Using a hook to trap holding registers write on a slave

[Jean-Simon LaRochelle]

I have to write a proxy for a Modbus device to remap the I/O to another device. One thing I would like to do more specifically is trap the holding registers write request on a slave to remap the I/O and use a master to write the remapped i/o to another device.

How could I modify the modbus slave example to do this ?

[Jean-Simon LaRochelle]

Ok I found the information I needed by looking at the master example that was using some hooks.

Looking at the comments in the hooks class after a few test I was able tofind the recipe I needed.

For my tests I modified the tcpslave_example like this:

    def on_write_request(data):
        slave, pdu = data
        print(f"PDU={pdu}")
       

    try:
        #Create the server
        server = modbus_tcp.TcpServer()
        logger.info("running...")
        logger.info("enter 'quit' for closing the server")


        slave_1 = server.add_slave(1)
        slave_1.add_block('0', cst.HOLDING_REGISTERS, 0, 100)
       
        hooks.install_hook("modbus.Slave.handle_write_multiple_registers_request", on_write_request)
        hooks.install_hook("modbus.Slave.handle_write_single_register_request", on_write_request)

Bingo!

[Luc JEAN]

Hello Jean-Simon,

Well done and thanks for sharing.

Best

luc

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