The HP48 calculator, introduced in 1990, features a compact 4-pin serial port with RS-232-compatible signal levels but a reduced voltage swing (±3.0–3.5 V transmit, ±0.3–15 V receive). The official HP 82208A cable connects this port to a PC DE-9 (9-pin D-sub female) serial port, carrying only three active signals — transmit data, receive data, and signal ground — alongside a shield connection on pin 1. The HP48 supports baud rates from 1200 to 9600 and configurable parity (none, odd, even, mark, or space); hardware handshaking lines such as RTS and CTS are not available on the 4-pin connector. Signal direction follows the PC/DTE convention: DE-9 pin 2 carries data received by the PC (transmitted by the HP48), and pin 3 carries data transmitted by the PC (received by the HP48).
9 PIN D-SUB FEMALE to the Computer.
4 PIN HP48 to HP Serial Port
| 9-Pin | HP48 | |
|---|---|---|
| Carrier Detect | 1 | |
| Receive Data | 2 | 2 |
| Transmit Data | 3 | 3 |
| Data Terminal Ready | 4 | |
| System Ground | 5 | 4 |
| Data Set Ready | 6 | |
| Request to Send | 7 | |
| Clear to Send | 8 | |
| Ring Indicator | 9 |
Notes
- The HP48 4-pin connector is proprietary; standard RS-232 handshaking lines (RTS, CTS, DSR, DTR, DCD, RI) are absent from the calculator end.
- DE-9 pin 1 carries the cable shield rather than an active DCD signal.
- On a DE-9 to DB-25 adapter, the TX and RX lines swap relative to a direct DB-9 connection: DB-25 pin 2 receives data from the HP48 and pin 3 sends data to it, with signal ground on pin 7 and shield on pin 1.
- The HP48’s transmit output is idled (shorted to signal ground) when the serial port is closed, minimising current draw from the calculator’s batteries.
- Maximum supported baud rate is 9600 baud (approximately 844 characters per second); the port is not rated for higher speeds.
- The companion cable for Macintosh serial ports is the HP 82209A.
Signals
- Carrier Detect (pin 1) — shield connection at the DE-9 end; not an active RS-232 DCD signal in this cable
- Receive Data (pin 2) — data received by the PC; carries the TX output of the HP48
- Transmit Data (pin 3) — data transmitted by the PC; carries the RX input of the HP48
- Data Terminal Ready (pin 4) — not connected to the HP48 4-pin connector
- System Ground (pin 5) — signal ground common to both devices; connects to HP48 pin 4
- Data Set Ready (pin 6) — not connected
- Request to Send (pin 7) — not connected
- Clear to Send (pin 8) — not connected
- Ring Indicator (pin 9) — not connected