9 PIN D-SUB MALE at the Computer.
The HP 48 scientific calculator (introduced 1990) and the HP 95LX palmtop computer (1991) both use a proprietary 4-pin connector with 2 mm pin spacing for serial communication. The interface carries asynchronous RS-232-compatible signals at 1200–9600 baud using only transmit, receive, signal ground, and chassis shield — there are no hardware handshaking lines. Because the HP 48’s TX output swings only ±3.0–3.5 V rather than the ±5 V minimum required by TIA-232-F, a level-converter cable may be needed for reliable connection to fully compliant RS-232 hosts.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | SHIELD | - | Chassis/Shield Ground |
| 2 | TXD | OUT | Transmit Data |
| 3 | RXD | IN | Receive Data |
| 4 | GND | - | Ground |
Notes
- The 4-pin connector on both the HP 48 and HP 95LX is spaced at 2 mm pitch (not 0.1 inch / 2.54 mm). Fitting a standard 0.1-inch connector will damage the calculator’s pins.
- Pin 1 (SHIELD) should be connected to the metal shell of the RS-232 cable connector (chassis ground), not to signal ground.
- There are no hardware handshaking lines (RTS/CTS, DTR/DSR); software flow control (XON/XOFF) must be used.
- The HP 48 supports baud rates of 1200, 2400, 4800, and 9600; parity options are none, odd, even, mark, and space; data framing is 1 start bit + 8 data bits + 2 or more stop bits, LSB first.
- The HP 48’s TX output is rated ±3.0 V minimum / ±3.5 V typical (into ≥3 kΩ load, ≤500 pF), which falls below the TIA-232-F minimum of ±5 V at the driver output. A level-converter interface cable is recommended for use with strictly compliant RS-232 hosts.
- Wiring to a DB9 (DE-9) female on the PC: HP48 pin 2 → DB9 pin 2 (RxD); HP48 pin 3 → DB9 pin 3 (TxD); HP48 pin 4 → DB9 pin 5 (GND); HP48 pin 1 → DB9 shield.
- Wiring to a DB25 on the PC: HP48 pin 2 → DB25 pin 3 (RxD); HP48 pin 3 → DB25 pin 2 (TxD); HP48 pin 4 → DB25 pin 7 (GND).
- The HP 95LX uses the same 4-pin 2 mm connector with pins labelled A–D: A = Shield (→ DB9 housing), B = TX (→ DB9 pin 3), C = RX (→ DB9 pin 2), D = Signal GND (→ DB9 pin 5).
Signals
- SHIELD — Chassis/shield ground; connect to the metal shell of the RS-232 connector, not to signal ground.
- TXD — Transmit data output from the calculator to the host. Voltage swing ±3.0 V min / ±3.5 V typical (HP 48).
- RXD — Receive data input to the calculator from the host. Input range +1.0 V to +15.0 V (mark) / −15.0 V to −0.3 V (space); impedance 5–7 kΩ; absolute maximum ±25 V (HP 48).
- GND — Signal ground reference for TXD and RXD.
References
- HP 48 I/O Technical Interfacing Guide — Hewlett-Packard Company, June 1990; primary electrical specification for the HP 48 serial port.
- HP 48 FAQ: Appendix D — Hardware Additions — community-maintained FAQ covering the 2 mm connector, wiring, and compatible cable construction.
- HP 95LX RS-232 serial connection information — Daniel Hertrich; HP 95LX 4-pin serial connector pinout and cabling details.
- HP-95LX I/O Applications Note — Hewlett-Packard; official application note for the HP 95LX serial interface (HP Calculator Literature Archive).