9 pin D-SUB female connector to the computer
25 pin D-SUB female connector to the printer
This cable connects a PC’s DE-9 serial port to a DB-25 serial printer using the RS-232 interface standardised as TIA-232-F. Because both a PC and a serial printer are DTE (Data Terminal Equipment) devices, a null-modem crossover cable is required rather than a straight-through cable. Data flow is asymmetric — the computer sends print data to the printer, whilst the printer returns only flow-control handshaking signals. Hardware handshaking is implemented by looping DE-9 pins 1 and 4 together at the computer side, and tying DE-9 pins 6 and 8 to DB-25 pin 20 (printer DTR) to allow the printer to signal busy or paper-out conditions.
| 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 |
Notes
- The DE-9 connector on the computer side is male (as found on PC serial ports standardised under TIA-574); the DB-25 connector on the printer side is female.
- DE-9 pins 1 (DCD) and 4 (DTR) are looped together at the computer end; this is a local handshake stub that keeps the PC’s serial port satisfied without a true modem signal.
- DE-9 pins 6 (DSR) and 8 (CTS) are connected together and wired to DB-25 pin 20 (printer DTR), so the printer can assert its DTR to pause the host via both DSR and CTS simultaneously.
- XON/XOFF software flow control is an alternative to this hardware handshaking scheme and requires only pins 2, 3, and 5 (GND) to be wired.
- Signal voltage levels follow TIA-232: logic 0 (SPACE) = +3 V to +15 V; logic 1 (MARK) = −3 V to −15 V.
Signals
- Receive Data (DE-9 pin 2 ↔ DB-25 pin 3) — carries data from the printer’s transmit output to the computer’s receive input; the crossover here is the defining characteristic of a null-modem cable.
- Transmit Data (DE-9 pin 3 ↔ DB-25 pin 2) — carries print data from the computer’s transmit output to the printer’s receive input.
- Clear To Send + Data Set Ready (DE-9 pins 8+6 ↔ DB-25 pin 20) — CTS and DSR at the computer are tied together and driven by the printer’s DTR output, enabling the printer to signal a busy or paper-out condition to the host.
- Carrier Detect + Data Terminal Ready (DE-9 pins 1+4 looped) — DCD and DTR at the computer side are looped locally; this handshake stub keeps the port active without requiring an external modem signal.
- Ground (DE-9 pin 5 ↔ DB-25 pin 7) — signal ground reference common to both ends.