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MPONI - PROBLEM - 3 wire Modbus
Is the modbus protocol not merely half duplex???

I have an MPONI from Cutler Hammer, and the technician will not allow a two wire connection for the communication. Why is this? If it is truly Modbus, why is a two wire connection not allowed? Will it work? Has anyone had any experience with this in the past? Please give insight and help. Thanks in advance.

By Carl Burgess on 13 February, 2007 - 10:23 pm

Modbus is a master-slave protocol, so the slave doesn't talk until it's spoken to. So yes, a half-duplex link is all that is required.

We frequently come across 'turn around' issues with RS485. A lot of RS485 systems (PCs are the worst) automatically enable/disable their drivers based on whether there is something in the transmitter buffer to send. Sometimes they can be a little slow to turn off, so that the slave device repsonds before the master switches from 'Transmit' to 'Receive'. For that reason I prefer to use 4-wire RS485 (or RS422) if possible which avoids the problem. A lot easier to debug an errant link if both halves of the conversation are on different pairs also.

Hope this helps,

By Benoit Galarneau on 13 February, 2007 - 10:29 pm

When Using 2 wires modbus on a device that can also do 4 wire, you will generally have to set an adjustable delay between receive and transmit on each devices (delay value is a case by case and has to be incremented till no more failures). Some devices are too prompt on transmitting after receiving, so the answer is not properly received by the other device...

If the technician insists on 3 wires, it must be half duplex (2 wires) with a signal ground (3rd wire).

The necessity for a signal ground comes from experience. While it's worth reading Mr. Fahrion's (B&B Electronics) entire document at for good advice on installing RS-485 serial links, the excerpt from his document on "2-wire RS-485" is below:

"There's no such thing as '2-wire RS-485.' The next time you hear someone describe their RS-485 system as '2-wire', please do me a favor and give them a poke in the ribs. RS-485 requires a differential pair and a signal return line. Sure, it seems like it works without the ground, but I can assure you that your system is in peril until you connect that dedicated signal ground connection. With this customer, I was easily able to demonstrate that the noise level dropped dramatically when we added a signal ground connection."


I didn't read Mr. Fahrion's comments because I've seen more failed 3-wire RS485 installations, than I have 2-wire installations. What's usually the cause of the 3-wire installation? The ground.

I agree, in a perfect world the ground should be used, but most often it's used incorrectly and causes more harm than good. In those cases, I simply remove the ground, and all is good.

By Henrik Maier on 15 February, 2007 - 11:17 pm

3 wire is the correct way to wire up a Modbus for RS-485 layer. The third wire is a "must" requirement as per Modbus standard. From the
official "MODBUS over serial line specification and implementation guide V1.0":

"On standard MODBUS system, all the devices are connected (in parallel) on a trunk cable constituted by 3 conductors. Two of those conductors (the "Two-Wire" configuration) form a balanced twisted pair, on which bi-directional data are transmitted, typically at the bit rate of 9600 bits per second."... "In fact a third conductor must also interconnect all the devices of the bus : the common."

The already mentioned B&B document (previous post) and other tech notes published by that company explains why the common is technically required.