The TAS-A units do not have the same traditional COM port previously used in 9900BX and TAS6xx series units.
For retrofit installations it is recommended to utilize the P1 connector and wiring from the original 9900BX/TAS6xx to the aircraft harness, as the pinout will be pin for pin. There will need to be additional wiring to provide WAAS GPS input to the TAS-A/Skytrax6xx that didn't previously exist.
The installation manual recommends using an "Unused" RS232 port on the P2 connector as a COM port for computer communication and configuration. RS232 ports 2 through 4 are available on both the P1 or P2 connectors. You can only use one or the other, as these ports are paralleled together internally.
In retrofit installations, any "unused" RS232 ports may have their RX line jumpered to ground @ the P1 connector, per the guidance in the original installation. If this is the case, using it's equivalent port on the P2 connector for PC communication will only be successful if the ground jumper on the respective P1 port is removed. For this reason, it is advised to wire the PC communication port to either RS232 Port 5, or RS232 Port 6. (Ports 5 and 6 didn't previously exist on a 9900BX/TAS6xx, and are only available on the P2 connector, so the RX jumper becomes a moot point.
For NEW installations, the use of the P2 connector is recommended for all of the installation wiring.