PLC´s communication

  • Thread starter Juan Manuel Colonia
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Thread Starter

Juan Manuel Colonia

I have a project where we have a PLC Quantum to handle a heat recovery system for a couple of gensets. This PLC has to communicate with a
remote DCS Fisher Rosemount. The problem that I am having right now is that the PLC Quantum has abled a modbus plus port while the DCS has a modbus port. They are not communicating each other. I would like to know what are the options to make them communicate and the costs of the solutions or where to go for a quote. I already went to the local agent but they haven´t given any solution yet.

I would really appreciate a reply to this message.

Thanks for the help and regards,

Juan Manuel Colonia
Hello Juan,

The Quantum PLC you have has 1 Modbus Port and 1 or 2 Modbus Plus ports. The Modbus Port is above the Modbus Plus port/s. If the Modbus port is currently being used for something else there is a bridge device called the BM-85 to bridge between Modbus Plus and Modbus.

If you need further information let me know.

Bradley G. Hite
Intertech Inc.
mailto:[email protected]
ICQ# is 38746036
Teaching Practical Skills For a Technological World

R A Peterson

Modicon makes whats called a bridge mux (part number BM85) which allows you
to go from MB+ to Modbus. Its kind of pricey ($1500 or so IIRC) but easy to use and setup.

Modbus Plus (MB+) and Modbus will not talk to each other. Modbus runs on a serial link at data rates such as 19.2 Kbps, 9600 bps, while Modbus
Plus runs at one megabit.

One potential solution is to use a bridge mux, which has both a MB+ and Modbus serial ports. Using MSTR communication instructions, you can
read and write registers to a device with a Modbus port. I've used this technique with the older model Modicon 984 processor to communicate
with a GE-Fanuc 90-30.

Brian Peck
Northeast Utilities

The Quantum MODBUS port is normally in "slave" mode and, I guess, the DCS MODBUS port is in the same mode - so they obviously cannot talk - only a
"master" can "ask questions" and then the "slave" "replies". First find out if I am right about the DCS MODBUS port. If so, you can turn the Quantum into a master by using the XMIT loadable
function (see more at or ask your local Schneider support). If you want it the other way around (DCS master, PLC slave), find out how you make the DCS to act as master (at Fisher Rosemount).

Hasta la Vista,
Meir C. Saggie Chief Engineer
AFCON Control and Automation Ltd., P.O.BOX 3120 Petach Tikva, Israel, 49130

T: +972-3-9392404 F: +972-3-9244249 E: <[email protected]>