The internal serial port of the Newton MessagePad 2x00 exposes the same ports as the external interconnect. In order to prevent two devices (one internal, one external) from diving the port at the same time, there are "Serial Port Select Pins" for Serial Channel 0 and Serial Channel 3.
Pin 1 PortSelect:
This is the control signal to select between a peripheral in the internal Serial SLot and the Newton Interconnect Port. When the signal is low, the Newton Interconnect POrt may drive the Serail Port 3 Signals. When the signal is high, the Internal Serial Slot Peripheral may drive the Serial Port 3 Signals.
Pin 8 gpSerPortSel
This signal controls the LTC 1323 line driver for Serial Port 0 that drives the 26 pin Newton Interconnect Port. When Low, the Newton Interconnect Port can drive Serial Port 0. When high, the internal Serial Device can drive serial port 0 on the 32 pin connector.
Serial Port 0 uses gpSerPortSel on pin 8. When gpSerPortSel switches high, the card must respond with a low on DCD (5).