MODBUS RTU for Boiler Control Module


Thread Starter



I have a boiler control module which communicates to a local display Via MODBUS RTU. We have installed a Building Management System which also communicated to the Boiler.

The problem is that it does not support two (2) devices and we need to hit a switch which allows us to go between the boiler display or BMS system.

From my understanding, the boiler is the master which serves data to the slave\client devices. Can I purchase a device which will allow me to use one Master to two (2) slaves devices? Someone mention that I could OR these, but the manufacture said that since both also send commands - from the screen and BMS HMI that things will be missed.

Looking for ideas on this
Thanks for any info.
> Can I purchase a device which will allow me to use one Master to two (2) slaves devices?

You shouldn't have to. A Modbus RTU master can read/write to any one of multiple RTU slaves on an RS-485 multidrop network. That's conventional Modbus.

But I suspect that
- the boiler module is actually a modbus RTU slave
- the display is a Modbus RTU master
- Building Management System is a Modbus RTU master

That's two masters and one slave. Modbus RTU can have only one master. Your present system allows you to switch which of two masters will talk to the boiler control module slave.

A contributor, Mr. Miille, offers some kind of workaround box that moderates two masters on one RTU network. Maybe he'll pop up with an answer for dealing with two masters and one slave, if that's what you really have.

>could OR these
a multidrop RS-485 network has multiple devices 'in parallel' on a wired network, but the Modbus Protocol polls each node/drop/slave individually, which is a long way from a boolean OR function.
I'd suggest replacing the local display with a PC-based HMI designed to replicate its screens and functionality, but which can operate on two comm channels at once. Then on the second channel, communicate as a master to your BMS as a slave.

You'll have to do some script to relay the data to the BMS, and make sure commands originating from the BMS are sent to the boiler, but nothing insurmountable.
Hello and thanks for the reply.

What I've been told by the manufacture, is that if more then one thing (slave) is connected the boiler control module does not have an output capable of support another device.

That might be a serial connection limitation.

RS-232 is point-to-point and supports only one device talking to another device. There used to be A-B switches for 9 pin D connectors that switched RS-232 from port to port.

RS-485 is multi-drop.

Sometimes a work-around for point-to-point is to use an RS-232/485 converter, but that assumes the ability of the RS-232 devices now sitting on an RS-485 network to be addressable. Sometimes they're not and conversion to 485 is not a work-around.

I think you need to hire a local systems integrator guy to sort this one out for you and tell you definitively whether there's an option.
What are the specs on your boiler control module and BMS? Converters and/or protocol gateways are always an option, but that depends on the limitations of your hardware. Two slaves communicating with one master should theoretically not be a problem. Many systems have many more than that. If it's not a limitation in the RS-232 line, it could simply be a simple configuration troubleshooting issue.

There are embedded controllers like the one here, from ICP-DAS USA, that take your RS-232/ 485 connection from your PLC and communicate to your multiple slave units through DAQ , using RS-485.

If you have the model number of your current equipment we can determine if it's just a connectivity problem.
For situations with two Modbus RTU masters communicating to on or more RS-485 slave devices, we have a new module which will allow this combination to work.

The TSH-735 will allow 2 Modbus RTU master devices connected to individual RS-485 ports to talk to Modbus RTU slave(s) connected to a 3rd RS-485 port.