This adapter will enable you to connect a 25 pin serial cable to a 9 pin connector at the computer.
9 PIN D-SUB FEMALE to the Computer
25 PIN D-SUB MALE to the Serialcable
The DE-9 connector was standardised by TIA-574 (formalised 1990) as a compact alternative to the 25-pin DB-25 specified in TIA-232 (formerly RS-232), with both connectors carrying the same nine active signals. This straight-through adapter maps each signal pin-for-pin between the two connector bodies; no crossing of signal lines takes place. The underlying electrical standard, internationally equivalent to ITU-T V.24 combined with V.28, specifies unbalanced single-ended signalling with MARK (logic 1) at −3 V to −15 V and SPACE (logic 0) at +3 V to +15 V, over a recommended maximum cable length of 15 m (50 ft) or 2500 pF.
| 9-Pin | 25-Pin | |
|---|---|---|
| Carrier Detect | 1 | 8 |
| Receive Data | 2 | 3 |
| Transmit Data | 3 | 2 |
| Data Terminal Ready | 4 | 20 |
| System Ground | 5 | 7 |
| Data Set Ready | 6 | 6 |
| Request to Send | 7 | 4 |
| Clear to Send | 8 | 5 |
| Ring Indicator | 9 | 22 |
Signals
- Carrier Detect (DCD) — indicates that the DCE has detected a carrier signal on the line (ITU-T V.24 circuit 109/CF); DE-9 pin 1, DB-25 pin 8.
- Receive Data (RxD) — serial data received by the DTE from the DCE (V.24 circuit 104/BB); DE-9 pin 2, DB-25 pin 3.
- Transmit Data (TxD) — serial data sent from the DTE to the DCE (V.24 circuit 103/BA); DE-9 pin 3, DB-25 pin 2.
- Data Terminal Ready (DTR) — DTE signals readiness to communicate (V.24 circuit 108/CD); DE-9 pin 4, DB-25 pin 20.
- System Ground (SGND) — signal reference ground common to both connectors (V.24 circuit 102); DE-9 pin 5, DB-25 pin 7.
- Data Set Ready (DSR) — DCE indicates it is ready and connected (V.24 circuit 107/CC); DE-9 pin 6, DB-25 pin 6.
- Request to Send (RTS) — DTE requests permission to transmit (V.24 circuit 105/CA); DE-9 pin 7, DB-25 pin 4.
- Clear to Send (CTS) — DCE grants permission to transmit in response to RTS (V.24 circuit 106/CB); DE-9 pin 8, DB-25 pin 5.
- Ring Indicator (RI) — DCE signals that an incoming call is being received (V.24 circuit 125/CE); DE-9 pin 9, DB-25 pin 22.
Notes
This is a straight-through signal adapter, not a null modem. Each signal passes directly between the two connector bodies with no line crossings. The DE-9 (TIA-574) and DB-25 (TIA-232) use different pin numbers for several signals (notably TxD and RxD swap between pin 2 and pin 3), yet the same signal is carried on both sides of the adapter. The adapter does not alter voltage levels or signal directions. Ring Indicator (DE-9 pin 9, DB-25 pin 22) is not present on all equipment; the remaining eight signals cover the standard DTE-DCE interface.