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Monday, February 18, 2019
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Two Master Controllers in One Loop
Possibility to install two master controllers on one Modbus/RTU Loop
By Mohammad Abughoush on 29 January, 2019 - 11:02 pm

I have a technical question.

I have a chiller that contains 3 controllers inside its control
panel. Those controllers are daisy chain connected through Modbus/RTU (RS485) loop end with PLC (as a Modbus master controller).

It is required to control /monitor the chiller signals via BMS.

So, if I consider the DDC (BMS Panel) as Master controller, and the chiller controllers are slaves, shall I override (remove the network wires from) Modbus chiller PLC master controller, and consider the other three controllers as slaves?

please support me.

Thanks and regards,

That is not a question that can be answered.

First you must document the process equipment being controled, and the intent of the existing controllers, in addition to the wiring hookup drawings.

If the chiller is comprised of 3 Modbus RTU slave controllers and a Modbus RTU master PLC, I would treat this system as a "black box". If you remove the PLC from this network, the chiller could very well cease to function.

Instead, you should investigate whether the PLC has another RS-485 or Ethernet port in which to connect your BMS, separate from the port connected to the 3 controllers.

I would also suggest contacting the manufacturer of the chiller for recommendations on proper interfacing to an external BMS, if this is even supported.