I was thinking about a KT74 as well so I contacted BK (not Burger King!) to ask and they replied with a list of items that are "configurable" (see below)
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The KT74 can't "set" the SIL and SDA values,,,, that's all part of the GPS input and how the KT74 is configured. Our KT74 is STC'ed for certain GPS units (see attached AML). Configuration of the KT74 is accomplished via the front panel buttons. We have a selections for:
1. GPS source - Aviation Format NMEA 0183, Nex Na mini, FreeFlight, KGX, Garmin ADS-B+ Out, BendixKing GPS Express (KSN770).
2. Interface Speed -- 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115K
3. GPS Certification -- (From the GPS manufacturer) Level B, Level C, Level D and uncertified.
4. GPS NAC Velocity -- Unknown, 3 meters per second, 10 meters per second.
5. Measurement Units -- meters and feet
6. GPS Reference Offset -- Auto set by GPS, Manual Set here and Unknown.
The above information is not inclusive of all configurable items,,, it's just the ones I thought would be applicable to your question.
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As the KT74 can set the right bits for Levels B, C and D to conform with the standard, I would have thought that setting the correct bits for an uncertified unit would have been viable too. Perhaps I am just being naive.
Anyway BK have passed my question on to Trigg (who they openly identify as being the design authority etc) to see what they say. Not looking good but may be still worth pressing a little further.
The way it is looking the solution may be to the unit as a Mode S and see what comes out if a non-certified GPS is connected (obviously with the collaboration of CAA/FASVIG etc), may be as an adjunct to the trial.