Hi,

Currently we have a Modbus network linking several dozens industrial air-con units which are able to communicate their status and receive commands via Modbus. We would like to utilise two different Building Management System software to monitor the same network of air-con units but were wondering if there could be any conflicts caused by directly hooking up two PCs (each running a different BMS) to the single network.

Any advice would be most appreciated!

Thanks.

Leon

 

Hello Leon;

Basically, a serial Modbus network will be designed to support a single master; Modbus + and Modbus/TCP can be multi-master buses. However, there are ways around these limitations, using "multiplexers" or "arbitration units" from different providers, Phoenix and Digi amongst others if you want to remain in the serial level without going to serial to Modbus/TCP converters.
There have been a number of discussions on the same subject on this forum, you could look at this one from a few years ago, there are a few interesting answers:

http://modbus.control.com/user/1026224198/index_html

Hope this helps,
Daniel Chartier

 

A Modbus network can only have one master. To connect multiple Modbus masters to your network you will need some sort of gateway.

There are two basic methods: You can install a "data concentrator" with three serial ports. One port will be a Modbus master that will connect to
your existing network and collect data from the air-con units and store it in a "database". The data collected can then be accessed by both BMS via two Modbus slave ports.

Another method is to use a multiplexer type connection that functions like a "traffic cop" to let the two BMS access to the single network one at a time.

Both methods work and as always there are pros and cons to each approach.

In the first method, there is an address translation involved. Registers 1 thru 10 in one air-con unit might occupy registers 1-10 in the data concentrator and registers 1-10 in the second air-con unit might appear as registers 11-20 and so on. Another problem can be controls. Writes to the data concentrator need to be passed on to the proper air-con unit transparently.

In the second approach, there are unavoidable communication delays. If one BMS is talking the other must wait until the first message is completed.

The MARC Omnii-Comm http://www.miille.com/din266-p00.pdf can be configured as a data concentrator and the MARC Multii-Port http://www.miille.com/din366-10p.pdf works as a "traffic cop".

Jerry Miille