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Technical Notes
MODBUS TCP/IP Ethernet to Serial Bridge Recommendations and Performance
By Martin Berutti
Product: IOD-4101 - Modbus TCP/IP Master Driver (General Modbus TCP/IP Communications)

This technical note answers questions about recommendations and performance of the use of Modbus Ethernet to Serial gateway devices.

For new installations, we recommend the use of the Schneider Electric TSXETG100 Ethernet to Serial Bridge. To follow are some general guidelines for prediction of IO updates into DeltaV using the Virtual IO Module Network Gateway with the Modbus TCP/IP Driver and either the TSXETG100 or the older 174CEV30020 Ethernet to Serial Bridge devices.

  • Modbus TCPIP Command/Response time to and from a network slave device is typically 20ms. The response from both devices should be similar.
  • Modbus Serial Command/Response, at 19200 baud, is approximately 0.5ms per byte. The Modbus Serial Command message will be 8 bytes in all cases (including header and CRC) for a total transmission time of 4ms.
  • The Modbus Serial Response for 100 analog registers will be 205 bytes (including header and CRC) for a total transmission time of 102.5ms. Therefore, each Modbus Serial Command/Response will take 106.5 ms.
  • All Modbus slaves will take some time from receiving the Command before sending the Response back to the Master.
  • Total Modbus message turnaround time will be 20ms + 106.5ms + Slave response per DeltaV dataset, for Modbus Ethernet to Serial gateway installations.

The above statements are general guidelines only. We recommend testing with the actual slave device in order to predict installed data response times.