The lights and the DC output voltage are a very simple rectifier circuit that then feeds the DC output terminal and a voltage divider to drive the bi-color LED. In my testing I verified the actual input base voltages that the would trigger the red and green lights. I spot checked some of the buffers DC outputs, though given the light performance, I didn't expect them to be off. The red light starts coming on right as the base PP signal goes below 4 volts, and the green light starts to illuminate around 5.26 volts. Those numbers typically varied by around 5%. I will say, if you get a red light with a Legacy base, I'm 99% sure you have a low signal output that should probably be addressed. The only way to be sure is to measure the actual command base output signal.
All the voltage measurements were done with a Fluke 8012A bench meter, the 'scope measurements were done using the ATTEN ADS 1102CAL digital 'scope. The test "base" signal was 455khz from an HP 3311A Function Generator. In the development, I verified the operation with a couple of the original TMCC bases, one BASE1L, and a couple of Legacy bases. That was to insure that my test setup would reflect the real world. Obviously, I couldn't use a real command base for the actual production testing as I had no reliable way to vary the voltage from the base to test the buffer response at various signal levels.
I remember that Dale had spent significant time talking about the measurement using a similar diode circuit and in fact was including a cheap HF multimeter to get consistent results with every kit he produced to measure the TMCC/Legacy base output voltage.
I'm thinking maybe we need an independent cheap TMCC/Legacy test board just to verify the actual output of the command bases.
