This null-modem style cable connects a DE-9 male RS-232 port on a host computer to the DB-25 female serial port found on Epson thermal and impact receipt printers such as the TM-T88 series. Because both a PC and a serial printer are DTE (Data Terminal Equipment) devices, a crossover (null-modem) wiring arrangement is required rather than a straight-through cable. The interface conforms to TIA-232-F (formerly EIA RS-232), using standard asymmetric RS-232 voltage levels (MARK = −3 V to −15 V; SPACE = +3 V to +15 V). Hardware flow control is handled via DTR/DSR handshaking, with loopback stubs at each end satisfying unused modem-control lines.
9 pin D-SUB male connector to the Computer
25 pin D-SUB female connector to the Printer
| Computer Name | Computer Pin | Direction | Printer Pin | Printer Name | Description |
|---|---|---|---|---|---|
| RXD | 2 | ← | 2 | ||
| TXD | 3 | → | 3 | ||
| DTR | 4 | 6 | |||
| Ground | 5 | ↔ | 7 | ||
| DSR | 6 | 20 | |||
| LOOP | 7,8 | — | |||
| — | 4,5 | LOOP |
Notes
The DE-9 connector on the computer side is male (pins); the DB-25 connector on the Epson printer side is female (sockets), as specified for the TM-T88 series. Data flow is asymmetric: the host computer transmits print data to the printer; the printer only returns flow-control handshake signals. The loopback at DE-9 pins 7 and 8 (RTS shorted to CTS) satisfies the computer’s own CTS requirement without requiring a signal from the printer. The loopback at DB-25 pins 4 and 5 (RTS shorted to CTS on the printer side) similarly satisfies the printer’s CTS input. Baud rate, data bits, parity, and flow-control mode are selectable on the printer via DIP switches (4800–115200 bps on TM-T88IV); the cable wiring is independent of the chosen baud rate.
Signals
- RXD — Receive Data (computer DE-9 pin 2): data received by the computer from the printer; wired to DB-25 pin 2 (TxD output of the Epson printer).
- TXD — Transmit Data (computer DE-9 pin 3): data sent from the computer to the printer; wired to DB-25 pin 3 (RxD input of the Epson printer).
- DTR — Data Terminal Ready (computer DE-9 pin 4): asserted by the computer to signal readiness; wired to DB-25 pin 6 (DSR input of the Epson printer, indicating host-ready status).
- Ground — Signal Ground (computer DE-9 pin 5): wired to DB-25 pin 7 (Signal Ground), providing the common voltage reference for all signals.
- DSR — Data Set Ready (computer DE-9 pin 6): read by the computer to determine printer status; wired to DB-25 pin 20 (DTR output of the Epson printer, which the printer asserts to signal busy or paper-out conditions).
- LOOP (DE-9 7,8) — RTS (pin 7) shorted to CTS (pin 8) at the computer end; allows the host’s CTS input to be satisfied locally without a printer-side signal.
- LOOP (DB-25 4,5) — RTS (pin 4) shorted to CTS (pin 5) at the printer end; satisfies the printer’s CTS input locally.