9 pin D-SUB female connector to the computer
25 pin D-SUB male connector to the Epson printer
This cable connects a PC’s DE-9 (9-pin D-sub) serial port to an Epson serial printer’s DB-25 (25-pin D-sub) female connector. Because both a PC and a serial printer are DTE (Data Terminal Equipment) devices, the cable uses a null-modem (crossover) arrangement rather than a straight-through wiring — Tx and Rx lines are swapped, and the DTR/DSR handshake signals are crossed between the two ends. Epson TM-series POS printers (such as the TM-T88IV) implement hardware flow control using DTR as a printer-status output (busy/paper-out) and DSR as a host-ready input, which this cable wiring accommodates. The printer’s DB-25 connector also carries an RTS output (pin 4) used as a secondary handshake signal, connected here to the computer’s CTS input (DE-9 pin 8).
| computer Name | computer Pin | Direction | Epson printer Pin | Epson printer Name | Description |
|---|---|---|---|---|---|
| Receive Data (RxD) | 2 | ← | 2 | Transmit Data (TxD) | |
| Transmit Data (TxD) | 3 | → | 3 | Receive Data (RxD) | |
| Data Terminal Ready (DTR) | 4 | → | 6 | Data Set Ready (DSR) | |
| System Ground (SG) | 5 | — | 7 | System Ground (SG) | |
| Data Set Ready (DSR) | 6 | ← | 20 | Data Terminal Ready (DTR) | |
| Clear To Send (CTS) | 8 | ← | 4 | Request To Send (RTS) |
Signals
- RxD (DE-9 pin 2) — Receive Data; data input to the computer, connected to the printer’s TxD output (DB-25 pin 2).
- TxD (DE-9 pin 3) — Transmit Data; data output from the computer to the printer’s RxD input (DB-25 pin 3).
- DTR (DE-9 pin 4) — Data Terminal Ready; asserted by the computer to signal readiness, wired to the printer’s DSR input (DB-25 pin 6).
- SG (DE-9 pin 5) — Signal Ground; common reference, wired to DB-25 pin 7.
- DSR (DE-9 pin 6) — Data Set Ready; received by the computer from the printer’s DTR output (DB-25 pin 20); the printer asserts DTR to indicate its status (ready, busy, or paper-out).
- CTS (DE-9 pin 8) — Clear To Send; received by the computer from the printer’s RTS output (DB-25 pin 4); used as a secondary hardware flow-control signal on Epson TM-series printers.
Notes
- The printer-side connector on Epson TM-series POS printers (e.g. TM-T88IV) is a DB-25 female.
- Signal levels comply with TIA-232: MARK = −3 V to −15 V (logic 1); SPACE = +3 V to +15 V (logic 0).
- Baud rate is selectable via DIP switch on the printer: 4800, 9600, 19200, 38400, 57600, or 115200 bps.
- Data format is configurable: 7 or 8 data bits; even or odd parity; asynchronous framing.
- Flow control can be switched between DTR/DSR hardware handshake and XON/XOFF software handshake via the printer’s DIP switch settings.
- DB-25 pin 25 (INIT — printer reset, controlled by DIP switch SW2-8) is not wired in this cable.
- This wiring differs from the generic 9-to-25 serial printer cable: it maps DTR (DE-9 pin 4) to DSR (DB-25 pin 6) and CTS (DE-9 pin 8) to RTS (DB-25 pin 4), rather than looping DE-9 pins 1 and 4 at the computer end.