Best practice RS-232

[frequent_flyer]

uAvionics AV-30 allocates one pin for reception of a GPS RS-232 bus. There is a power ground pin but no signal ground pin. The AV-30 will be connected to an RS-232 port of a Garmin GNC 355 which allocates separate pins for TX, RX, and ground for each RS-232 port.

I have suggested to the person making this installation that he should use shielded twisted pair with GPS TX to AV-30 RX, GPS RS-232 ground spliced to AV-30 power ground, and shield connected to shell at both ends.

RS-232 is quite tolerant and I expect the installation would work with a single unshielded wire between GPS TX and AV-30 RX pins. They share airframe ground. I would not want to wire it that way.

What would be best practice for wiring this interface?

 

[sjohnson]

I would use twisted pair, connected to the Garmin signal ground and the AV30 power ground. Shielding won't add much. If your installer insists on using it, make sure the shield is grounded on only one end, preferably the Garmin, and connect to power or frame ground (not signal ground). You want to avoid having the shield carry power or signal current. It is there for noise only.

 

[wireweinie]

This is just the way I wire these. I've done it over the last several years and haven't had issues so I'll keep on doing it this way.

Wire the data line at the 355 just the way Garmin shows in the diagram. Usually here, they call out three conductor shielded wire. One each, TX, RX, and data lo. Pigtail the shield at the 355 connector only, as it's being used just as a shield. On the AV30, position 2 is the GPS data input and it is shielded, with the shield being pigtailed at the connector. I use two conductor shielded wire here and tie the data lo wire to the AV30 power ground. When I connect the AV30 to the 355, I simply connect two wires of the three available (AV30 GPS data input to 355 GPS TX and data lo to data lo). I feel that this gives the best ground path for the data as it's ground path is directly from one unit to the other instead of through airframe ground.

Web