Table 6-10 Half-Duplex using Coded Control (HDX B) Signals
Pin Name (Mnemonic) CCITT / EIA Switch Description
1 Protective ground
101/AA - Connects to terminal chassis. Also connected to external ground through third wire of power cord.
2 Transmit data
103/BA S6 From VT102:
When switch S6 is on (closed), signals on TXD are serial characters and breaks generated by terminal. TXD is in mark state when no characters are transmitted.

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

3 Received data
104/BB - To VT102:
RXD receives serial characters from computer.
4 Request to send
105/CA S1 From VT102:
When switch S1 is on (closed), RTS is on if terminal is ready to transmit characters and off if terminal is receiving characters.
5 Clear to send
106/CB S2 To VT102:
When switch S2 is on (closed), CTS indicates when modem is ready to transmit data to the computer. 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 S2 is on (closed), switch S3 must be off (open).

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 the 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 receive data from computer. When off, terminal is transmitting data to computer or modem is disconnected from communication line; data received is ignored.
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 line disconnect. When on, modem can usually answer calls (modem auto answer option). When off, modem usually 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.