Control.com - Nerds in Control

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