The Data port on the back of the radio is active in all modes, else you would not be able to use it with a Terminal Node Controller for Packet or any other similar mode on FM in 2 Meters or 70 CM.
There are also settings for the audio level input and output in the settings menus for the data port separate from the mic level settings, and there is a Key line on that Data port that keys the radio through the Data port rather than the PTT on the mic input port on the front.
About 25 years ago, I home brewed an interface on perf board to make an FT-100 and later an FT-857D into a remote base for a repeater through the Data port for all voice modes, when I was in the Youngstown Ohio area. We used it on FM to link to other repeaters, or SSB to make repeater to HF (or 2M SSB or 6M SSB) contacts through the repeater. It was a 440MHz repeater.
All of the input and output pins were isolated and buffered, the Squelch pin went to the input of the Repeater Remote Base COR input, the Repeater Remote Base PTT line would key the radio, and the audio input and output pins would go to the remote base audio input and output lines. It could be used receive only, like to monitor for a call on another repeater.
