The output of the power transformer should eb plugged into the plkug marked RWP, in the rear of the modem. The digital interface is at the rear of the modem, marked RS-232. Table 3.2 details the pins and their funtions. Due to the fact that the Model 420 operates at bit rates beyond the rates called out in RS-232 specification. Transitions must occur at the positive edge of the selected transmit clock.
Space is positive, mark is negitive.
Transitions occur at the positive edge of the receive clock. Space is positive, mark is negitive. An output from the modem. When positive indicates the modem is prepared to except data to be transmitted. Indicates the modem is powered up.