Is the specification for Modbus Plus available?

 

Hello,

I have never found it.

Good luck,

Mark
http://peakhmi.com/

 

No, you have to be a Collaborative Automation partner of Schneider. The spec itself really wouldn't do any good unless you were building a device anyway, as the real network management is hidden within a "black box" chipset that partners have to purchase from Schneider. The spec really just documents how to interface to the chipset (not the network directly).

Darrin Hansen

 

> Is the specification for Modbus Plus available?

Plus it is NOT RS-485, despite the fact that many boards use RS-485 driver chips to drive an edge-triggered signal through isolation transformers. The spec is much like RS-485, but you won't be able to talk ModbusPlus from a PC with an RS-485 port. So the spec won't do you much good.

 

Thank you all for responding. I appreciate the help. Unfortunately I'm still in the dark.

My understanding is that the Modbus is a software protocol that can be used with different hardware methodologies. Please correct me if I am wrong about this.

My assumption is that Modbus Plus is an extension to the Modbus software protocol. Perhaps this is an incorrect assumption but, without information about Modbus Plus, I can only speculate as to what it is. I'm looking for information as to what the Modbus Plus is. Typically I would expect this to be in a specification.

My requirement is to hang a sensor on an existing RS485 network that communicates using Modbus Plus. Do I need to integrate a chip set into my design so that the sensor can communicate on the Modbus Plus?

 

> My understanding is that the Modbus is a software protocol that can be used
> with different hardware methodologies. Please correct me if I am wrong about this.

You are wrong - ModbusPlus is not Modbus. Some of the messages it moves are compatible with Modbus, but it includes 'multicast' productions which have no Modbus equivalent. ModbusPlus is more like Allen-Bradley DH+ or Siemens MPI than Modbus.

It is a 1mbps token bus system, and you need to include the official Schneider-Electric chip set (last I knew, a custom 8051 MPU) to gain compatibility. It also is not RS-485, despite wistful thinking that it is.

Perhaps with 6 months or more of reverse engineering you could add ModbusPlus to your product without adding the custom MPU (although you may violate a patent or two if you succeed), but the business case for such a fragile & futureless product-expansion is poor. Better to stick with Modbus/RTU in true RS-485 or look to Modbus/TCP on Ethernet.

 

4-5 years ago I stopped by the Modbus booth at the national ISA show.

I told the booth guy that I'd done a fair amount of work with various Modbus RTU or TCP devices with HMI and SCADA systems, but then asked him what the deal was with Modbus Plus because I never run into it.

His explanation was that it was that Modbus Plus was a licensed, token sharing protocol for PLC to PLC communication and that it was not intended for device to device field instrument communication, like the open standard Modbus is.

Schneider's Modbus Plus web page confirms this. "The Modbus Plus networks can be used with Quantum™, Momentum™ and Premium™ PLCs."

Modbus Plus applications

-PLCs (Modicon Quantum, Momentum, Premium, TSX Micro, Compact)
-Drives
-IP 20/IP 67 I/O
-Operator terminals

Check it out yourself: http://tinyurl.com/dyujykb

 

> My understanding is that the Modbus is a software protocol that can be used
> with different hardware methodologies. Please correct me if I am wrong about this.

This is true but irrelevant, as you'll see in a moment.

> My assumption is that Modbus Plus is an extension to the Modbus software protocol.

This is where you start to go off the tracks. They have nothing in common but the name. One is indeed an open, easily implemented protocol used by thousands of devices on the market. The other is a closed proprietary bus which you must in effect get permission to use. They have as much in common as St. Petersburg, Florida and St. Petersburg, Russia.

 

Thanks again one and all. I appreciate the help. Looks like I need to design the chip set into my product or purchase a converter.

So, the next question is, does anyone have experience with or can anyone recommend an RS232 to Modbus+ converter?

 

Look at:

http://www.prosoft-technology.com/P...s/Modbus-Plus-to-ASCII-Generic-Serial-Gateway

William (Bill) L. Mostia, Jr. PE
Fellow
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

 

If your employer is dead set on you needing to do this despite Modbus Plus being both a closed network and not really meant for this application, why not just use some of the OTS MB+ I/O bricks the other poster mentioned? Seems like the most direct solution with a minimum of new parts to invent.

 

Hello,

I used the BM85 bridge from Schneider and it works without problems.
If you want to embed the converter in your HW try the from www.hms.se.

Regards,
Andrzej
www.modbus.pl

 

Our MBP-100 gateway can convert RS-232 and/or RS-485 Modbus RTU to Modbus Plus.

http://www.iccdesigns.com/products/gateway/mbp-100.html

Regards,
Darrin Hansen
ICC, Inc.