A null modem cable connects two DTE (Data Terminal Equipment) devices — typically two computers — directly together by crossing the transmit and receive lines, eliminating the need for an intermediate modem. The RS-232 standard (TIA-232-F, internationally equivalent to ITU-T V.24 + V.28) defines the signal circuits used, but null modem wiring itself is not specified by that standard; several incompatible variants exist in practice. This full-handshake 9-pin variant crosses RTS and CTS between the two connectors and ties DTR to both DSR and Carrier Detect at the far end, satisfying software that monitors modem-status lines. The DE-9 (9-pin D-sub) pinout for PC serial ports is defined by TIA-574.
9 PIN D-SUB FEMALE to Computer 1
9 PIN D-SUB FEMALE to Computer 2
| D-Sub 1 | D-Sub 2 |
|---|---|
| Receive Data | 2 |
| Transmit Data | 3 |
| Data Terminal Ready | 4 |
| System Ground | 5 |
| Data Set Ready + Carrier Detect | 6+1 |
| Request to Send | 7 |
| Clear to Send | 8 |
Notes
- DSR & CD are jumpered to fool the programs to think that they are online.
- This is a full-handshake null modem: TxD and RxD are crossed end-to-end, RTS and CTS are crossed end-to-end, and DTR at each end drives both DSR and DCD (Carrier Detect) at the far end.
- The minimum (no-handshake) null modem uses only three wires: TxD↔RxD crossed and Signal Ground straight (DE-9 pins 2↔3 and 5↔5). Software must not check hardware handshake lines with that variant.
- Both connectors are DE-9 female (as fitted on a PC) so the cable must use male plugs at both ends.
- Signal levels follow ITU-T V.28: SPACE (logic 0) = +3 V to +15 V; MARK (logic 1) = −3 V to −15 V at the driver output.
Signals
- RxD (Receive Data) — serial data received by the DTE; DE-9 pin 2. Crossed to TxD (pin 3) of the far connector.
- TxD (Transmit Data) — serial data transmitted by the DTE; DE-9 pin 3. Crossed to RxD (pin 2) of the far connector.
- DTR (Data Terminal Ready) — DE-9 pin 4. Indicates the DTE is powered and ready; connected to both DSR and DCD (pins 6 and 1) at the far end.
- SGND (Signal Ground) — DE-9 pin 5. Common reference for all signal voltages; connected straight through.
- DSR + DCD (Data Set Ready + Carrier Detect) — DE-9 pins 6 and 1 tied together. Driven by DTR (pin 4) from the far connector, so each computer’s modem-status inputs are asserted when the remote computer is ready.
- RTS (Request to Send) — DE-9 pin 7. Crossed to CTS (pin 8) of the far connector to provide end-to-end hardware flow control.
- CTS (Clear to Send) — DE-9 pin 8. Driven by RTS (pin 7) from the far connector.
References
- ITU-T Recommendation V.24 (02/2000): List of definitions for interchange circuits between DTE and DCE — International Telecommunication Union
- ITU-T Recommendation V.28 (03/1993): Electrical characteristics for unbalanced double-current interchange circuits — International Telecommunication Union