Application Note: PTT Keying
The firmware of the Pi may configure GPIO pins as inputs with pull-ups.
Once the kernel boots, the driver correctly configures the pins. This is not generally a problem, except possibly for PTT. This will manifest itself by the PTT keying the transmitter before boot-up of the Raspberry Pi.
GPIO 12 & 23 are used to drive PTT and the pull-up is sufficient to turn the transistor on and key the radio, even though the red PTT LED is not lit.
This only happens during boot-up or shutdown, but may be an issue in some applications. Adding a 2.2k pull-down on the Pi's 40 Pin Header solves the issue.
Mini DIN-6 Connector:
Pin-16 to Pin-14
Pin-30 to Pin-32
Note: An original UDRC only uses GPIO 23, so Pin-30 to Pin-32 is prescribed.
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