MODBUS with Different Power Supplies


Thread Starter



The following is in Modbus. Two different power supplies 12V and 24V.

I will add a dew/temperature transmitter (Comet) to an existing flow-meters network (Micro-Motion). The flow-meters network works with 24V while the new device will work with 12V. Is it possible to work like that? Using two different power supplies?

Thank you!!
Any device using Modbus will have a hardware comm port (RS-232/RS-485/Ethernet) and that comm port will have the correct voltage needed for its specified operation.
It is not clear , is it means you have two system with different power supply level:

- Device 1 (Commet) is using 12 VDC

- Device 2 (Micromoton) is using 24 VDC and the two will be connected each other using Modbus. I assumed using Modbus RTU over RS-485 simplex or duplex.

is that you mean?

If so, it would be no problem at all, two system with separate power supply system can be connected through RS-485 serial communication.

If not you mean, please explain more details

Lynn A Linse

The supply voltage probably won't be an issue, but you may suffer burn-outs if your grounding is not properly managed. I'm going to guess that your 12vdc devices are using wall transformers (wall-warts). If the transformer is isolated by 500v or 1000v (common), then you shouldn't have trouble - especially since the larger 24vdc supply probably is isolated. But if the 12vdc is by simple 2-pin plugs, so grounded with reference to the 'N' line, you could have problems.

If you have many of the dew monitors on a single RS-485 line, you could consider adding one RS-485 isolated repeater between the flow meter and the sensors. If you have many flow meters, each with one sensor, that would get expensive, so trying to understand and validate the correct grounding exists.