7 External Connections
RS-232-C devices are classified by their function as either Data Terminal Equipment (DTE) or Data Communication Equipment (DCE). Generally, data terminal equipment is defined as the communication source and data communication equipment is defined as the device that provides a communication channel between two DTE-type devices.
Modem
DCE
Telephone
line
Modem
DCE
Figure 16 --- Use of DTEs and DCEs in a communications link
DTE- and DCE-type devices have complementary pinouts to allow
terminals and modems to be connected directly using a one-to-one cable as shown in Figure 16. In many applications, DCEs are unnecessary, and in these cases a cable called a "null modem cable" or "modem eliminator cable" is used to directly connect two DTE-type devices. A typical null modem cable is also shown in Figure 17.
(3) | RxD | | | | | | TxD | (3) | (3) | RxD | | RxD | (3) |
| |
(2) | TxD | | | | | | RxD | (2) | (2) | TxD | | TxD | (2) |
| |
(4) | RTS | | | | | | CTS | (4) | (4) | RTS | | RTS | (4) |
| | |
(5) | CTS | | | | | | RTS | (5) | (5) | CTS | | CTS | (5) |
| | |
(20) | DTR | | | | | | DSR (20) | (20) | DTR | | DTR (20) |
| | | |
(6) | DSR | | | | | | DTR | (6) | (6) | DSR | | DSR (6) |
| | | |
(8) | DCD | | | | | | DCD (8) | (8) | DCD | | DCD (8) |
| | | | | |
(22) | RI | | | | | | RI | (22) | (22) | RI | | RI | (22) |
| | | | | |
(7) | GND | | | | | | GND (7) | (7) | GND | | GND (7) |
| | | | | |
| | | | |
| Typical DTE-to-DCE cable | | Typical DTE-to-DTE null modem cable |
Figure 17 --- Cabling requirements for RS-232-C devices
(cables using 25-pin connectors shown)
32 | DSCLP/SSCLP-100 User's Manual |
