Modbus RTU 4 or 2 Wire RS485


Thread Starter


Why somebody use 4 wire network for Modbus RTU.
Modbus is not full duplex so 2 wire should be ok!!
Any advantages?


Jerry Miille

A 4 wire hookup is much simpler to troubleshoot and to use. The master transmitts to all the slaves and all the slaves transmitters are connected to the masters receiver. The slaves only "hear" messages from the master and the master can only hear the slaves responses.

With a 2 wire hookup, everyone hears all the traffic. Slaves receive not only polls from the master but also can hear every response from all other slaves. Unless the hardware specifically disables it, everyone also can hear themselves talking. If you have an interrupt per character type system, a 2
wire system will significantly add to the interrupt overhead.

With that said, Modbus will indeed work on a 2 wire system but some manufacturers equipment may not.

Jerry Miille
The only difference really is that echo suppression has to be handled properly in all of the devices on the 2 wire network. 4 wire has 2 sets of differentials and does not have to worry about suppressing the echo. I've dealt with many headaches on echo suppression over the years but it seems to be fading. 4 wire RS485 and RS422 has always been more robust for me.

Ganesh Okade

4-Wire communication is differential and hence any noise on the bus lines cancel each other unlike in the case of two wire which makes it more noise resistant. Hence 4-wire is still used - it has nothing to do with duplex communication.

Ganesh Okade
Sunlux Technologies Ltd.
I would also be interested in this. I am doing Modbus for the first time. I have 6 PLC's each with a Sinec L2 and am trying to conenct to a PC with an RS232/485 adaptor. Wiring diagram shows 4 wires (and earth) - do I need this.

Also does anyone have a low cost SCADA package for Modbus RTU - I only need to see about 10 registers in each PLC


Lynn at Alist

See for some explanation of RS-485 and grounding issues.

You need to handle the earth for RS-485 signal reference and current return somehow - either you use the 5th signal wire or all 6 of your PLC must reference the RS-485 to the same physical ground. Since you use RS232/485, it will depend on the converters you buy. In a lab, RS-485 4-wire without concern for grounding works fine, but systems that don't use the 5th signal ground wire when it should be used will have long-term robustness problems.

Best Regards

Lynn August Linse,
[email protected]
Technical Director
Robust DataComm Pte Ltd (

Ganesh Okade

Hello Adrian,
The wiring diagram shown is obviousy for 4-wire communication. If all your devices support 4-wire then goahead and use it - its has better noise immunity and you need not worry about echo as others have noted. If not convert the 4-wire system to 2-wire and use it in 2-wire mode.

What is your budget for the SCADA package. Are you looking for a single screen which displays the 10 registers and maybe log them?

Ganesh Okade
Sunlux Technologies Ltd.