Question on Serial Grounds

[lr172]

I have noticed several diagrams showing only the Transmit or Receive wire connections between devices like GPS's and A/P's. I always assumed that I needed the serial ground for these connections, but I guess they are all attached to a common ground bus.

Do I need the ground connection when the devices do not have a dedicated serial ground connection point? I am guessing that if they are both using a common device/airframe ground that it doesn't matter.

I know that I need them for the RS-232 connections for computer connectivity.

Thanks for the help here.

Larry

[MarkW]

My answer should not be conclusive for you but I went and looked at my wiring harness spreadsheet.
All Garmin components and all RS-232 has a ground if it is a in or out only.
I have a couple that are both in and out without a gnd.

[jdeas]

RS-232 is a ground referenced signal so the better the ground reference between the two devices, the less chance of having communication errors (better noise rejection).

Your PC's RS-232 port is normally attached to devices with different power supplies (no common ground) so the serial port's cross ground connection is imperative.
In your plane the need is less clear given the common battery system and short, relatively low speed serial connections.

*I did hook them up (belt and suspenders) but I doubt I would have noticed any difference if I had omitted them.

[lr172]

thanks for the help guys!

[Walt]

RS232, ARINC 429 etc. are shielded on both ends, digital dala lines are noise emitters and can be a problem especially if run in bundles with audio wires if not shielded.