A DB-25 to DB-25 serial printer cable connects a computer to a serial printer, both of which are DTE (Data Terminal Equipment) devices. Because both ends are DTE, a null-modem (crossover) arrangement is required rather than a straight-through cable, swapping the Transmit Data and Receive Data lines between the two connectors. Hardware flow control is achieved by looping the handshake signals locally at each end: CTS and DSR are tied to DTR at the computer side, whilst Carrier Detect is looped with RTS at the printer side, allowing the printer to signal busy or paper-out conditions without requiring a full modem handshake. The cable carries RS-232 signals as defined by TIA-232-F (formerly EIA-232), with signal levels of +3 V to +15 V (logic 0) and −3 V to −15 V (logic 1).
25 pin D-SUB female connector to the computer
25 pin D-SUB female connector to the printer
| D-Sub 1 | D-Sub 2 |
|---|---|
| Receive Data | 2 |
| Transmit Data | 3 |
| Clear To Send + Data Set Ready | 5 + 6 |
| Carrier Detect + Data Terminal Ready | 8 + 20 |
| Ground | 7 |
Notes
Both connectors are DB-25 female. Pin 2 on each side carries Transmit Data from that device’s perspective, so the cable crosses pins 2 and 3 to connect each transmitter to the opposite receiver. Signal ground (pin 7) is connected straight through. The loopback connections (CTS+DSR tied to DTR at the computer; CD tied to RTS at the printer) satisfy hardware handshake requirements without additional control wiring. XON/XOFF software flow control may be used as an alternative if both the host and printer support it.
Signals
- Receive Data (pin 2) — data received by this device from the far end; connected to the opposite end’s Transmit Data line
- Transmit Data (pin 3) — data sent by this device to the far end; connected to the opposite end’s Receive Data line
- Clear To Send + Data Set Ready (pins 5 + 6) — CTS and DSR looped together at the computer side and driven by the printer’s DTR (pin 20), signalling printer ready/busy status
- Carrier Detect + Data Terminal Ready (pins 8 + 20) — CD and DTR looped together at the printer side; DTR (pin 20) is the printer’s primary flow-control output
- Ground (pin 7) — signal ground reference common to both ends