A two-wire (minimum) serial cable connects a DTE device (typically a PC) to a DCE device (typically a modem) using the DB-25 D-subminiature connector defined by TIA/EIA-232-F. Only three signal paths are active — Transmit Data, Receive Data, and Signal Ground — while the hardware handshake lines (RTS, CTS, DSR, DTR, DCD) are looped back locally at each connector end to satisfy UART flow-control requirements. The DB-25 connector was the original RS-232 interface, with signal assignments standardised in successive EIA revisions through TIA/EIA-232-F (1997).
25 PIN D-SUB FEMALE to the Computer
25 PIN D-SUB MALE to the Modem
| Female | Male | Dir | |
|---|---|---|---|
| Shield Ground | 1 | 1 | |
| Transmit Data | 2 | 2 | OUT |
| Receive Data | 3 | 3 | IN |
| System Ground | 7 | 7 | |
| Jumper these: | |||
| Request to Send | 4 | OUT | |
| Clear to Send | 5 | IN | |
| Data Set Ready | 6 | IN | |
| Carrier Detect | 8 | IN | |
| Data Terminal Ready | 20 | OUT | |
| Request to Send | 4 | OUT | |
| Clear to Send | 5 | IN | |
| Data Set Ready | 6 | IN | |
| Carrier Detect | 8 | IN | |
| Data Terminal Ready | 20 | OUT |
References
- Interface Circuits for TIA/EIA-232-F (SLLA037A) — Texas Instruments application note covering TIA-232-F electrical characteristics, DB-25 and DE-9 signal assignments, and DTE/DCE interface circuits.
- Interface Circuits for TIA/EIA-232-F — Internet Archive — archived copy of TI application note SLLA037A.