13 - What is the benefit of being completely digital?
Unlike the "semi digital" DCS system, the Fieldbus system is a "pure digital" system. Instead of the system's pressure or temperature transmitters converting a sensed digital process value to an analog 4-20 mA signal before feeding it to the DCS and the rest of the system chain, Fieldbus keeps the signal purely digital all the way from the transmitter to the digital input of the control valve. Fieldbus devices should not be mixed up with what we today call smart devices which are a hybrid of 4-20 mA and slow digital communication. Keeping the signal digital until the very end of the chain allows for infinitely more complex and precise signal processing. And the industry standard, inexpensive cable that links all the Fieldbus components together can be hundreds of meters long and remain totally free of the noise pickup and signal degradation associated with analogue signal transmission.
In a 4-20 mA analog system, a single value is transmitted by an infinite variation of a current. A signal error just changes a valid signal to another valid signal, noise and other signal distortion cannot be detected in an analog system. The signal from even the most accurate analog transmitter may be totally inaccurate by the time it reaches the controller. Much more noise is required to distort a digital signal such as in Fieldbus, which only has two valid states, one and zero. Secondly, error checking is also used to filter out scrambled messages and make sure they are sent again. All data is checked and guaranteed free from distortion due to noise or impedance mismatch that may affect analog signals undetected.
Each analogue link - transmitter, signal conditioner, isolation amplifier, cable, input card, output card, I/P converter - whispers an 'analogy' of what it hears to the next, but something is always lost or added.
What emerges at the end may not even resemble the original message. No matter how long the chain, at its end digital equipment listens only for patterns of zero and one, which it reassembles into the original message, ignoring all other whispered information as noise. Hence the integrity of digital transmission: no accuracy lost, any noise added.
The integrity of a digital signalling results in better accuracy and security. The latter is being very important in the process industry where expensive equipment, life and the environment may be at stake. Fieldbus has taken this integrity even further. Measurement and control variables passed between function blocks have not only a value, but also a status, which include signal quality, limit information and a sub-status. The signal quality informs if e.g. a measurement is Good, totally Bad or perhaps Uncertain, e.g. out of range by a few percent. Other function blocks use this to e.g. put control in manual in case of fault, calculation function blocks may also take this information into consideration, e.g. exclude it when calculating an average.