A null modem cable connects two DTE devices (such as two computers) directly over RS-232 serial without an intervening modem. The DB-25 connector layout is defined by TIA-232 (originally EIA-232, internationally equivalent to ITU-T V.24 and V.28), which assigns TxD to pin 2, RxD to pin 3, and DTR to pin 20. Null modem wiring is not itself defined by TIA-232; the standard only covers DTE-to-DCE interfaces. This full-handshake variant crosses TxD and RxD, crosses RTS and CTS, and ties DTR to DSR plus Carrier Detect at the far end, satisfying software that monitors modem status lines.
25 PIN D-SUB FEMALE to Computer 1
25 PIN D-SUB FEMALE to Computer 2
| D-Sub 1 | D-Sub 2 |
|---|---|
| Receive Data | 3 |
| Transmit Data | 2 |
| Data Terminal Ready | 20 |
| System Ground | 7 |
| Data Set Ready + Carrier Detect | 6+8 |
| Request to Send | 4 |
| Clear to Send | 5 |
Signals
- TxD (Transmit Data, pin 2) — data transmitted from DTE to the far end; crossed to RxD (pin 3) at the remote connector.
- RxD (Receive Data, pin 3) — data received by the DTE; fed from TxD (pin 2) of the remote connector.
- RTS (Request to Send, pin 4) — asserted by a DTE when it is ready to transmit; crossed to CTS (pin 5) at the far end.
- CTS (Clear to Send, pin 5) — controlled by the remote DTE’s RTS; crossed to RTS (pin 4) at the remote connector.
- DSR (Data Set Ready, pin 6) — in this wiring, driven by the remote DTR (pin 20); indicates the far DTE is ready.
- SGND (Signal Ground, pin 7) — common signal reference; connected straight through.
- DCD (Carrier Detect, pin 8) — tied to DSR (pin 6) and fed from the remote DTR (pin 20); allows software that polls DCD to proceed.
- DTR (Data Terminal Ready, pin 20) — asserted by a DTE when it is ready; drives DSR and DCD (pins 6 and 8) at the remote connector.
Notes
- DSR and DCD are jumpered to the remote DTR to satisfy software that expects modem status lines to be asserted before communicating.
- RTS and CTS are crossed end-to-end, enabling hardware flow control across the link.
- Signal Ground (pin 7) is connected straight through; Protective Ground (pin 1) is not required for this cable.
- Ring Indicator (pin 22) is not connected in this variant.
- Maximum recommended cable length is 15 m (50 ft) or 2500 pF total capacitance at rated speed per ITU-T V.28.