determining RS485 pins on a connector


Thread Starter



I am to connect a modbus device to a plc. my plc has a rs485 port on itself, but the problem is that i don't know the pinout. Can anybody tell me How to determine different pins on a rs485 port.

PS: My plc is a Sprecher and i have tried to connect modbus pairs to all pins to test which ones are D+ and D- but i couldn't get any result. it seems that i have a 4 wire port and i should connect it to a 2 wire device. the problem is that i don't know which pins are rxd+ rxd- txd+ and txd-

Lynn August Linse

Unfortunately there is no absolute way to know, plus different vendors select + or - for different reasons, so one vendor's + might be another vendor's -.

In general, the + will be at least 100 or 200mV higher than the - when the RS-485 line is idle (no one talks).

In general, with a good volt-meter you can detect the TX pair voltage fluctuate when a device talks (doesn't help for a slave device). For example most Fluke meters have a little "+/-" symbol that flickers when it detects voltage changes too fast to show.

When given an unknown RS-485 device, I would open the product and check the PCB; you can likely see how the terminal pins connect to the RS-485 driver chip, the data sheet of which is likely on the web. For true 4-wire, likely there is a terminator resister (& optional jumper to disable) on the RX pair. There will be none on the TX pair.

Lynn August Linse

I would add, you might require a 5th signal-ground wire. True '2-only' or 4-only' wire RS-485 generally only works for products from a single vendor, as then they have designed the power-supply & grounding system is such a way the signal-return is not required.

For multi-vendor systems, assume it won't work (or won't work RELIABLY 24/7) if you don't link the signal ground together somehow.