This straight-through cable connects a DTE (computer) DE-9 serial port to a DCE (modem) DB-25 port under the TIA/EIA-232-F and EIA/TIA-574 standards. The DE-9 connector was standardised by EIA/TIA-574 and became the dominant PC serial port with the IBM PC/AT in 1984, replacing the earlier DB-25 on host machines while modems retained the 25-pin interface. All nine signals — including the full hardware-handshake lines RTS, CTS, DSR, DTR, DCD, and RI — are carried straight-through with pin-number remapping between the two connector sizes.
9 PIN D-SUB FEMALE to the Computer
25 PIN D-SUB MALE to the Modem
| Female | Male | Dir | |
|---|---|---|---|
| Shield | 1 | - | |
| Transmit Data | 3 | 2 | OUT |
| Receive Data | 2 | 3 | IN |
| Request to Send | 7 | 4 | OUT |
| Clear to Send | 8 | 5 | IN |
| Data Set Ready | 6 | 6 | IN |
| System Ground | 5 | 7 | - |
| Carrier Detect | 1 | 8 | IN |
| Data Terminal Ready | 4 | 20 | OUT |
| Ring Indicator | 9 | 22 | IN |
Notes
The DE-9 end is female (fits the male port on the PC); the DB-25 end is male (fits the female DCE socket on the modem). All signals are wired straight-through — each logical signal maps directly from its DE-9 pin number to the corresponding DB-25 pin number per the standard assignment tables. RS-232 logic levels are Mark (logic 1) = −3 V to −15 V and Space (logic 0) = +3 V to +15 V; maximum cable capacitance is 2500 pF per TIA/EIA-232-F.
Signals
- Shield — Protective (chassis) ground on DB-25 pin 1; no corresponding pin on the DE-9.
- TxD — Transmit Data; serial data sent from DTE to modem.
- RxD — Receive Data; serial data received by the DTE from the modem.
- RTS — Request to Send; DTE asserts to signal readiness to transmit.
- CTS — Clear to Send; modem asserts to grant DTE permission to transmit.
- DSR — Data Set Ready; modem asserts to indicate it is powered and ready.
- GND — Signal Ground; common reference for all signal voltages.
- DCD — Carrier Detect; modem asserts when a carrier signal is detected on the line.
- DTR — Data Terminal Ready; DTE asserts to indicate it is ready and to keep the modem off-hook.
- RI — Ring Indicator; modem asserts when an incoming ring signal is detected.
References
- Interface Circuits for TIA/EIA-232-F (SLLA037A) — Texas Instruments, September 2002
- Interface Circuits for TIA/EIA-232-F (Rev. A) — SLLA037A — Texas Instruments, 2002