Details of Modbus Plus protocol


Thread Starter

Sandeep Shroff

Dear List members

I am looking for information on Modbus Plus protocol. I have checked the site for the same but could not get details on the following points. The website site does not give any information of Modbus Plus.

1) Is Modbus Plus protocol an open protocol? (To my understanding Modbus ASCII/ RTU & TCP are open protocols while Modbus Plus is Schneider Automations proprietary protocol. Is this true?)

2) What physical layer does this protocol work on?

3) Where can I find technical information necessary for driver development? Whom should I contact to get the protocol details? (If this not a proprietary protocol?)

4) Can I put a device supporting Modbus RTU protocol (supporting serial communication) to Modbus Plus network? How?

Thanking you in anticipation.

Sandeep Shroff

Here is a paragraph from Chapter 5 of my book "Automation Network Selection":

"Unlike the open Modbus ASCII and RTU modes, Modbus Plus is a proprietary protocol but is based upon the same set of Modbus application commands. It functions over twisted shielded pair cable using ANS/EIA/TIA-485 and ISO/IEC 3309:1991 HDLC (High-level Data Link Control) multidrop protocol."

Schneider has licensed this protocol to a few cooperative marketing partners, but it is proprietary. You cannot use a device with Modbus RTU protocol on a Modbus Plus network.

Modbus/TCP has essentially replaced Modbus Plus, and is very widely used.

Dick Caro
Richard H. Caro, CEO
CMC Associates
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