About MODBUS 3

2014-08-25

This part is for MODBUS for TCP/IP.

In MODBUS part 2 (for Serial Port Line), we saw MODBUS had master / slave model, master sent request to slave. In TCP, MODBUS has Server / Client mode. Client sends req, Maser gives the response.

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A MODBUS Request is the message sent on the network by the Client to initiate a transaction,
A MODBUS Indication is the Request message received on the Server side,
A MODBUS Response is the Response message sent by the Server,
A MODBUS Confirmation is the Response Message received on the Client side

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The listening TCP port 502 is reserved for MODBUS communications. It is mandatory to listen by default on that port. However, some markets or applications might require that another port is dedicated to MODBUS over TCP. For that reason, it is highly recommended that the clients and the servers give the possibility to the user to parameterize the MODBUS over TCP port number. It is important to note that even if another TCP server port is configured for MODBUS service in certain applications, TCP server port 502 must still be available in addition to any application specific ports.

(From MODBUS TCP/IP)

See Also

1: About Modbus 1
2: About Modbus 2