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Random avionics internals

It would actually be easy enough to make a slide-in lookalike KDI572.

All the box does is decode a serial data stream from the KN63 box. It is a sync clock+data one. Then there are two DC signals (called “discretes” in avionics language) which select which of the two NAV radios is to be used for the DME channel number.

The Sandel SN3500 EHSI does all this. It snoops on the KDI572 or DME wires and picks up the data and displays it in the corner of the screen. My writeup shows this. Sandel additionally check that the NAV radio # matches the NAV1/NAV2 selection on the EHSI, to avoid you selecting say NAV2 while using the DME from NAV2, which could confuse the hell out of somebody.

It’s just hard work doing this with 4000 series CMOS logic and other hardware, and then you need a pile of components to drive that EL display. I went to look for a high voltage generator and could not see any inductors, and then found they do it with a Cockroft-Walton voltage multiplier which somebody builds as a little module

In a previous life I used to work in high voltage and we went up to 500kV DC (stable to ~1ppm – that’s another story) using this method.

King had very good engineers in those days.

With a microprocessor, the whole job becomes relatively trivial. The only slight issue is that nobody would be able to legally install it, except on homebuilts, and I doubt any of those have a DME Also I wonder if you could do it on G-reg (LAA) ones; would you get it past the LAA inspector?

Administrator
Shoreham EGKA, United Kingdom

“(LAA) ones; would you get it past the LAA inspector?”
It would probably need LAA Engineering approval, rather than an Inspector. But would the inspector know about the change?

Maoraigh
EGPE, United Kingdom

At some point I was dreaming of building an “open-source aircraft” (that was before starting wrenching on them and understanding what it takes to align all the bolts..), but maybe there would be room for an open-source avionics stack - it definitely looks like the skills are there :-)

Peter wrote:

would you get it past the LAA inspector?

Yes. Most inspectors would support you doing this, it’s very much in the spirit of grass-roots aviation. It just has to be done properly. LAA inspectors are essentially free lance mechanics and are there to help you get stuff done safely, not stop you doing things. We don’t have adversarial relationships with them, and they don’t have to be in an “organisation”, and they are not employees of the CAA or LAA.

Last Edited by alioth at 09 Jan 20:19
Andreas IOM

EEVBlog did a tear down of a Microair transponder:



Andreas IOM

That was fun to watch

That guy is pretty good, especially with his non PC narrative

Impressive CNC machined case!
Very clever incorporation of combline filters in the CNC case.
Very labour intensive construction. Must take bloody ages to put together, set up, test, etc.
But some RF guy had a whole load of fun which is what really matters

One day if I can get a dead GTX330 I will take it apart and I am sure Garmin won’t be doing this kind of stuff in theirs In fact there must be piles of dead King Mode A/C transponders kicking around avionics shops, so if anybody wants to send me one…?

At some point I was dreaming of building an “open-source aircraft”

I vaguely recall hearing about some project like that. The problem is that

  • assembling stuff takes a lot of time and skill, so getting the designs, even PCB gerbers, for nothing doesn’t really help
  • open source [anything] is great until the originator gets himself a girlfriend and then it is abandoned, until she does a runner
Administrator
Shoreham EGKA, United Kingdom

I have a GTX320 (which I put some photos of in this thread, I haven’t stripped it like Dave did for the Microair above, since I intend to use it one day for testing a homebrew transponder tester – but guess what, I have to build an interdigital filter to build that :-) There is an engineering firm in the Isle of Man who can CNC me a box and precisely cut the rods).

Andreas IOM

@Peter, it could be a group of people doing something! :)

But, as everything these days is “smart” and comes with code, the problem is going to be with the software QA.
For the certified avionics that is independent external entity, and with the open source it will be, well, interesting! :)

EGTR

I was going to strip down a Shadin Microflo but the only spare I have has the ex-Socata EASA paperwork and a seal on it, so I better not But I did see one some years ago and it was 1970s tech, Z80 IIRC. Another very tempting DIY project for a homebuilt plane, or it would be easy to build that functionality into something else like an OAT gauge so you have a backup for the Shadin.

I doubt GA avionics has much software QA. Maybe autopilots do.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I doubt GA avionics has much software QA. Maybe autopilots do.

For certified GA? They should!
DO-178, right?

EGTR
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