Just FYI, there is a 10k pull-up resistor connected to +3V3 on the PTT signal in the GD-77. I didn't look at your TNC schematics yet.KC7RBW wrote: ↑Wed Mar 25, 2020 6:49 amThanks for the help so far, Jens!
Here's what I learned by poking around with a cheap multimeter:
This first part is interesting, but I don't think this is the problem...
- Speaker mic
- PTT not pressed
- 2.5mm sleeve (GND) and 2.5mm tip (Speaker) are connected with a slight resistance
- Nothing else is connected
- PTT pressed
- 3.5mm sleeve (PTT) and 2.5mm sleeve (GND) are connected with almost no resistance
- 2.5mm sleeve (GND) and 2.5mm tip (Speaker) are still connected with a slight resistance
- 3.5mm sleeve (PTT) and 2.5mm tip (Speaker) are of course also connected the same
Here's where I think the problem really is...
- TNC2
- Not transmitting (PTT not pressed)
- Nothing is connected
- Transmitting (PTT pressed)
- 3.5mm sleeve (PTT) and 2.5mm sleeve (GND) are connected with a slight resistance
- Nothing else is connected
So I think what's happening is that the TNC2 cable is kicking the radio into USB mode, which lowers the voltage the radio puts between PTT and ground for that detection (presumably to avoid triggering PTT while connected to USB).
- GD-77
- With generic cables
- Shorting 3.5mm sleeve (PTT) and 2.5mm sleeve (GND) triggers PTT
- 3.22V measured between these two points
- With TNC2 TRRS cable
- Shorting TNC2 TRRS sleeve and Ring 1 does not trigger PTT
- Around 0.35V measured between these two points
I don't know how the radio decides this. I can't figure how it's possible since in both cases, as far as I know, the wires are open-ended until they are connected to something. But I tested it many times while trying to figure out what was going on and the results above are completely consistent.
Is there maybe a capacitance being measured to detect if long wires are present?
Cheers.