8 PIN MINI-DIN MALE to the Computer
4 PIN FEMALE Connector to the HP48
The HP 82209A is the official cable supplied by Hewlett-Packard for connecting the HP 48 calculator to a Macintosh serial port. The Macintosh end uses a mini-DIN-8 connector, introduced with the Macintosh Plus (1986), whose serial ports implement RS-422 balanced differential signalling governed by TIA/EIA-422-B. Because the HP 48 uses RS-232-compatible single-ended signal levels (±3 V to ±3.5 V output), only the unbalanced negative (−) data lines of the Macintosh port are used; the positive differential lines are biased to ground. The HP 48’s proprietary 4-pin connector carries Shield, TX (output), RX (input), and Signal GND.
| Mac | HP48 |
|---|---|
| TxD- | 3 |
| RxD- | 5 |
| GND+RxD+ | 4+8 |
| Shield | SHIELD |
Notes
The Mac column refers to mini-DIN-8 pin numbers (Pin 3 = TxD−, Pin 5 = RxD−, Pin 4 = GND, Pin 8 = RxD+). The HP48 column refers to the calculator’s 4-pin proprietary connector (Pin 2 = TX output from HP 48, Pin 3 = RX input to HP 48, Pin 4 = Signal GND, Pin 1 = Shield). Tying RxD+ (Mac Pin 8) to GND biases the Mac’s differential RS-422 receiver input into single-ended RS-232-compatible mode. The HP 48 serial format is 8 data bits, 1 start bit, and one or more stop bits; mark corresponds to a negative voltage and space to a positive voltage, consistent with RS-232 convention.
Signals
- TxD− — Transmit data (negative/unbalanced line), Mac Pin 3; carries data from the Macintosh to the HP 48 RX input (HP 48 Pin 3)
- RxD− — Receive data (negative/unbalanced line), Mac Pin 5; carries data from the HP 48 TX output (HP 48 Pin 2) to the Macintosh
- GND+RxD+ — Mac Pin 4 (signal ground) and Pin 8 (RxD+) joined together and connected to HP 48 Pin 4 (Signal GND); shorting RxD+ to ground biases the differential receiver for single-ended operation
- Shield — Connector shell / cable screen; connected to HP 48 Pin 1 (SHIELD)