Installing and Administering PPP

Appendix A 145
Modem Connections
RS-232 Interface
RS-232 Interface
The types of signals carried by the RS-232 interface are listed below:
Signal Description
Signal Ground (SG) Common electrical ground path for all interface
circuits.
Transmit Data (TD) Command codes or data sent from DTE to DCE;
won't be sent unless RTS, CTS, DSR and DTR
are set to on.
Receive Data (RD) DCE responses to DTE commands or data
received from a remote DCE.
Request To Send
(RTS)
Set to on, the DTE tells the DCE to remain in
transmit mode; set to off, the DTE tells the
DCE to receive.
Clear To Send
(CTS)
Set to on, the DCE reports it is ready to receive;
set to off, the DCE tells the DTE not to
transmit.
Data Set Ready
(DSR)
Set to on, the DCE reports it is ready to operate.
Data Terminal
Ready (DTR)
Set to on, the DTE tells the DCE to connect to a
communications channel (required to be on for
automatic answering by DCE). Set to off, the
DTE tells the DCE to break the connection
when it has finished the current transmission.
Ring Indicator (RI) Set to on, the DCE reports it detects the
presence of a ringing signal on the
communications channel. Set to off, the DCE
indicates the absence of a ringing signal.
Carrier Detect (CD) Set to on, the DCE reports receiving a signal
from the communications channel that a
connection can be established.