Problem with single coil functions about arduinomodbus HOT 4 OPEN

Hi @Nickyy-b,

Could you please provide more insight on how things are wired/connected?

I also believe since the Pro Mini only has a single Serial instance, any sketches should avoid Serial.println(...) statements and let the Modbus library have full control of the Serial port.

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Thanks for replying, sorry for the delay in returning.
I connected the client (Mega) and the server (Pro mini) directly Rx to Rx and Tx to Tx, that programming I attached is from mega, and the Serial window is the answer from Pro mini (server) returns to mega. The programming of the server is modified only with a char that makes a led flash, just this, I'm not using serial.print, strangely only this function of reading a single coil is returning me -1, the others read / change multiple registers / Coils work normally, I can change to read less than 10, 3 for example, but ModbusRTUClient.coilread returns me -1, can you help me?

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@Nickyy-b please share the exact sketches you are using. Are you able to also use a logic analyzer to trace what is being sent and received on the wire?

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The codes !!
Edited.zip

What didn't catch was the function in "if serial == d", this function returns me -1. Do you think I can replace it with the requestFrom function or RTU.beginTransmission, without any problem?

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