The serial port pin-out is cunning designed so that you can connect to a PC with a simple modem cable, or daisy-chained with other TTi devices.

1 Power to optional PC-02
2 TXD Transmitted data from instrument to PC
3 RXD Received data to instrument from PC
4 nc
5 GND Signal ground
6 nc
7 RXD2 Secondary received data
8 TXD2 Secondary transmitted data
9 GND Signal ground

Note that the pins 2, 3 and 5 match the IBM PC DE-9 connector standard.

Pins 7, 8 and 9 connect to the next device in the chain. They replace the RTS, CTS and RI functions. The signal directions are the same, avoiding buffer damage if the device is connected to the PC port pins.

It is a clever way of implementing a GPIB-style chain on RS232, but both GPIB and RS232 have largely been replaced by USB, or Ethernet.