12.3.5 Minor setback on Avionics wiring

This entry is part 40 of 67 in the series 13 - Electrical / Instruments

Tim at Approach FastStack saved me a ton of frustration when he discovered that I would have to have two displays. So here’s how the all the avionics will connect to the displays:

HXr-1Serial 1: Nav-1
Serial 2: AHRS-1
Serial 3: AHRS-2
Serial 4: EIS
Serial 5: Audio Panel
Serial 6: Com-1
Serial 7: Transponder
Serial 8: AutoPilot

Serial 1: Unused
Serial 2: AHRS-2
Serial 3: AHRS-1
Serial 4: EIS
Serial 5: GPS
Serial 6: Com 2
Serial 7: VPX
Serial 8: AutoPilot

But once I started getting everything connected, I discovered that the VPX should be connected to the number 1 display. That display will be on prior to engine start and the only way to see if there are any electrical problems is if the display that the VPX is connected to is powered on.

So a quick call to Tim and he said “no problem”. He’ll send me a new configuration plug that will put the VPX on EFIS-1, Serial 7 and the transponder on EFIS-2, Serial 1. Excellent! Two days later, I get the new config plug, and start getting the HXr’s setup. But the transponder isn’t talking to the EFIS-2!

Now here’s the big problem with remote avionics: You don’t even know if it’s on! With panel mount stuff, there are all sorts of lights and such that tell you if it’s on. So when the transponder isn’t talking, there’s no way to know if it’s dead, or on and not talking. I check power and I can see 24v to the connector that connects to the transponder. And the ground is good. So I can assume that the box is getting power. But I still can’t verify that it’s actually on.

What to do?

I connected the old config plug and powered everything up. Transponder is talking away. New config plug and the transponder is quiet.


Well, back in my early years, I used to spend a fair amount of time troubleshooting computer peripherals that communicated using RS-232. Which, coincidentally, is how the transponder talking to the EFIS. So I dug around in my old tools and luckily enough, I still my old breakout box. So I connected this in-line between the transponder and the EFIS and here’s what I saw using the old config plug:

2014-06-13 IMG_20140613_172249713 (Large)

You can’t see it in the picture but the transmit and receive lights are flashing away.

With the new plug, those lights are out.

I do some tracing and it seems to me that the config plug is wrong. I email Tim and within an hour he responds that they made a mistake (and apologizes all over the place). I get a new config plug within a two days and all is well.

I just goes to show: it’s always something.

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