Modbus TCP’s Connection-Orientation

  • In Modbus, data transactions are traditionally stateless, making them highly resistant to disruption from noise and yet requiring minimal recovery information to be maintained at either end.
  • Programming operations, on the other hand, expect a connection-oriented approach. This was achieved on the simpler variants by an exclusive ‘login’ token, and on the MODBUS Plus variant by explicit ‘Program Path’ capabilities which maintained a duplex association until explicitly broken down.
  • Modbus TCP handles both situations. A connection is easily recognized at the protocol level, and a single connection may carry multiple independent transactions.
  • In addition, TCP allows a very large number of concurrent connections, so in most cases it is the choice of the initiator whether to reconnect as required or re-use a long-lived connection.

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