Table 6-7 Asymmetric Full-Duplex with Modem Control (FDX C) Signals
Pin Name (Mnemonic) CCITT / EIA Switch Description
1 Protective ground
101/AA - Connects to terminal chassis. Also connects to external ground through third wire of power cord.
3 Received data
104/BB - To VT102:
RXD receives serial characters from the computer.
6 Data set ready
107/CC - From VT102:
When DSR is on:
  • DSR indicator is on
  • Modem connects to a communication line
  • Modem is ready to use modem control signals.
When DSR is off:
  • DSR indicator is off
  • Terminal performs a disconnect.
7 Signal ground
102/AB - SGND is common ground reference potential for all connector signals except protective ground. SGND connects to protective ground conductor.
8 Receive line signal detector
109/CF - To VT102:
Also called carrier detect, RLSD is on when communication line signals are good enough to transfer data. When off, data received is ignored.
13 Secondary clear to send
121/SCB S3 To VT102:
When switch S3 is on (closed), SCTS indicates when modem is ready to transmit data. When on, modem is ready to transmit data and CTS indicator is on. When off, modem is not ready to transmit data and CTS indicator is off.

NOTE: When S3 is on (closed), switch S2 must be off (open).

14 Secondary transmit data
118/SBA S7 When switch S7 is closed, STXD transmits serial characters and breaks generated by terminal. STXD is in mark state when no characters are transmitted.

NOTE: When S7 is on (closed), switch S6 must be off (open).

19 Secondary request to send
120/SCA S8 To VT102:
When switch S8 is closed, SRTS is on if terminal is on-line and off if terminal is off-line.

NOTE: Switch S5 must be off (open).

20 Data terminal ready
108.2/CD - From VT102:
DTR is on when terminal is on-line and off when terminal is off-line or performing a disconnect. When on, modem can usually answer calls (modem auto answer option). When off, modem disconnects and does not answer calls.
23 Speed select
111/CH S10 From VT102:
SPSD is on when receive speed SET-UP feature selection is greater than 600 baud. SPSD is off when speed is 600 baud or less.