Hi all, i have an old teleprinter with an rca logo on the front that is sitting on an extel interface box. the extel model is AN 11R 155 it was used with a upi or api wireline. the interface has a crystal selector switch and i have no idea what that is for or why it used xtal's on wireline. the printer is dot matrix but not 9 pin. What two wire interface and protocol did this use? i'm thinking eather 20 ma current loop or some odd subcarrier audio line. anyone know? Bill