribution toward improving the use of resources. The information below will summarize the importance of PROFIBUS as a central connecting link on the automation of data flow.
The PROFIBUS, in its architecture, is divided in three main variants:
It is the high-speed solution for PROFIBUS. It was developed specifically for communication between automation systems and decentralized equipments. It is applicable on control systems where the access to I/O-distributed devices is emphasized and substitutes the conventional 4 to 20 mA, HART systems or in 24 Volts transmissions. It uses the RS-485 physical medium or fiber optics. It requires less than two minutes to transmit 1 I/O Kbyte and is largely used in critical time controls.
Currently, 90 percent of the applications involving slaves PROFIBUS utilize the PROFIBUS DP. This variant is available in three versions: DP-V0 (1993), DP-V1 (1997) e DP-V2 (2002). The origin of each version occurred alongside the technological advancement and the long-time growing demand for the applications
Figure 2 – PROFIBUS Versions.
The PROFIBUS-FMS provides the user with a wide selection of functions when compared to other variants. It is the solution in universal communication standard that may be used to solve complex communication tasks between PLCs and DCSs. This variant supports the communication between automation systems besides the exchange of data between intelligent equipments, generally used in control level. Recently, since its primary function is the peer-to-peer communication, it is being replaced by application in Ethernet.
The PROFIBUS PA is the solution PROFIBUS that attend to the requisites of process automation, where automation systems and process control systems connect with field equipments, like pressure and temperature transmitters, converters, positioners, etc. It may also replace the 4 to 20 mA standard.
There are potential advantages for using this technology, which carry functional advantages such as the transmission of reliable information, variable status dealing, failure safety systems, and the feature of auto-diagnosis equipment, equipment rangeability, measuring with high resolution, Integration with high speed discreet control, etc. In addition to the economical benefits pertinent to the installations (cost reduction up to 40% comparable to the conventional systems in some cases), maintenance costs reduction (up to 25 %. against the conventional systems), smaller startup time, it offers a significant increase in functionality and safety.
PROFIBUS PA permits measurement and control through one line and two single cables. It also powers the field equipment in intrinsically safe areas. PROFIBUS PA allows maintenance and connecting/disconnecting equipment even during operation without interfering in other stations in areas potentially explosive. PROFIBUS PA was developed in cooperation with the Control and Process Industry (NAMUR), in compliance with the special requirements on this application area:
- The original application profile for the process automation and interoperability of field equipments from different manufacturers;
- Addition and removal of bus stations even in intrinsically areas without affecting other stations;
- A transparent communication through the couplers of the segment between the PROFIBUS PA automation bus and the PROFIBUS-DP industrial automation bus.
- Power and data transmission on the same pair of cables based on the IEC 61158-2 technology;
- Use in potentially hazardous areas with “intrinsically safe” or “intrinsically unsafe” explosive-proof protection shield.
The connection of the transmitters, converters and positioners in a PROFIBUS DP network is made by a DP/PA coupler. The twisted pair cable is used as power supply and data communication for each equipment, which makes the installation easier and lower the cost of hardware, thus resulting in less initiation time, problem-free maintenance, low engineering software cost and highly reliable operation.
Posteriormente, em outras edições, o PROFINET será abordado.
All of the PROFIBUS variants are based in the OSI (Open System Interconnection) network communication model, in compliance with the ISO 7498 international standard. Due to the field requirements, only levels 1 and 2 and also level 7 on the FMS have been implemented, for efficiency reasons.
3 – PROFIBUS Protocol Communication Architecture.
On the three variants the two lower levels are very similar, their biggest difference being the interface with the application programs. Level 1 defines the physical medium. Level 2 (data transportation level) defines the access protocol to the bus. Level 7 (application level) defines the application functions.
This architecture ensures fast and efficient data transmission. The applications available to the user, as well as the behavior of the several types of PROFIBUS-SP devices, are specified on the user interface.
The PROFIBUS-FMS has levels 1, 2 and 7 defined, where the application level is made up of FMS (Fieldbus Message Specification) messages and the lower layer interface (LLI). The FMS defines a large number of powerful communication services between the masters themselves and master and slaves. The LLI defines the representation of FMS services on the level 2 transmission protocol.
The PROFIBUS PA communication protocol uses the same communication protocol as the PROFIBUS DP. This is because the communication and the messages services are identical. In fact, the PROFIBUS PA = PROFIBUS DP – communication protocol + Extended Acyclic Services + IEC61158 that is the Physical Layer, also know as H1. It allows a uniform and complete integration between all the automation levels and the plants of the process control area. This means that the integration of all the plants areas can be accomplished with a communication protocol that uses different variations.
RS485: THE MOST USED PROFIBUS PHYSICAL MEDIUM
The RS 485 transmission is the transmission technology most used on PROFIBUS, although the fiber optics may be applied on long distances (over 80 Km). Its main characteristics are:
- NRZ Asynchronous Transmission;
- 9.6 kBit/s to 12 Mbit/s configurable baud rates;
- Twisted pair shielded cable;
- 32 stations per segment, 127 stations max;
- Distance dependent on the transmission rate (Table 1);
- 12 MBit/s = 100 m; 1.5 MBit/s = 400m; < 187.5 kBit/s = 1000 m;
- Expansible distance up to 10 Km with the use of repeaters;
- 9 PIN, D-Sub connector.
Normally it is applied in areas involving high transmission rate, simple installation and low cost. The bus structure allows the addition and removal of stations without affecting others stations with further expansions, without affecting stations in operation.
When the system is configured, only one transmission rate is selected for all the devices in the bus.
There is the need for an active termination on the bus, at the beginning and the end of each segment, according to Figure 4, while both terminators must be energized in order to keep the transmission signal integrity.
Figure 4 – Cabling and Termination for RS-485 Transmission in the PROFIBUS
For cases with more than 32 stations or for dense networks, repeaters should be used. The maximum length of cabling depends on the transmission rate, according to Table 1.
PROFIBUS PA TRANSMISSION TECHNOLOGY
The transmission technology is synchronous with Manchester codification in 31.25 Kbits/s (voltage mode); it is defined according to the IEC 61158-2 and was elaborated aiming to satisfy the requisites by chemical and petrochemical industries: intrinsic safety and the possibility of power the field equipment through the bus. The work options and limits in potentially explosive areas were defined as per the FISCO (Fieldbus Intrinsically Safe Concept) model.).
Table 2 shows some of the IEC 61158-2 features:
Twisted pair with armor
Bus, tree, point to point.
Via bus or external medium
Number of equipments
Maximum : 32(non-Ex)
1900 m, expansible to 10 Km with 4 repeaters.
Codification, with voltage modulation
Table 2 – Featureas of IEC 61158-2 Transmission Technology
FIBER OPTICS TRANSMISSION
The fiber optics solution meets the needs for eliminating noises, potential differences, long distances, ring architecture and physical redundancy and high speed transmission.
Monomode Glass Fiber
2 – 3 Km mean distance
Multimode Glass Fiber
Long Distance > 15 Km
Short Distance > 80 km
Short Distance > 500 m
Table 3 – Types of Fiber and their Characteristics.
THE COMMUNICATION SYSTEM AND ITS SAFETY AND ACCESS LAYER
The efficiency of communication is determined by the level-2 functions, which specify the tasks of access to the bus, data frame structures, communication basic services and many other functions.
Level-2 tasks are executed by FDL (Fieldbus Data Link) and FMA (Fieldbus Management), the first one performing the following tasks:
- Control of access to the bus (MAC-Medium Access Control);
- Telegrams structure;
- Data safety;
- Availability of data transmission services:
- SDN (Send Data with no acknowledge)
- SRD (Send and Request Data with reply)
The FMA provides several management functions, such as:
- Configuration of operation parameters;
- Events report;
- Activation of services of access points (SAPs).
The PROFIBUS protocol architecture and philosophy ensure to every station involved in the exchange of cyclic data the time sufficient to execute their communication task within a defined time period. For this, they use the “token” passage procedure from the bus master stations when communicating between themselves, and the master-slave procedure to communicate with the slave stations. The token message (a special frame to enable the right of passage from one master to another) must circulate one time for each master within the maximum rotation time defined (which is configurable). On PROFIBUS the token passage procedure is used only for masters to communicate with each other.
Figure 5 – Multi-Master Communication
Figure 6 – Master-Slave Communication
The master-slave communication enables the active master (with the token) to access its slaves through the reading and writing services.
PROFIBUS uses different subsets of level-2 services in each of its profiles (DP, FMS, PA). See Table 4.
Send Data with Acknowledge (Envia dados com confirmação)
Send and Request Data with reply (Envia e recebe dados com resposta)
Send Data with No acknowledge (Envia dados sem confirmação)
Cyclic Send and Request Data with reply
Addressing services with 7 bits identify the network participants, and on 0 to 127 range the following addresses are reserved:
- 126: standard address attributed via master;
- 127: used to send frames in broadcast.
PROFIBUS-DP AND HIGH SPEED COMMUNICATION RATE
The PROFIBUS-DP profile was developed to answer in cyclic communication in a quick way among the distributed devices. In addition, PROFIBUS DP provides functions for acyclic access services, like configuration, monitoring, diagnostics and field equipment alarm management.
In 12Mbit/s, the PROFIBUS-DP needs only 1 ms to transmit 512 input bits and 512 output bits distributed by 32 stations. This profile is recommended for discreet control, which requires high speed processing. Figure 7 shows the PROFIBUS-DP typical transmission time, according to the number of stations and the transmission speed, where each slave has 2 input bytes and 2 output bytes and the minimal slave interval time is 200 µs.
Figure 7 – Bus Cycle Time for a DP Mono-master System
The telegrams are defined by the FDL, as follows:
- Telegrams without data field (6 bytes control);
- Telegrams with one fixed length data field (8 bytes data and 6 bytes control);
- Telegrams with variable field data (from 0 to 244 bytes data and 9 to 11 bytes control);
- Fast recognition (1 byte);
- Token telegram for access to bys (3 bytes).
The integrity and the safety of the information are kept in all of the transactions, as the frame parity and checking to reach the HD=4 hamming distance.
Figure 8 illustrates the user data transference principle, while recalling that on the DP side, the data are transmitted in asynchronous way under 485 and on the PA side, in bit-asynchronous way on H1.
Figure 8 – Principle of Users Data Transfer Used by FDL
In order to exchange data with a slave, it is totally essential that the master follows the sequence below during the startup:
- Station address;
- Request for diagnostics;
- Slave parameterization;
- Diagnostics request check before cyclic data exchange, as confirmation that the initial; parameterization is OK;
- Cyclic data exchange;
- Global control.
Figure 9 – Mandatory and Optional Services between Class 1 and 2 Slave and Master
Figure 9 shows mandatory and optional services between a DP slave and class 1 and 2 masters that masters and slaves should have:
TYPES OF DEVICES
Each DP system may include three different types of devices:
- CLASS - 1 DP MASTER (DPM1):
It is a main controller that exchanges information cyclically with the slaves. The programmable logic controllers (PLCs) are examples of the master devices.
- CLASS - 2 DP MASTER (DPM2):
Are engineering stations used for configuration, monitoring or supervisory systems like, e.g., ProfibusView, AssetView, Simatic PDM, Commuwinll, Pactware, etc
A DP slave is a peripheral device such as: I/O devices, actuators, IHM, valves, transducers, etc. There are also devices that have only one input, one output or combination of both. Here are included the PA slaves, as they are seen by the system as DP slaves.
The amount of input and output information depends on the type of device, being allowed up to 244 input bytes and 244 output bytes.
The data transmission between the DPM1 and the slaves is executed automatically by the DPM1 and is divided in three phases: parameterization, configuration and data transfer.
Safety and reliability are indispensable to add to PROFIBUS-DP the protective functions against parameterization errors or the transmission equipment failure. According to this effect, the monitoring mechanism is implemented in the DP master and the slaves too, monitoring the time specified during configuration. The DPM1 Master monitors the slave data transmission with the Data Control Timer. A time counter is used for each device. The timer expires when a right data transmission does not happen within the monitoring time and the user is informed when this happens. If the automatic reaction to an error (Auto Clear = true) is enabled, the DPM1 master ends the OPERATION status and protects every slave output of and changes its status to “CLEAR”. The slave uses the watchdog timer to detect failures on the master or on the transmission line. If no data is exchanged with the master within the watchdog timer interval, the slave will automatically change its outputs to the fail safe state.
The extended DP functions make possible acyclic reading and writing, and interruption recognition functions that may be executed in parallel and independently from cyclic data transmission. This permits the user to access the parameters in an acyclic way (via class-2 master) and that the measuring values of a slave may be accessed by supervision and diagnostics stations.
Currently these extended functions are widely used in the online operation of PA field equipments by engineering stations. This transmission has lower priority than the cyclic data transfer (one that requires high speed and high control priority).
PROFIBUS DP RESPONSE TIME
The response time in a PROFIBUS DP system depends essentially on the following factors:
- MaxTSDR (response time which a station may respond);
- The communication rate that was selected;
- Min Slave Interval (time period between two polling cycles), when a slave may exchange data with a slave. It depends on the ASIC used, although in the market we can find ASICs with 100 µs intervals.
For practical purposes, at 12Mbits/s we may assume that the time of message cycle (Tmc) that involves the prompting telegram + TSDR + slave response, where N is the number of slave inputs and outputs, is:
Tmc = 27µs + N x 1.5µs
For example: a master with 5 slaves and each slave with 10 input bytes and 20 output bytes, at 12Mbits/s would have a Tmc around 72µs/slave. The bus cycle time is obtained by the addition of all the message cycles:
Tbc = 5 x 72µs = 360µs
More details about the system times can be consulted in the Profibus Standards.
The use of PROFIBUS on typical devices and process control applications are defined in compliance with the PROFIBUS-PA profile, which defines the parameters of the field equipments and their typical behavior, independent from the manufactuer, and is also applicable to pressure and temperature transmitters, positioners. It is based on the functional blocks concept, as they are standardized in such a way to guarantee the interoperability between field equipments.
The measuring values and status, as well as the setpoint values received by the field equipment on the PROFIBUS-PA, are transmitted cyclically with the highest priority via class 1 master (DPM1). On the other hand, the parameters for visualization, operations, maintenance and diagnosis are transmitted by engineering tools (class 2 masters, DPM2) with low priority through the acyclic services via connection C2. Cyclically, a sequence of diagnostics bytes is also transmitted. The description of the bits of these bytes are on the equipment GDS file and depend on the manufacturer.
The approximate time of cycle (Tc) may be calculated as:
Tc ≥ 10 ms x number of equipments + 10 ms (acyclic class 2 master services) + 1.3 ms (for each set of 5 bytes cyclic values).
Think about a situation with 2 control loops with 5 pressure transmitters and 5 valve positioners. The cycle time would be around 110 ms.
PROFIBUS PA NETWORK ELEMENTS
Basically, on a PROFIBUS network the following elements may be cited:
- Masters: the elements responsible for the bus control. They have two classes;
- Class 1: responsible for the cyclic operations (reading/writing) and the control of the open and closed loops of the control/automation (CLP) system;
- Class 2: responsible for the acyclic access to the PA equipment parameters and functions (engineering or operation station: ProfibusView, AssetView, Simatic PDM or Communwingll, FieldCare or Pactware);
- Couplers: they are devices used to translate the physical characteristics between PROFIBUS DP and PROFIBUS PA (H1: 31.25 kbits/s). And also:
- They are transparent for the masters (with no physical address on the bus);
- They operate with safety applications (Ex) and (Non-Ex), by defining and limiting the maximum number of equipments on each PA segment. The maximum equipment number depends, among other factors, of the addition of the quiescent currents, the equipment failures (FDE) and the distances involved on cabling;
- They may be powered up to 24 Vdc, according to the manufacturer and area of classification;
- They may work with the following communication rates, depending on the manufacturer: P+F (93.75 kbits/s and SK2/SK3: up to 12Mbits/s) and Siemens (45.45 kbits/s).
Figure 10 – Basic Coupler Architecture
- Link devices: They are devices used as slaves on PROFIBUS DP networks and master on PROFIBUS PA (H1: 31,25kbits/s). They are used to reach high speeds (up to 12Mbits/s) on the DP bus.
- The have a physical address in the bus;
- They accept up to 5 couplers, but limit the number of equipments to 30 on each Non-Ex bus and 10 on each Ex bus. With that, they increase the DP network address capacity.
Figure 11 – Basic Architecture with Couplers and Links (IM157)
- Terminator: consists of a 1µF capacitor and one 100Ω resistor connecting each other and parallel to the bus, with the following functions:
- Shunt from current signal: the communication signal is transmitted as current, by is received as voltage. The conversion is made by the terminator;
- Protection against communication signal reflection: it must be located on the two bus terminations, one at the end and the other generally on the coupler.
- Cabling: it is recommended to use twisted cable 1x2, 2x2 or 1x4 shielded types, and also:
- Diameter: 0.8 mm2 (AWG 18);
- Impedance: 35 to 165 Ohm on 3 to 20 Mhz;
- Capacitance: lower than 30 pF per meter.
Table 5 – PROFIBUS PA Cables Data
PROFIBUS NETWORK ADDRESSING
Regarding to the addressing, there are two architecture to be analyzed where fundamentally stand out the couplers and the address attribution to the links devices, as seen on Figures 12 and 13.
Figure 12 – Addressing with Couplers
Figure 13 – Addressing with Links
In the Figure 13 the addressing capacity is significantly increased by the presence of the link devcices, as they are slaves for the DP and masters on the PA.
Regarding to the topology, there may be the following distributions: tree (Figure 14), bus (Figure 15) and point-to-points (Figure 16):
Figure 14 – Tree topology
Figure 15 – Bus Topology
Figure 16 – Point-to-Point Topology
INTEGRATION WITH PROFIBUS SYSTEMS
In order to integrate an equipment on a PROFIBUS system, it is used the equipment GSD file. Each type of equipment has its own GSD file (electronic datasheet), which is a text file with hardware and software revision details, bus timing and information on the cyclic data exchange. See example on Figure 17.
Figure 17 – GSD file for the LD303 – Pressure Transmitter
In addition to the GSD file, it is commonly offered the Device Description files (DDs), where the parameters are detailed, methods and menus that will make possible to configure cyclically the field equipment. These files follow the EDDL standard defined by PROFIBUS International. Still there are the FDT and DTM standards for configuration, monitoring and calibration.
PROFIBUS-PA PROFILE 3.0
A PROFIBUS system may be operated and monitored independently of equipments and manufactures. This statement will be true if all of the functionalities and parameterizations, and the manner of access to this information are standardized. These standards are determined by the PROFIBUS-PA profiles.
These profiles specify how the manufacturers should implement the communication objects, variables and parameters, according to the class of work of the equipment. And there is also the parameters classification:
- Process dynamic values: those with respect to the process variables, whose information is described in the GSD (device data master) and will be read cyclically by the class 1 masters and also non-cyclically by the class 2 masters;
Class 1 Master : Class 1 – in charge of cyclic operations (readings/scripts) and control of open and closed loops in the system.
Class 2 Master : Class 2 – in charge of acyclic access of the parameters and PA equipments functions (engineering station like, for instance, ProfibusView, AssetView, Simatic PDM, ComuWinII, FieldCare, Pactware, etc.).
- Standard values of configuration/operation: those that are exclusively accessed for reading and writing, via acyclic services. There are parameters that are mandatory for implementation and others that are optional to manufacturers;
- Specific manufacturer parameters: those that are exclusive to the functionality for a manufacturer equipment and may be accessed in a non-cyclic way, as they are also defined in accordance with the profile structure standards.
Currently, the PROFIBUS-PA is defined according to the PROFILE 3.0 (since 1999), where exist information for the several types of equipment, like pressure and temperature transmitters, valve positioners, etc.
These equipments are implemented in accordance with the function blocks model, where parameters are grouped and ensure uniform and systematic access to the information.
Several blocks and functions are necessary, depending of the operational mode and phase. Basically, the following blocks may be cited:
- Analog input and output Functional Blocks: they describe, during the operation, functionalities like exchange of cyclic input and output data, alarm conditions, limits, etc;
- Physical Block: brings equipment I.D. information related to the hardware and software;
- Transducer Blocks: they pack sensor information that will be used by the functional blocks, and also receive information from these to shoot actuation in final control elements. Normally, an input equipment (e.g., a pressure transmitter) has a transducer block (TRD) linked to an analog input block (AI) through a channel, and an output equipment (e.g., a valve positioner) and an analog output block (AO) that receives a setpoint value and enables it via channel to a transducer block (TRD) that will activate the final element (for example, positioning a valve).
Some equipment have several AI and AOs blocks that are called multi-channel equipments, where there must be several TRD blocks associated to the hardware.
The PROFIBUS-PA still differentiate the profiles in classes:
- Class A Equipment: includes information only from the physical and function blocks. In this class the equipment is limited to the basic necessary for the operation: process variable (value and status), unity and tag;
- Class B Equipment: stores extended functions from physical blocks, transducer and function blocks.
A powerful feature supported by the PROFILE 3.0 is defining each segment in accordance with the GSD files. These files ensure that any PROFIBUS system is accepted by the equipment, regardless of its characteristics. By that, each manufacturer may develop his particularities as functional blocks, going further than what is defined on the profile. This adds value to the equipments and makes possible the development competition and offering additional features in the different equipments. These specific equipment particularities may be accessed through standard interface concepts, with basis on EDDL (Equipment Descriptive Device Language) or FDT (Field Device Tool). These interfaces give the user configuration, parameterization, calibration versatility and flexibility, and mainly download and upload mechanisms in the phase of project planning and commissioning.
Figure 18 – Analog Input Block AI
Figure 19 – Totalization Block TOT
Figure 20 – Analog Output Block AO
PROFISAFE: THE PROFILE OF SAFETY
The demand for more resources in the automation and process control area through the advent of digital technology and the fast expansion of Fieldbus favored the development of the technology devoted to the diagnostics and treatment of safe failures. Mainly, aiming at the protection of people, equipments and the environment, always having for goal the ideal safety system.
This safe system requires, in other words, that the data and information may be validated in relation to its values and time domain, one that must be applicable to the system in its entirety. This implies ensuring that the data received was sent correctly and the sender is also de right transmitter. Furthermore, that this be the expected information, at a given moment and that the incoming information was sequentially correct, etc.
Currently, the most typical example of international safety standard, one that involves the most part of system developers and implementers with safety is the IEC 61508. This standard shows the activities involved in the entire life cycle of programmable electronic systems concerning safety. Therefore, it deals both to hardware and software requirements.
The danger with accidents in industrial processes is large and the probability of their happening depends on the probability of system failures. The implication of failures depends on the application’s safety types and requirements.
The PROFIBUS “PROFIsafe” application profile – The Profile for a Safety Technology describes safety communication mechanisms between peripherals liable to fail-safe situations and safe controllers, like IEC 61508 and EN954-1, as well as in the experience of the manufacturers with fail-safe mechanisms and the community of PLC manufacturers.
Following, their main concepts will be presented.
This profile supports safe applications in a wide area of field applications. And, instead of using special buses for the safety functions, it permits the implementation of safe automation through an open solution in the PROFIBUS standard, which guarantees cost-effective cabling, consistency of the system regarding the parameterization and remote diagnostics functions. It guarantees safe decentralized control systems through fail-safe communication and safe mechanism devices and equipments
See below a few examples this safety profile application areas:
- Manufaturing Industry;
- Quick protection to persons, machines and environment;
- Emergency stop functions;
- Light barriers;
- Input control;
- Drivers with integrated safety;
- Control of processes in general;
- Chemical and petrochemical areas;
- Public transportation;
The PROFIBUS open technology meets a number of requirements, by the most varied applications in terms of safety according to PROFIsafe:
- Independency between relevantly safe communication and safe communication;
- Applicable to SIL3 (IEC61508), AK6 (DIN V 19250) levels and the control category 4 (KAT4) (EN 954-1);
- Redundancy is only used to increase trust ability;
- Any master or DP link may be used;
- Upon implementation, DP masters, ASICs, links and couplers must not be modified, as long as the safety functions are installed above the OSI layer 7, i.e., with no change or accommodation on the DP protocol;
- The implementation of safe transmission functions must be restricted to the communication between the equipments and should not restrict their number;
- It is always a communication relation 1:1 between the F devices;
- The transmission times must be monitored.
In practice, safe applications and standards share the PROFIBUS DP communication systems simultaneously. The safe transmission functions include every measure that may be deterministically found in possible dangerous failures. These may be added to the standard transmission system, aiming at minimizing its effects. They included, e.g., the random malfunctioning functions, EMI effects, systematic hardware or software failures, etc.
For instance, during communication it is possible that part of a frame is lost, part of it may be repeated, or yet, that it appears in the wrong order or even late.
On PROFIsafe, a few preventive measures may be taken, aiming at enclosing possible causes of failure or that these may happen with safety, should they occur:
- Consecutive numbering of every safe message: this minimizes the loss of communication, the insertion of bytes on the frame and the wrong sequence;
- Watchdog timer system for messages and their acknowledgment for controlling delays;
- A password between remittent and receptor, to avoid linking standard and safe messages;
- Additional telegram protection by including 2 to 4 CRC bytes, thus avoiding the corruption of user data and the linking of standard and safe messages.
These measures must be analyzed and taken on a single fail-safe data unit. See below the F message model.
PROFIsafe is a one-channel software solution that is implemented as an additional layer above layer 7 on the devices. A safe layer defines methods to increase the probability of detecting errors that may occur between two equipments/devices communicating in a fieldbus
A great advantage is that it can be implemented without changes, to protect the users investment.
On the physical means RS485 or H1 (31.25kbits/s) the cyclic communication mechanisms are used. The acyclic communication is used for irrelevant levels of data safety. It ensures very short response times, which are recommended in intrinsically safe manufactures and operations, in compliance with the requirements of the process control area.
PROFIsafe uses the error detecting mechanism to keep the desired safety levels. This profile detects communication errors such as duplicated frames, lost of frames, incorrect frame sequences, corrupted frames, frame delays and wrong frame addressing. The PROFIsafe profiles use the redundancy of information to validate the communication between two devices. The relevant safe information is t is transmitted in conjunction with the process data, that is, this data is embedded in the PROFIBUS DP basic frame. This type of frame may deal with a maximum 244 bytes process data. PROFIsafe reserves 128 bytes from this total for the safety data. Beyond that, 4 or 6 bytes are treated separately as status and control bytes, depending on the quantity of safe data transmitted. Always two control bytes are sent from each frame, one for status and the other with the frame sequence. The four remaining bytes are saved for the checksum generated to protect redundant safe information. A small amount of relevant safe data transmitted involves one 16-bit CRC and 4 bytes control. For transmissions over 12 safe data bytes (until 122), one 32-bit CRC is used with 6-byte control.
Figure 21 shows the DP frame model that includes in the information the frame known to that frame, in addition to fail-safe data (a maximum 128 bytes in 244 bytes, due to its limitation of 64 words exchanged in a single time between the Host and the DP master), as well as the parity safety resources and FCS (Frame Checking Sequence).
Figure 21 - Risk considerations according to IEC 61508.
Figure 22 shows the F message model (safe message), where may be seen the bytes for integrity control and error minimization previously described as preventive measures.
Typical system using default and security communication sharing the same bus and protocol
The Table 6 shows details on how to deal with safe failure, communication, timer-outs, CRCs, message numbering, etc.
Table 6 – PROFIsafe Architecture
Through the monitoring and control of information between safe masters and slaves, such as: synchronization, F protocol cycle, watchdog timers, order of messages, frame repetitions, SIL monitor (one that counts corrupt messages in a given period of time), it is possible to guarantee safety to the integrity levels, as follows.
Table 7 – SIL Monitor
GSD & PROFIsafe files:
Equipments with the PROFIsafe features include in its GSD file the following key word:
F Device supp = 1; 1 = F device
We covered a few details about the PROFIBUS protocol, what it encloses in terms of resources and benefits to automation and the control of continuous and discreet processes. Its potentiality is outstanding at world level both in applications and the management of publicity and support jointly with Regional Associations and Competence Centers. Another detail is the concern by companies in the continuous offer of products according to the market demand and guarantee future investments with total interoperability and exchangeability.
- PROFIBUS technical description;
- PROFIBUS GuideLine;
- PROFIBUS-DP/PA - ProfiSafe, Profile for Failsafe Technology.
- IEC 61508 – Functional safety of electrical/electronic/programmable electronic safety-related systems;
- PROFIBUS PA – SMAR Manuals;
- SMAR PROFIBUS Training Material, 2003, César Cassiolato;
See some articles / applications with SMAR products in Profibus: