Trig TT-22 iEFIS control with certified Garmin GPS

[flyboy62000]

I cannot find the pinout data on the Trig CAN converter and with the unavailability of the SP-12 am trying to find out if it’s possible to wire my certified Garmin GPS-175’s RS-232 (ADSB format) output to the transponder’s GPS pin while still controlling the squawk code input, squat switch, and other settings with the iEFIS control via CAN. Alternatively, is there another way to send data from the Garmin to the TT-22 and meet the ADSB position requirements?

Adam

[rainier]

Sadly Garmin does not publish their propriety ADS-B data format and it is not known. Some bits have been reverse engineered (not by us) but not enough to get all the data needed for the transponder. While Garmin apparently has made the format available to some manufacturers they chose not to respond to our queries.

The SP-12 is currently awaiting supply of the NexNav GPS chipset module. We did place an order last year and paid for it too. We have yet to receive them and the manufacturer is unable to tell us when they will be available. They were manufactured in India but for some reason the production was moved to the UK and that seemingly is not working out well.
Trig's TN-72 can be used - it has a RS232 feed (it uses the same GPS chipset also used by several other companies as it basically was the only game in town). You can use this feed as the GPS source in the EFIS and it will then be used for the Trig interface via the CAN bus similar to the SP-12.
However, I do not know how many TN-72 Trig still has in stock as they would be facing similar problems to us.

If anybody knows of a possible approved GPS source we could look into - please let us know.

[flyboy62000]

Would the CAN converter module allow me to just wire the GPS’s RS-232 data directly to the TT-22’s pin 5 or does it use this pin to provide GPS data from the CAN bus? I’m assuming the converter uses the transponder TMAP I/O for control communication to/from the iEFIS system. I already know the Garmin output is compatible with the TT-22 input.

Alternatively, if it’s possible to feed the data via iEFIS RS-232 from a TN-72 it should also be possible to feed the same channel (iEFIS RS-232 #6) with from the Garmin RS-232 Aviation Format at 9600 baud to accomplish the same thing, right?

Thank you,

Adam

[rainier]

Would the CAN converter module allow me to just wire the GPS’s RS-232 data directly to the TT-22’s pin 5 or does it use this pin to provide GPS data from the CAN bus? I’m assuming the converter uses the transponder TMAP I/O for control communication to/from the iEFIS system. I already know the Garmin output is compatible with the TT-22 input.

Alternatively, if it’s possible to feed the data via iEFIS RS-232 from a TN-72 it should also be possible to feed the same channel (iEFIS RS-232 #6) with from the Garmin RS-232 Aviation Format at 9600 baud to accomplish the same thing, right?

Thank you,

Adam

The current firmware in the transponder interface sets the GPS data source to TMAP so connecting anything to pin 5 will have no effect. Theoretically this can be done but would require that the TMAP GPS stream in the interface be removed and as part of the configuration we would need to select protocol and baudrate so that would need to be added to the EFIS transponder setup plus the protocol between EFIS and interface would need to accommodate the new setup.
It would in this case probably be better just to get the head from Trig and not use the EFIS to control the transponder.
I don't think Garmins aviation format is compliant as ADS-B source. You would need to select the Garmin ADS-B format.

The main requirement for a certified source is the availability in the data stream of a very old way of determining the position accuracy estimate and resulting horizontal and vertical protection levels. This method was only ever used in a very early commercial GPS receiver once but somehow made its way into the certification requirements. Of course newer GPS receivers run complete rings around the old devices and are far more accurate and have much better ways of predicting accuracy but sadly the way things work in aviation the goal is that every aircraft will use the same method and arrive at the same result even if its wrong. As long as everybody is wrong at the same time and the same way it's fine.

The UK CAA ran an interesting test using a variety of cheap commercial GPS receivers as ADS-B source and you can find the report online. In a nutshell - they are just fine. But sadly we can't use them at least not to full potential. The TSO-C199 document provides a method to convert the new method of calculating accuracy to the old way. In this way it is permitted to use a commercial (COTS) GPS receiver provided certain minimum criteria can be matched but the SIL/SDA value has to be set to 1/1 which is not sufficient for an approach and your horizontal protection limit has to be broadcast at half a nautical mile which is a little ridiculous.