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MODBUS broadcast with Connect WAN 3G IA

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BASIC PROBLEM: We are trying to reset the watchdog timer on our remote PLCs by way of a broadcast message.

We have a Telvent OasysDNA SCADA system that uses MODBUS/RTU to communicate with Modicon PLCs at our field sites. Communication is by radio, ADN (land line), and now we have some circuits that are cellular, using your Connect WAN 3G IAs.

So for each cell circuit we have 1 Digi connected to the central SCADA system, and it talks to up to 5 Digis at field sites. Physical connections are using the serial port.

Most messages from the SCADA system are to specific MODBUS addresses, and that is working splendidly. However, we also have a broadcast (going to 00) every minute to reset the watchdog timer on the PLC and that does not seem to be working. Apparently the broadcast gets ignored or thrown away? or is there some configuration setting we're missing?

So, with the assumption the broadcast doesn't work, there was the idea that we could simulate with a Python script. Which seems like it would consist of 1. a. Either get the incoming broadcast message [Q: would it be possible to get only those or would we have to read all, and would that interfere with the regular message processing] or b. have timer going in Python script to generate watchdog 2. a. Read IA table to get destination addresses [Q: is that possible?] or b. have hard coded table of destinations for that device 3. send message out to each destination
asked Mar 18 in Modbus and Industrial Automation by marnold New to the Community (0 points)

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