The VT-MODEM-3 has eight DIP switches. These switches establish the mode of operation for the RS422/RS485 port. Set these switches to match the type of wiring connected to the RS422/RS485 port. It is not necessary to cycle power to the modem if DIP switch changes are made.
Wiring and DIP Switch Settings for 2 Wire RS485
Wiring and DIP Switch Settings for 4 Wire RS485