I’m having a bizarre issue with the PTT GPIO output that I’m wondering if anyone has come across before. Logic high on the PTT GPIO is way lower voltage than it should be. Now seeing it across two different known-good Pi3s.

Setup:
– Raspberry Pi 3
– 2x Kenwood TK-805d with I/O mods
– Opto-isolated COS (3.3V active-high)
– Easy Digi opto-isolated PTT

The issue is that for some reason no matter which GPIO I select as PTT, the logic high signal is only a few hundred mV. Additionally other GPIOs will also go high at a few hundred mV when they should stay at 0V. The level is not consistent between trials either. Scope captures below.

It should be noted that the COS input GPIO is working fine. I hear audio passed via the sound card as expected when a signal is received and all the levels look ok.

I was thinking this could be a fried GPIO controller in some way, but I’m seeing identical behavior after switching in another known-good Raspberry Pi 3. Input pin works fine, audio is passed, but PTT GPIO voltage is way too low.

Really scratching my head on this one. Unless I managed to destroy the GPIO outputs in exactly the same way on two different boards this points to something software-related… but I’m not sure how it could.

COS signal working at a full scale 3.3V, as a sanity-check:

PTT output on pin 12 (with output set to pin 12), note different voltage levels at different times but both way off. Keep in mind this is with nothing external connected to the pin other than the oscilloscope probe.

On an unrelated pin (15) that should be off or floating, we somehow get a higher output signal than on pin 12:

Any input would be appreciated!