In older industrial plants, which have been in operation for a long time and have been upgraded with process analytic technology, mostly 4..20mA type interfaces were used as a standard to convey results to process control systems [PCS]. In newer projected plants, digital infrastructures are used for this purpose.
How do these communication standards compare? What advantages and drawbacks have to be considered? Are there standards superior to others?
There is no simple answer to these questions: trading off the benefits of analogue vs. digital process communications is difficult because the required infrastructures differ significantly. While analogue data transmission requires a wire pair between the instrument and PCS for every value that is exchanged, the user can easily measure the data exchanged on every single pair and correct it if necessary. In addition, an offset of 4mA makes sure, that a break in the line is recognized.
The protocol of modern digital bus protocols allows the transport of multiple signals on a single cable, e.g. by using Modbus TCP on an Ethernet infrastructure or Profibus on an RS485 infrastructure, and even includes error detection.
The drawback is that protocols cannot be monitored easily and there is a variety of protocols in the field, using different bus and media types.
A special challenge arises if newer measurement techniques have to be integrated into a plant. Adapting different instruments on a PCS may require a step to transform information from one communication standard to another one. The necessary conversion sometimes leads to a degradation of the data quality. As an example, the measurement equipment may produce a digital result in floating point format, which has to be converted by a 12-bit DAC for adoption to a 4-20mA data line. On the PCS side, the information again has to be converted back into a floating point value, using a 12 bit ADC. It is obvious that this conversion leads to an error accumulation.
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