MKR Zero RS485 shield and Modbus RTU

Official Hardware MKR Family MKR Shields
1

Hello Everyone,

I feel like a fool, I really do. I've got 2 x MRK zeros with RS485 sheilds and I'm testing using the example programs ModbusRTUClientToggle and ModbusRTUServerLED.

My setup is :
Y connected to Y
Z connected to Z
Half duplex ON on both
The YZ resistor ON on the Client board

It couldn't be simpler

It kind of works but the LED flashes erratically and with Serial Monitor running on the Server port I get a lot of errors like those below.

Failed to write coil! Response not from requested slave
Failed to write coil! Invalid data
Failed to write coil! Invalid CRC
Failed to write coil! Connection timed out
Failed to write coil! Response not from requested slave
Failed to write coil! Invalid data
Failed to write coil! Invalid CRC
Failed to write coil! Connection timed out
Failed to write coil! Response not from requested slave

With my 'scope on Y and Z running a Modbus RTU analyser I see much the same thing.

I'm at a loss, can anyone advise me?

Thanks,
Mike

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