RS232 shielding question

[YellowJacket RV9]

I am working on interfacing my GRT Sport with my Garmin GTR-200 comm. The Garmin wiring harness included a shielded RS232 input wire, but it does not reach the EFIS. To test things, I spliced an unshielded wire to the center of the shielded wire from the radio, and ran that to the EFIS. The serial link works fine, but I am getting interference in the radio with the EFIS on.

The GTR-200 manual shows the RS232 serial line grounded at both ends with the shield. First, would it be acceptable to splice a new shielded wire to the end of the first? And second, where should I ground it at the EFIS side, as there is no grounded metal backshell?

Thanks for any input!

Chris

[Jesse]

Generally shielded wires are only geounded on one end. Splicing shouldn't be a problem.

[rbibb]

Originally Posted by Jesse

Generally shielded wires are only geounded on one end. Splicing shouldn't be a problem.

That depends. General rule is:

If you want to keep EMI inside the cable, ground both ends. If you want to keep the EMI outside the cable, ground one end.

So if a signal is small in magnitude (think audio signal like microphone circuit) ground one end to keep interference from outside corrupting the signal inside the cable (including interference created by a ground loop if you were to ground both ends).

If you are connecting something like a strobe signal (large AC signal) go ahead a ground both ends as goal is to make sure what is inside doesn't radiate to the outside (groundloop induced corruption of the large strobe signal is a non-issue).

In the case of an RS-232 signal you have a fairly large signal [5V or more Peak to peak) running at almost 10KHZ baseband frequency (or more)]. Radiated harmonics could cause all sort of issues. Hence, ground both ends. Use a connector with a metal hood and round the shield to whatever pin the EFIS designates. As usual, AeroElectric Connection shows good techniques for doing so.

[YellowJacket RV9]

Thanks guys for the input. A little more investigating this AM may have changed the culprit. It now appears my Dynon D10 is the source; not the GRT systems as I had thought. This was just the first time I had had my headset plugged in and noticed the interference, and it is only on some of the lower frequencies (<124 Mhz). I can definitively start and stop the interference by powering it on and off.

I will talk with Dynon, I hear they have an EMI filter dongle for their d-sub connectors that may help.

Chris

[Jesse]

I was going to mention that, but it sounded from the OP that the static came when you added that wire. I have a lot of serial wires unshielded with no problems. That doesn't help solve your problem, though

[YellowJacket RV9]

Yea I found it odd that I had three other serial lines unshielded with no issue. Apparently I plugged my headset in for a different reason at the same time I connected that serial line. Goes to show the imprtance of methodical troubleshooting.

Apprently the EMI is a known D10 issue.

Chris

[lrgabriel]

Originally Posted by rbibb

General rule is:

If you want to keep EMI inside the cable, ground both ends. If you want to keep the EMI outside the cable, ground one end.

This isn't really the case.

Terminating the shield at one point vs. multiple depends on the electromagnetic coupling mode you're trying to defend against (electric or magnetic)[1]. Electric coupling is a function of the capacitance across the insulator between the signal wire and the shield[2], while magnetic coupling is picked up by the loop formed by the cable and the ground plane[1].

Low frequency signals interfere via magnetic coupling, and high frequency signals via electric[1][2]. Therefore, when trying to defend against low frequency interference (like when you're trying to protect a low frequency audio signal), you use single point shield termination to avoid creating a loop/current path for magnetic coupling to occur. For high frequencies, it's important to reduce the voltage accumulating on the shield, so multiple grounding points are used[2].

[1] Fr?bel, Anke. "Cable Shielding to Minimize Electromagnetic Interference." Cotbus University of Technology, Germany.
[2] Renner, E. NAVSO P-3181 Aircraft and Avionics Cabling E3 Design Manufacturing Guidelines. U.S. Navy, 1992 para 4.3

[ricardoramos]

I know it may sound dumb, but I need some clarification on shielding on data cables.

As a broadcast engineer, I am used to ground cable shields at one end only. This is a very standard practice in balanced audio cabling.

I came to a halt when wiring RS232s on my 10. I have dual G3Xs and a VPX-Pro. I need to connect the MFD to the VPX using the RS232 and I wonder if should I use a 3-conductor shielded cable (TX, RX and SIGNAL GND) and have them all connected respectively with the shielding grounded on the backshells OR should I use a 3-conductor shielded cable and have the cable shielding acting as SIGNAL GND connector ???

Clarifications will be much appreciated.

Ricardo
Brazil

[SMO]

Page 21-9 of the G3X installation manual shows the RS232 connection to VP with a 3 wire shielded cable, one of the 3 wires goes to a signal ground pin on each end, and the cable shield is separately grounded externally on each end.

[ricardoramos]

Thanks for the reply, Mark. I totally overlooked that. Makes sense now.