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Modbus Slave Behaviour

posted by RSPriya, 29.04.2008, 12:57

Hi,

I am developing Modbus Master on RS485 Communication.

As per the Modbus Application Protocol Document, when the Master sends a request to a slave,
Slave will provide either Modbus Positive response or Exceptional Response (if Function Code not valid or Invalid Address or data or Error in Execution).

But when tested with one Slave Meter EM3460, the meter is responding only for Read holding register function code. When polled for Read Coil/Discrete Input (which is not present in that meter), it is not giving me even the Exceptional response.(just timeout happening)

I would like to know whether it is not mandatory that Slave should provide Exceptional Response even if it doesnt support that function code?

What is the standard way of Modbus Slave Communication?.

Thanks in Advance,
Priya

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