Using Telemetry to MODBUS
interface modules

There is a very ingenious trick open to the clever radio telemetry engineer for reducing the transmissions on a system, while maintaining an almost complete link between two PLCs using MODBUS protocols. This is done by using two MODBUS interfaced telemetry modules in place of two radio modems.

These modules, an example being the Elpro 105U-C, hold a number of registers which are interfaced to the PLC via the MODBUS comms. Only a change of value or state causes a transmission, when things are stable transmissions are kept to an absolute minimum.

A further bonus with using these units is the in-built ability to interface to standard radio telemetry units which become additional inputs and outputs for the PLC(s) with the advantage of being remote. All this conducted on one radio channel.

Here a word of caution; Analogue signals may appear to not change but slight fluctuations (noise being sufficient) will shift an analogue value and a change is recognised at just one bit. As fluctuations, especially noise, will change faster than the radio can transmit, the radio channel may become overloaded even with only one analogue value being transmitted.