If PLCs form part of a total control system and radio telemetry is used to get the data to the PLC, then "telemetry to MODBUS" interface devices are very neat solutions. The only prerequisite is that the amount of data being relayed (including the rate of change of each value) does not flood the radio channel (see "Using Telemetry to MODBUS interface modules").
This pit-fall can be demonstrated in the real-life example of a system where it was planned to use an Elpro105U-C to collect data from 11 outstations as well as to link in a second PLC via a second 105U-C.
The amount of traffic required between the two PLCs alone was fifty continually changing values in one direction, and eleven in the other with update to occur within 20 seconds. With each value sent as a separate transmission of 1 second, it does not take long to realise this requirement was becoming a dream.
If the sole purpose of your radio telemetry system (or dedicated radio system) is to link a bunch of PLCs, then point-to-point Radio Modems come into their own. They are firstly cheaper than their fully fledged radio telemetry counterparts, and the data is sent immediately rather than waiting for a radio channel to become clear.
There is only one limiting factor with using radio modems being the speed at which the data link must work. MODBUS links are usually forced to work at speeds far higher than required. There are two dangers with this. The first is data becoming corrupted with noise. Slowing a link by two reduces the chance of noise corruption by four. Second, with the speed of the link being high, any retries performed by the link are usually not noticed until there has been a number of unexplained failures or glitches.
A good test for the viability of a radio link is to reduce the speed of the MODBUS link to 2400 baud and if all the registers are updated within a required time limit then radio modems will work superbly.
The above system was re-planned to use radio telemetry on one radio channel to gather the data and present it to the master PLC. A pair of radio modems was then used in a point-to-point link operating on a separate radio channel for the data exchange between the two PLCs.
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