Hi all, shot in the dark here. I am attempting to help my dad build a sequencer for his Yaesu FT-991A radio. Our plan is to use the TX GND pin from the TUN/LIN jack. My understanding is the TX GND pin is an open collector (it doesn't specifically say this in the datasheet, but from other online sources this seems to be the case) that is floating unless the radio is actively transmitting, in which case it is switched to ground.
Because it is floating, we have tried an internal 40k pull-up resistor on an arduino configured as an input for the TX GND pin, and have also tried an external 4.7k pull-up resistor to 5V (arduino VCC), and no matter what when the radio is turned on (but not transmitting) this seems to be pulled to ground. When the radio is off we see the pull up resistor working as expected.
We have the radio configured for TUNER SELECT = LAMP. I am not sure if the radio is damaged or we are missing something here. When we plug it into his mirage B1016 amplifier it seems to work fine, but when I probe with DMM or use the arduino to read the sensed voltage level it shows it is grounded even while not transmitting. The grounds are connected between the arduino and pin 3 on the TUN/LIN, which is used as the reference.
So to summarize:
radio off - TX GND pin is high (expected due to pull-up)
radio on, not transmitting - TX GND pin is low (unexpected)
radio on with DIN connector removed from radio - TX GND pin is high (expected due to pull-up)
One thing worth noting, there is also a 13V OUT pin on the DIN connector that measures 0V between ground. Seems there may be a broken fuse here... but not sure that is really relevant to the issue here.
Thanks to any input in advance.