This cable connects a DTE (such as a PC with a DE-9 serial port, per TIA-574) to a DCE (such as a modem with a DB-25 port, per TIA/EIA-232-F, the international equivalent of ITU-T V.24 + V.28) when hardware handshaking is not required. Only three signals travel end-to-end: Transmit Data, Receive Data, and Signal Ground. On each connector the remaining handshake lines — RTS, CTS, DSR, DCD, and DTR — are looped back locally so that software requiring active modem-status lines does not stall waiting for them.
9 pin D-SUB female connector to the Computer
25 pin D-SUB male connector to the Modem
| Female | Male | Dir |
|---|---|---|
| Shield Ground | 1 | |
| Transmit Data | 3 | 2 |
| Receive Data | 2 | 3 |
| System Ground | 5 | 7 |
| Jumper these: | ||
| Request to Send | 7 | |
| Clear to Send | 8 | |
| Data Set Ready | 6 | |
| Carrier Detect | 1 | |
| Data Terminal Ready | 4 | |
| Request to Send | 4 | |
| Clear to Send | 5 | |
| Data Set Ready | 6 | |
| Carrier Detect | 8 | |
| Data Terminal Ready | 20 |
Notes
The DE-9 end uses a female connector as found on a PC serial port (TIA-574); the DB-25 end uses a male connector as found on most external modems (TIA/EIA-232-F). On the DE-9 side, RTS (pin 7) and CTS (pin 8) are jumpered together, and DSR (pin 6), DCD (pin 1), and DTR (pin 4) are jumpered together, providing local loopback for handshake signals. On the DB-25 side, RTS (pin 4) and CTS (pin 5) are jumpered together, and DSR (pin 6), DCD (pin 8), and DTR (pin 20) are jumpered together. The cable carries only TxD, RxD, and Signal Ground across its length. The maximum recommended cable length under TIA/EIA-232-F is 15 m (50 ft) or 2500 pF total capacitance at rated speed.
Signals
- TxD (Transmit Data) — serial data transmitted from DTE to DCE; DE-9 pin 3 to DB-25 pin 2 (ITU-T V.24 circuit 103)
- RxD (Receive Data) — serial data received by DTE from DCE; DE-9 pin 2 to DB-25 pin 3 (ITU-T V.24 circuit 104)
- RTS (Request to Send) — DTE signals readiness to transmit; looped locally to CTS on each connector (ITU-T V.24 circuit 105)
- CTS (Clear to Send) — DCE permission to transmit; looped locally to RTS on each connector (ITU-T V.24 circuit 106)
- DSR (Data Set Ready) — DCE ready indication; looped locally with DTR and DCD on each connector (ITU-T V.24 circuit 107)
- DTR (Data Terminal Ready) — DTE ready indication; looped locally with DSR and DCD on each connector (ITU-T V.24 circuit 108)
- DCD (Carrier Detect) — received line signal detector; looped locally with DTR and DSR on each connector (ITU-T V.24 circuit 109)
- GND (Signal Ground) — common signal reference; DE-9 pin 5 to DB-25 pin 7