9 PIN D-SUB FEMALE to Computer
25 PIN D-SUB FEMALE to Printer
A serial printer cable adapts the DE-9 (9-pin) serial port found on IBM PC/AT-compatible computers to the DB-25 (25-pin) serial interface fitted to many printers. Both connectors follow TIA/EIA-232-F, which standardised the DE-9 DTE pin assignments (EIA/TIA-574) alongside the original DB-25 layout. Data lines are crossed so that the computer’s transmit pin connects to the printer’s receive pin and vice versa; handshake signals are similarly paired to satisfy flow-control requirements at each end.
| D-Sub 1 | D-Sub 2 |
|---|---|
| Receive Data | 3 |
| Transmit Data | 2 |
| Clear To Send + Data Set Ready | 8 + 6 |
| Carrier Detect + Data Terminal Ready | 1 + 4 |
| Ground | 5 |
Signals
- TxD — Transmit Data; driven by the DTE (computer), received by the printer.
- RxD — Receive Data; driven by the printer (or looped), received by the computer.
- RTS — Request To Send; asserted by the computer to indicate readiness to transmit.
- CTS — Clear To Send; asserted by the printer (or looped from RTS) to permit transmission.
- DSR — Data Set Ready; asserted by the printer to signal that it is ready.
- DTR — Data Terminal Ready; asserted by the computer to signal that it is active.
- DCD — Carrier Detect; on a printer cable this pin is typically tied to DTR at the printer end to satisfy the UART.
- GND — Signal Ground; common reference for all logic levels.
Notes
The DE-9 connector on the computer side is a male plug (DTE); the DB-25 connector on the printer side is typically a female socket. RS-232 voltage levels are: Mark (logic 1) = −3 V to −15 V; Space (logic 0) = +3 V to +15 V. Cable capacitance must not exceed 2 500 pF per TIA/EIA-232-F. Where a printer does not implement full handshaking, CTS and DSR are commonly looped back at the printer end (shorted together with DCD and DTR) so that the computer’s UART does not stall waiting for flow-control assertions.