from the Automation List department...
Modicon 984-685E Modbus Communications
Communications systems and equipment. topic
Posted by Joe Hodges on 29 January, 2002 - 3:05 pm
I am trying to connect a Modicon 984-685E PLC to an ABB Advant 450 controller using Modbus. Will the XMIT function work on the 685E? Is there another way to communicate using Modbus without XMIT?


Posted by grantt1 on 29 January, 2002 - 3:20 pm
A Modicon Bridge Multiplexor would work quite nicely. p/n NW BM85 000. Additional Modbus devices could also be added to the Modbus ports on
this device


Posted by kmorin on 29 January, 2002 - 5:07 pm
By "connect" are you trying to get the '984-685 to be a peer, a slave or a master for the ABB 450? as Modbus is a master slave
relationship, Mb+ begins peer to peer - the goal needs to be defined who starts the conversation and who answers.

The '984 will issue Mb strings in ascii or rtu [essentially 7 or 8 bit framing] via Mb+ from the BM85-00 by using the older MSTR block in
ladder. IF you are using the Concept interface package then the 'xmit' may do the same or "remote read array" may be more functional as
there are more parameters included in a fill-in-the-blanks layout.

As for the 450 end you'd have to define the port and protocol that your model supports. ie. Mb or Mb+ and ASCII or RTU and the addressing
it will accept. If it will accept a modbus address then which of the two protocols will is answer? If standard Modbus then the BM-85 will
act as local modem or master, if the 450 will wire to Mb+ then make it a drop on the network and peer-cop the data unless you have a large
amount.

I don't work with this controller but understand that many external communications are supported. However I'm confident of the Mb remarks with regard this contoller.


Posted by Jim Dungar on 29 January, 2002 - 5:13 pm
You need to use the Modicon bridge multiplexer, or an equivalent Modbus Plus to Modbus bridge device.

The Modicon 685E does not support the XMIT command. There is no way for it to intiate Modbus communications.

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