RS485/RS422 protocol, use of common mode signal & 4 wire vs 2 wire communication

Hello,

I am really confused here. I will be helpful for help.

  1. Does RS485 and RS422 are interchangeable?
  2. What is difference between RS485/RS422 2 wire communication and 4 wire communication
  3. Does common mode signal wire is must for this communication protocol? And can it be connected to ground if can be floating?
  1. Although they are very similar they have small differences. The most important: RS-422 has only one single Commanding Device (master) and up to 10 Listeners (slave). In most cases RS-422 is used for only one (commanding) device to one other (listening) device. RS-422 is only defined with 4 wires (separate pairs for transmission and reception).
    RS-485 can have multiple commanding devices and up to 32 devices listening on the bus. In most cases RS-485 does use only two wires.

  2. 2-wire communication is half-duplex while 4-wire communication is full-duplex meaning that with two wires at a given time one device can write to the bus. With 4 wires the controlling device may write while one of the listeners is writing back an answer. As 4 wires usually means RS-422 and that is usually used only for point-to-point connections both communication partners can write at the same time.

  3. I don't know what you mean with "common mode signal wire". If you mean that you need two wires per signal the answer is yes because the signal is transmitted a difference between the two wires and it doesn't have a fixed offset to a common ground. This makes it immune (mostly) to inducted noise signals.
    You don't need a ground wire between the communication partners and it's usually not a good idea to have such a wire if the partner not also share the power supply.

Couple of articles on wiring RS484 for half-duplex and full-duplex and terminations.

MAXIM AN763 Guidelines for RS485 wiring.pdf (247 KB)

Selecting and using RS-232,, RS-422, RS-485 Serial Data Standards AN723.pdf (251 KB)

Selecting an RS-485 Transceiver in Electronic Power Meters, Considerations for AN3776.pdf (114 KB)

@pylon thanks. Thats what I wanted to know. what i meant is I have RS422 connector that has positive, negative, common mode and shield wires. I am sure it will work fine with only 2 wires but I have connected shield wire to ground. Connecting Common mode wire to ground do any good for long distance communication?

As far as I know RS-422 is not defined for 2 wires only for 4 (one differential pair for each direction). I also never heard of a common mode wire to be in a RS-422 cable. What is that wire used for in your setup?
Do you have a link to that RS-422 cable?