A two-wire (minimum) serial cable connects a DTE device such as a computer to a DCE device such as a modem using a DB-25 connector at each end, as defined by TIA-232-F (internationally equivalent to ITU-T V.24 and V.28). Only the three essential circuits — Transmit Data (circuit 103), Receive Data (circuit 104), and Signal Ground — are wired end-to-end; all hardware-handshake lines are looped locally at each connector to satisfy software that asserts or polls those signals. This configuration is appropriate when neither the DTE nor the DCE requires RTS/CTS or DTR/DSR flow control across the link.
25 pin D-SUB female connector to the Computer
25 pin D-SUB male connector to the Modem
| Female | Male | Dir |
|---|---|---|
| Shield Ground | 1 | 1 |
| Transmit Data | 2 | 2 |
| Receive Data | 3 | 3 |
| System Ground | 7 | 7 |
| Jumper these: | ||
| Request to Send | 4 | |
| Clear to Send | 5 | |
| Data Set Ready | 6 | |
| Carrier Detect | 8 | |
| Data Terminal Ready | 20 | |
| Request to Send | 4 | |
| Clear to Send | 5 | |
| Data Set Ready | 6 | |
| Carrier Detect | 8 | |
| Data Terminal Ready | 20 |
Notes
The cable carries only three end-to-end connections: pins 1 (Shield Ground), 2 (TxD), 3 (RxD), and 7 (Signal Ground). Note that pin 1 (Shield/Protective Ground) is distinct from pin 7 (Signal Ground) and is typically connected to the cable screen rather than the signal return.
At the female (DTE/computer) end, RTS (pin 4) is jumpered to CTS (pin 5), and DSR (pin 6) is jumpered to DCD (pin 8) and DTR (pin 20), so the computer sees its own handshake signals reflected back. The same local loopbacks are applied at the male (DCE/modem) end. Software that monitors DCD or DSR will see those lines asserted by the loopback rather than by the remote device.
The maximum recommended cable length per TIA-232-F and ITU-T V.28 is 15 m (50 ft) or 2500 pF total capacitance at the rated signalling rate. The electrical interface uses unbalanced single-ended signalling: a SPACE (logic 0) is +3 V to +15 V at the driver; a MARK (logic 1) is −3 V to −15 V.
Signals
- Shield Ground (pin 1) — Protective/chassis ground; connected to the cable screen. Distinct from Signal Ground.
- Transmit Data / TxD (pin 2) — Serial data output from DTE to DCE (ITU-T V.24 circuit 103).
- Receive Data / RxD (pin 3) — Serial data input from DCE to DTE (ITU-T V.24 circuit 104).
- Request to Send / RTS (pin 4) — DTE requests permission to transmit (V.24 circuit 105). Looped locally to CTS at each end; not carried across the link.
- Clear to Send / CTS (pin 5) — DCE grants transmit permission (V.24 circuit 106). Looped locally to RTS at each end.
- Data Set Ready / DSR (pin 6) — DCE ready indication (V.24 circuit 107). Looped locally with DCD and DTR at each end.
- Signal Ground (pin 7) — Common signal return for all interchange circuits (V.24 circuit 102).
- Carrier Detect / DCD (pin 8) — Received line signal detector (V.24 circuit 109). Looped locally with DSR and DTR at each end.
- Data Terminal Ready / DTR (pin 20) — DTE ready indication (V.24 circuit 108). Looped locally with DSR and DCD at each end.