ARINC 410 Interface subsystem

Well it’s about time I posted this interface. I’m not going to show you the whole thing because it goes on for pages and pages. Instead I’m going walk you through one channel in and one channel out.  I’m going to assume that you have a older radio head you know the ones with out the fancy LED displays.  Most (here I go assuming again) use the ARINC 410 interface which is a 2 of 5 code. I described it here in an older blog.

Gables G-3717 Dual Nav Head

Nav Pinout

The Analysis: Well the first thing we found out about ARINC 410 that the signals are low active which means they must be ‘pulled up’  if the radio head does not have pull up resistors installed. As you see from the NAV-1 table we need the following information:

Signal Name	ARINC410
Whole MC TENS	A, B, E
Whole MC UNITS	A, B, C, D, E
FRAC. MC TENTHS	A, B, C, D, E
FRAC. MC HUND.	B, C
DME TENS	A, E
DME UNITS	A, B, C, D, E
DME TENTHS	A, B, C, D, E
DME HUND	C
FREQ SEL COM.
DME Indicator	2
ILS Energize
ILS TEST SW
POWER
PANEL LTS	28v
DIAL LTS	28v
VOLUME	3

The Plan and a little more Analysis:  I need to get the 4 connectors for the Gables G-4390 (five heads in one panel).   So the one connector is a PTO2A-22-55P so I need my connector guy to give me the mate to this number. It looks like I only have to worry about the fields labeled DME and that a max of 5 bits. So I’m just going to throw away the other 3 bits. There is no off the shelf  2 0f 5  to ASCII decoder, so lets not worry well do that in the driver..

I picked a Arduino  Duemilanove to run the driver and talk to the flight simulator. It only has 14 digital I/O, so could use the Mega 2560 as it has a bunch of digital I/O.

The Plan:  Ok. So we have to talk to the different heads in the plane. And you could have a bunch. So we need a protocol to talk bi-directional to the heads.. here we go.

1. Write to the interface the head & channel you want to talk to.
2. Read the response from the head.

Bi-directional communication must use two ports one out and one in.. but its easy to combine them and use tri-state drivers.  First I want to use the 74LS125 or 126. This is an quad package, with each of the buffers having separate controls.   So I can get away with 8 bits..  first since the max we need is to count 0 – 5 thats 3 bits. So lets use the first 3, that give us 5 more to select a max of 32 heads. So lets take a pair of 74LS138 3 to 8 decoder one of them has to be hooked up to the D0, D1, D3 to decode which of the words you want A to E. The second 138 is hooked up to the D4, D5, D6 this will give us 8 radios, really 7, because I’m going to use 0 for power status & LED controls..  Next you have to AND the Radio + Chanel to drive the 5 74LS125 Tri-State Control signals..   So if you use my signals you end up with this: EN_Chan1 + EN_Radio1 = EN_R1Ch1.

The Software:  Ok here goes I’m not going to write the code.. but here is what you need to do.

1. Initialize the registers.. #int radio = 0; int channel = 0; odd = 0; max_radio = (you radio count + 1(status)); freq [radio][chan]; freq2[radio][chan];
2. Start Odd / Even loop.. #for(odd = 0; odd < 2; odd++)
3. Start Radio loop.. # for(radio = 0; channel < max_radio;  radio++)
4. Start Channel loop.. # for(channel = 0; channel < 5; channel++)
5. write to the control register with radio, channel.
6. read the control register and store the results..
7. if ( odd = 0){ freq[radio][chan] = result from reg}
8.  if (odd = 1){ freq2[radio][chan] = result from reg}
9. if (odd > 1) write ERROR.
10. End of Channel Loop
11. End of Radio Loop
12. if(odd = 1){  now check for changes.. you want to do this with some loops..
13. if( freq[radio][chan] != freq2[radio][chan])  its changed.. now send to the simulator..
14. odd = 0;}
15. End of Odd /Even loop

FINS & FYI:  You will need a bunch of the 125’s and lots of pull up resistors. Just for doing my Gables G-4390 with out any more radios lets do some math.. 5 heads * 5 channels per * 5 bits = 5 cubed or 125 buffers (32 packages)  & 125 resistors this is not including status or light drivers..  I wire wrap (power tools), but that a lot of buffers. I am working on putting the whole thing into a Xilinx part. This is a product that I am going sell at Flite-Tronics.com. This still leaves the problem with audio, or driving other indicators, like ILS flags, etc.   If you attempt this I really recommend that you check out Avnet they have the best prices for semiconductors period.

I wish to thank Gables Engineering for the pin-outs and outline dimensions.