Working with RS485

[The Master]

I think i kinda understand the problems you get with ground loops but i cant seem to relate that to an RS485 connection. As far as i know you get 2 wires (A and B) and the output to the PIC chip only changes if there is more than 200mv between A and B and it changes to 1 or 0 depending on which input has the highest voltage. Does that mean that you dont need a 3rd wire running between devices if its on a small scale?

I re-read the app note about grounding and there are 2 drawings. I think i might need the one in Figure 4.4 Isolated RS-485 Device. The only thing im not sure about is that that drawing shows the ground isolated from the circuits ground. Is it not enough to have the circuit etc isolated from the mains ground?

[dhouston]

Does that mean that you dont need a 3rd wire running between devices if its on a small scale?

A 3rd wire is going to defeat your isolation. In many networks, power is supplied over the cable from a single source. That leads to mixing grounds and requires attention.If you will power each node individually, you don't have that problem.

I re-read the app note about grounding and there are 2 drawings. I think i might need the one in Figure 4.4 Isolated RS-485 Device. The only thing im not sure about is that that drawing shows the ground isolated from the circuits ground. Is it not enough to have the circuit etc isolated from the mains ground?

You have to either isolate it completely or deal with the mixed grounds. A little mixing of grounds is akin to being a little bit pregnant.

The simplest thing to do is run the ground wire between nodes but connect it to each local ground through 100-ohm current limiting resistors.

I recomment Jan Axelson's Serial Port Complete which covers RS232 & RS485. The subject is complex and really cannot be covered here.

[The Master]

I will google "Jan Axelson's Serial Port Complete" tomorrow. Im off to bed now.

What exactly do you mean by "isolate it completely" is that having each device isolated from the rest of them or having each device isolated from the mains using a transformer?

You seem to be saying that i shouldnt have the 3rd wire in this case but then you say the easiest thing is to have one. I think i might have misunderstood on of those paragraphs.

You really wernt joking when you said "Ground loops are very difficult for most people to understand". I definately fit in the "most people" category