MODBUS timings

One of the areas where transmission rates can be drastically reduced on radio modems used to link PLCs is to bring MODBUS timings into the realistic realm. There is this amazing desire by process control engineers to have MODBUS comms operate 'flat out'.

There is no need for this as delays are introduced by the radio systems so forcing high transmission rates (by severely cutting update times) achieves nothing more than straining the system. If the system is not capable of accepting these delays then the requirement is simply not suited to this type of link.

Personal experience has never indicated a requirement to have better than 2 seconds update between groups and yet many 'defaults' are set at 0.5S, that is a four-fold increase in radio traffic.

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