Serial between two 'systems' (embarrassing question)

[alexanderbrevig]

Hi again. For some reason I cant make my head up if I'm thinking straight. So to illustrate, here's a schematic:


I'm not sure if sharing V+ is OK. If not, could I share ground to make each system recognize HIGH sent from the other over a UART?

I've tried to google and search the forum but I don't have sufficient terminology or intuition for what search terms would be best. Sorry.

I'd appreciate any and all input.
Kind regards

[digsys]

With RS232, you do need to reference the 0Vs, and be SURE they really are at 0V relative to each other. RS485, gives you some latitude if you
can't guarantee they can be tied. As for V+, you only need to share it, if the remote connection has no supply, or you prefer to supply it from
the host (or vice-versa). So you only need 3 connections.

[MatthewEveritt]

Sharing V+ is generally a bad plan, whereas sharing ground is generally a good plan. Putting different power supplies in parallel is a recipe for damage.
In this case,  just cut the V+ and let the common ground provide a reference.
Typically serial devices recognise high and low signals by comparing them to their power supply,  so (very roughly) anything less than half Vcc is a 0, anything more than that is 1. There's generally enough 'wiggle room' in that to allow devices with separate but similar power supplies to understand each other. If not you need some active level translation.

[alexanderbrevig]

Thanks guys! Three wires it is. It was what I thought as well before I started pondering about it. I've shared both before but as you pointed out I only had one power source then.

I am sure GND will be the same for both systems so should be fine.
Thank you both