Accessory/ext PA PTT Control
This port is used to check and trigger the PTT from/to an external device. The "tip" is internally binded to PTT/DAH line so it is used to key the transceiver. And the "ring" is grounded when the transceiver is keyed and it can be used to key an external amplifier. You can think about this as "tip" is input, and "ring" is output.
The connector used is a stereo jack 3,5mm.
Fig. 8 - Accessory Connector J3-RF
External device side
As an example we can view how to connect an external linear amplifier (KXPA100) key control line.
This connection depends on your devices so for avoid damages in mcHF and/or external device double check the "sink current" values for both of them, and the connection schemes for each one and ensure everything is compatible.