I tried it with one here running R02.07.00.04, and wired a 2k ohm resistor between the GPIO pin and ground (since it's an open collector output), and only see a logic state change in the analog mode when a subscriber keys the repeater. In digital mode I don't see any change at all when a subscriber keys the repeater.
The help file states two things about the PL/Talkgroup Detect function:
"This is an output line used for permitting an output to be activated when the radio is receiving carrier and radio's Unmute Rule has been satisfied on the channel (applicable to Mobile configuration only).
This is an output line that indicates to the user whether the PL/DPL codes received on the channel match those configured in the repeater (applicable to Repeater configuration only)."
That first part about the unmute rule being satisfied sounds like what I want it to do, but I'm not sure how "Mobile Configuration Only" even applies to an SLR5700. I just want something that signals whenever a subscriber is talking on either timeslot in the digital mode.