SkyView serial ground ?

[RKellogg]

Help!
I am connecting SL30 serial (RS232) to SkyView harness. The SL30 has a pin for "serial ground" for the shield ground braid. The SkyView manual makes no mention of a serial ground. What should the serial ground be connected to at the SkyView harness? I am out of ground connections at the SV 37-pin harness connections.
Thanks for any help, it would be nice to get this right the first time... Maybe this is common knowledge, couldn't find it on search.
- Roger

[vlittle]

Easy, just ignore it. Not required in our small aircraft.

[Jesse]

Ditto what Vern said.

[Walt]

Originally Posted by vlittle

Easy, just ignore it. Not required in our small aircraft.

May be true to a degree, however serial lines can generate noise (usually in audio systems) especially if the lines run parallel in the same bundle. Best practice is to keep them sheilded unless they are very short runs and not bundled with senstive equipment. If the unit is out of pins just ground the shield as close the connector as possible.
Just my opinion.

[lrgabriel]

Well, I'm a rookie...but I ran twisted shielded pairs between my Dynon and serial devices, with the shield grounded to the connector backshells on both ends.

Also, I made sure the cases of everything (Dynon, radio rack, components, shelf) were all grounded together with actual wire runs.

No problems with data transmission or audio noise.

[RKellogg]

Thanks for the thoughtful responses! You guys are what makes VAF so valuable.

Wikipedia had a readable explanation. Quoting: "Because both ends of the RS-232 circuit depend on the ground pin being zero volts, problems will occur when connecting machinery and computers where the voltage between the ground pin on one end, and the ground pin on the other is not zero. This may also cause a hazardous*ground loop. Use of a common ground limits RS-232 to applications with relatively short cables. If the two devices are far enough apart or on separate power systems, the local ground connections at either end of the cable will have differing voltages; this difference will reduce the noise margin of the signals. Balanced, differential, serial connections such as USB,*RS-422*and*RS-485*can tolerate larger ground voltage differences because of the differential signaling."

It appears that the Tx and Rx wires do not make a loop but require a commom ground reference between devices. Since both devices are grounded to the same ground and the wire lengths are short, it is unlikely that the ground reference voltage between the two units will vary by the 3 to15 volts of the data signal. And I hear that twisted pairs are relatively immune to static problems.

Nevertheless, Garmin recommends a ground/shield for the serial data line. I have a spool of shielded twisted pair tefzel wire, and l local chassis ground is easy to do. After investing so much T&M in this project, it would be a shame to have flaky performance of the technology or some irritating droning noise in the headsets. Maybe it is overkill. Gotta get it done.

-Roger

[vlittle]

What about all of the serial links in the SkyView harness? To be consistent you would have to rewire these as well, but there is no need. Shielding the system audio cables, however, is highly recommended.

For local interconnect, the Rs232 shield is not essential. If you want to run off panel, it's more advisable. There are thousands of SV systems installed like this.

[RKellogg]

Vern,
That thought occured to me today as I was connecting wires. Those other four serial links are just twisted pairs. But I justified the shielding by its proximity to the audio components. Guess the chassis ground is pretty reliable for data links.
Looks like it works, per ground tests, but haven't flown it yet to verify airborne operability.
The handheld radio does pick up a lot of static on 122.7 when the SkyViews are powered up. Can't yet tell if the SL30 sees the same noise... more tests soon.
Roger

[vlittle]

The Skyviews generate audio noise, but this can be minimized by ensure the intercom and Skyview share the same ground, the Skyview audio lines are terminated with 50 ohm resistors and the Skyview audio level is set to 70% or higher.

Dynon recommends grounding the case of he Skyview separately but I found this to make it worse.