After getting the repeater working in op25/asterisk i'm still stuck with the problem of PTT control. I see on the app_rpt page there are instructions for modifying a CM108 usb sound card (which i attempted to order, but the order got cancelled because they are on backorder...). I was reading this page http://yo3iiu.ro/blog/?p=324 and was wondering if something similar could be done with the chan_usrp in asterisk (or soundcard_tx.py in op25). The part that interested me was using a serial connection to do the signaling for PTT. 

Just a thought,

Ryan KC9GMY