9 PIN D-SUB FEMALE to Computer 1
25 PIN D-SUB FEMALE to Computer 2
A null modem cable connects two DTE devices (such as two computers) directly, without an intervening modem, by crossing the transmit and receive lines and satisfying hardware handshake signals at each end. RS-232 serial interfaces are defined by TIA-232-F (internationally equivalent to ITU-T V.24 and V.28); the DE-9 nine-pin variant is standardised by TIA-574, whilst the original DB-25 form follows TIA-232. Null modem wiring is not itself defined by TIA-232 and several incompatible variants exist; the full-handshake arrangement shown here crosses RTS/CTS and connects DTR to both DSR and DCD at the far end, satisfying software that monitors modem-status lines.
| D-Sub 9 | D-Sub 25 |
|---|---|
| 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.
- The DE-9 (TIA-574) pin assignments are: pin 1 = DCD, pin 2 = RxD, pin 3 = TxD, pin 4 = DTR, pin 5 = SGND, pin 6 = DSR, pin 7 = RTS, pin 8 = CTS, pin 9 = RI.
- The DB-25 (TIA-232) pin assignments are: pin 2 = TxD, pin 3 = RxD, pin 4 = RTS, pin 5 = CTS, pin 6 = DSR, pin 7 = SGND, pin 8 = DCD, pin 20 = DTR.
- DE-9 pin 2 (RxD) connects to DB-25 pin 2 (TxD), and DE-9 pin 3 (TxD) connects to DB-25 pin 3 (RxD). These same-numbered pins carry opposite signals because TIA-574 and TIA-232 use opposite assignments for TxD and RxD.
- DE-9 pin 4 (DTR) connects to DB-25 pins 6 (DSR) and 8 (DCD), and conversely DE-9 pins 6 (DSR) and 1 (DCD) receive DB-25 pin 20 (DTR).
- DE-9 pin 7 (RTS) connects to DB-25 pin 5 (CTS); DE-9 pin 8 (CTS) connects to DB-25 pin 4 (RTS).
- Maximum recommended cable length is 15 m (50 ft) or 2 500 pF total capacitance at the rated data rate.
Signals
- RxD (Receive Data) — serial data received by the DE-9 DTE from the remote device; V.24 circuit 104 (BB).
- TxD (Transmit Data) — serial data transmitted by the DE-9 DTE to the remote device; V.24 circuit 103 (BA).
- DTR (Data Terminal Ready) — asserted by the DTE to indicate it is ready to communicate; V.24 circuit 108 (CD). In this cable, DTR drives DSR and DCD at the far end.
- SGND (Signal Ground) — common reference for all signal voltages; V.24 circuit 102.
- DSR (Data Set Ready) — indicates the remote DTE is ready; driven by the far end’s DTR; V.24 circuit 107 (CC).
- DCD (Data Carrier Detect) — indicates a carrier is present; driven by the far end’s DTR in this configuration; V.24 circuit 109 (CF).
- RTS (Request to Send) — asserted by the DTE to request permission to transmit; V.24 circuit 105 (CA). Connects to CTS at the far end.
- CTS (Clear to Send) — grants permission to transmit; driven by the far end’s RTS; V.24 circuit 106 (CB).