Appendix B Null Modem Cable Requirements 31
RS-449 Null Modem Cable
TABLEB-1 and TABLE B-2 list the signals and names for RS-499 and X.21 circuits
TABLEB-1 RS-449 Signals
Circuit Name Direction
TxD TransmitData ToDCE
RxD ReceiveData FromDCE
TxC TransmitClock ToDCE
TxCI TransmitClock In FromDCE
RxC Receive Clock From DCE
RTS Request to Send ToDCE
CTS Clear to Send From DCE
DCD Data Carrier Detect FromDCE
DTR Data Terminal Ready To DCE
DSR Data Set Ready FromDCE
SG Signal Ground
TABLEB-2 X.21 Signals
Circuit Name Direction
G Signal Ground
T Transmit ToDCE
R Receive FromDCE
C Control ToDCE
I Indication FromDCE
S Signal Element Timing From DCE
B Byte Timing FromDCE