A null modem adapter allows two DTE (Data Terminal Equipment) devices to communicate directly by crossing the transmit and receive lines, eliminating the need for an intervening modem. The RS-232 standard (TIA-232-F, October 1997 — internationally equivalent to ITU-T V.24 and V.28) defines DTE-to-DCE interfaces only; null modem wiring is not part of the standard, and multiple incompatible variants exist in practice. This DB-25 female-to-male adapter presents a female connector to the computer and a male connector to the serial cable, with the internal crossings achieved mechanically between the two connector bodies, allowing an ordinary straight-through serial cable to be used as a null modem link.
25 PIN D-SUB FEMALE to the Computer
25 PIN D-SUB MALE to the Serialcable.
| Female | Male |
|---|---|
| Shield Ground | 1 |
| Transmit Data | 2 |
| Receive Data | 3 |
| Request to Send | 4 |
| Clear to Send | 5 |
| Data Set Ready | 6 |
| Data Terminal Ready | 20 |
| Ground | 7 |
Signals
- Shield Ground (pin 1) — Protective/chassis ground; connected to connector shell.
- Transmit Data / TxD (pin 2) — Serial data output from DTE to DCE (ITU-T V.24 circuit 103/BA). Internally crossed to Receive Data on the opposite connector body.
- Receive Data / RxD (pin 3) — Serial data input to DTE from DCE (ITU-T V.24 circuit 104/BB). Internally crossed to Transmit Data on the opposite connector body.
- Request to Send / RTS (pin 4) — DTE requests permission to transmit (ITU-T V.24 circuit 105/CA). Crossed internally to CTS.
- Clear to Send / CTS (pin 5) — DCE grants permission to transmit (ITU-T V.24 circuit 106/CB). Crossed internally to RTS.
- Data Set Ready / DSR (pin 6) — Indicates the DCE (or far-end DTE) is ready (ITU-T V.24 circuit 107/CC). Crossed internally to DTR, and may be tied to DCD (pin 8) to satisfy software that monitors the received-line-signal detector.
- Data Terminal Ready / DTR (pin 20) — Indicates the DTE is ready (ITU-T V.24 circuit 108/CD). Crossed internally to DSR and typically also to DCD on the opposite side.
- Ground / SGND (pin 7) — Signal ground reference for all interchange circuits (ITU-T V.24 circuit 102).
Notes
- The table above shows the signal-name-to-pin-number passthrough mapping of each connector face. The actual null modem crossings (TxD↔RxD, RTS↔CTS, DTR→DSR+DCD) are achieved internally between the two connector bodies of the adapter, not by altering the cable.
- Using a straight-through DB-25 serial cable with this adapter is electrically equivalent to a full-handshake null modem cable with those crossings wired directly.
- Null modem wiring is not defined by TIA-232-F; several incompatible variants exist. Software that monitors DCD or DSR requires DTR to be connected to both DSR and DCD on the opposite side. Software that does not use hardware handshaking may work with a simpler three-wire (TxD, RxD, GND only) arrangement.
- Maximum recommended cable length for RS-232 at rated speed is 15 m (50 ft) or 2500 pF total cable capacitance.