Hi..

In my project I want to read registers from a modbus tcp slave device. In normal conditions everything is ok but I must handle an error bit while an reading or socket error at communication.
For example when turn off the power of slave device, a communication error occurs. So how can understand the error of modbus tcp.

Thanks already..

If you have control over the slaves you could toggle a bit and increase a counter on the slave side and read that back, make sure it increases. But if there is a way to access the on board diagnostics from Codesys that would be nice.

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Each modbus slave device has a property "byModbusError".
In your case you can use e.g. the following code:

Mb1State := Modbus_TCP_Slave.byModbusError;

Mb1State is then of the following type:

VAR
   Mb1State: IoDrvModbusTCP.MB_ErrorCodes;
END_VAR

In the library "IoDrvModbusTCP" you can find this ENUM (in the Structs folder) to see the details.