A DB-25 to DB-25 straight-through cable connects a DTE device (computer or terminal) to a DCE device (modem) using the full set of RS-232 handshake signals. The governing standard is TIA/EIA-232-F, formally titled Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange (1997 revision), which superseded the earlier EIA RS-232-C (1969) and RS-232-E (1991) revisions. The 25-pin D-subminiature (DB-25) connector was the original RS-232 physical interface; all ten signals are connected straight-through, pin-for-pin, between the female DTE connector and the male DCE connector. RS-232 voltage levels are Mark (logic 1) = −3 V to −15 V and Space (logic 0) = +3 V to +15 V, with a maximum cable capacitance of 2500 pF.
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 |
| Request to Send | 4 | 4 | OUT |
| Clear to Send | 5 | 5 | IN |
| Data Set Ready | 6 | 6 | IN |
| System Ground | 7 | 7 | - |
| Carrier Detect | 8 | 8 | IN |
| Data Terminal Ready | 20 | 20 | OUT |
| Ring Indicator | 22 | 22 | IN |
Notes
The female DB-25 connector attaches to the DTE (computer) port; the male DB-25 connector attaches to the DCE (modem) port, as specified by TIA/EIA-232-F. All pins are wired straight-through — no crossovers are required on a DB-25 to DB-25 modem cable. Pin 1 carries Protective Ground (chassis/shield), which is distinct from the Signal Ground on pin 7. Pins not listed (9–19, 21, 23–25) are not used by the standard modem interface.
Signals
- Shield Ground (Pin 1) — Protective/chassis ground; connects the cable shield to the equipment frame.
- Transmit Data / TxD (Pin 2) — Serial data transmitted from DTE to DCE.
- Receive Data / RxD (Pin 3) — Serial data received by DTE from DCE.
- Request to Send / RTS (Pin 4) — DTE signals readiness to transmit; used for hardware flow control.
- Clear to Send / CTS (Pin 5) — DCE grants permission for DTE to transmit; hardware flow control response to RTS.
- Data Set Ready / DSR (Pin 6) — DCE indicates it is connected to the telephone line and ready.
- System Ground / SIG GND (Pin 7) — Signal reference ground common to all data and control lines.
- Carrier Detect / DCD (Pin 8) — DCE indicates a valid carrier signal has been detected on the telephone line.
- Data Terminal Ready / DTR (Pin 20) — DTE signals that it is powered on and ready to communicate.
- Ring Indicator / RI (Pin 22) — DCE indicates an incoming ring signal on the telephone line.
References
- Interface Circuits for TIA/EIA-232-F (SLLA037A) — Texas Instruments application note covering TIA/EIA-232-F electrical characteristics, DB-25 and DE-9 connector signal assignments, and DTE/DCE interface design.
- Interface Circuits for TIA/EIA-232-F (SLLA037A) — Internet Archive — archived copy of the Texas Instruments SLLA037A application note.