Modbus and DC Power over same cable


Thread Starter

Saad Farooq

I am using a DIGI wireless modem for communication between GE Fanuc MircoPLC's. The modem gets digital data over Modbus RS485 and DC voltage for power.

The supplier provided Belden 4 pair cables for the Modbus data transmission and an AC-DC adapter for the DC voltage.

So now, we have to lay two cables. One is the Belden for the Modbus and the other is 220Vac power which is converted to DC by the adapter at the modem JB.

I was wondering if it is acceptable to just send the DC over the Belden cable instead of laying two cables?

Email: saadfarooq [at]


If I understand you right, you intend to send the DC power thru the cable, and not the 220VAC. Definitely don't send the 220VAC thru the cable.

I'd definitely recommend sending the DC power through the same cable. Devicenet works like this using two pairs (one pair for data, one for power).

I've done it many times with multidrop RS-485 Modbus/RTU networks, where I've used a single VDC power supply feeding all devices.

Makes for a much simpler and cleaner installation.