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.
The "Internal Serial Slot Designer's Guide states:
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.
The signals are most critical for Serial0, which has the LTC1323 driver on the main board which will attempt to drive the signal lines unless Pin 8 is high. For Serial3, there are few (if any) devices that ever connected to the external interconnect port and used Serial3, so the potential for conflict is less. Even still a "good citizen" will still honor these signals. A bus driver buffer that can tristate these signals when not selected by these logic signals should probably be used.
One interesting wrinkle is an observation by Matthias Melcher:
Serial Port 0 uses gpSerPortSel on pin 8. When gpSerPortSel switches high, the card must respond with a low on DCD (5).
Basically, Matthias has found a sort of handshake where the Newton checks to see if a modem is present, and if the modem answers properly, it will leave the modem port selected. I have not yet reproduced this, but I have seen a 5ms pulse when connecting to Serial0 on the Newton. This pulse may be an interrogation. I have yet try and respond on DCD to see if the port stays high (internal port selected).
