I'm going to develop a network of 10-30 electronic nodes based on MCU. The distance between them could 100m maximum. There is a single centrale power supply unit (PSU) that gives power to nodes through a low-voltage 12-18Vdc plus/minus cables.
I'm thinking about a two-wires half-duplex RS485. I'll have 4 cables (+/- for power and A/B for signals) and the advantage of differential signals that should be more immune to external electromagnetic interference.
However the classical scenario for RS485 is a network of nodes where each node has a *different local PSU*. Indeed, there are many considerations about grounding, shielding and avoiding ground loops. Should I connect the ground between nodes? Should the ground be connected to earth through a short or a resistor or a capacitor? Should a shield be used? Where the shield should be connected? Should the shield be connected to ground/earth on one/both sides? And so on... I read some documents and there are many different "point of views"... :-)
However my situation is different. I'll have a *central* PSU, so I must cable ground and the positive rail. The remote nodes aren't locally connected to earth (directly or through any other resistive, inductive or capacitor coupling).
FIRST QUESTION In this situation, is a single-ended signal (RS232) similar to differential signals (RS485)? I know differential is better because an interference is coupled to both signals as a common mode voltage that the differential receiver doesn't consider. But I have ground cable too. So an interference is coupled to
*ground and signal* at the same time. The single-ended receiver measures the difference between signal and ground, so the common mode voltage created by the interference is subtracted anyway.SECOND QUESTION Shield is typical connected to ground (or earth?) only at one side to avoid ground loop antenna. This is true if the remote node has a local earth connection. In my case the remote node is "floating" respect the earth, so there's no risk of ground loop antenna if the shield is connected to ground at both sides. Is this correct?