Psion Series 3a - 6 Pin SIBO connector
The Psion Series 3a external expansion connector is a 6-way two-row 0.1-inch pitch (2×3) header introduced with Psion’s SIBO (Sixteen-Bit Organiser) platform in the early 1990s. It carries the proprietary SIBO synchronous serial bus — a 2-wire half-duplex interface using a CLK and DATA line at a nominal 3.84 MHz clock, yielding up to approximately 312 kbytes/s — rather than RS-232 signals; the connector operates at 5 V TTL levels. RS-232 communication became available only via the 3-Link adapter, which contains active circuitry (including an ASIC5 chip for level conversion) together with ROM software, and is therefore not a simple passive cable. The Siena variant of the SIBO family draws power from the host through pin 3 rather than supplying it.
| Pin | Signal |
|---|---|
| 1 | SIBO serial data |
| 2 | SIBO data clock |
| 3 | Siena=3DVin(6VDC) or 3c/3mx=3D5V out (30mA max) |
| 4 | signal and power ground |
| 5 | SIBO peripheral interrupt |
| 6 | SIBO external switch on |
Notes
- The connector is a 2×3 two-row 0.1-inch pitch header; pin numbering follows the standard Psion SIBO HDK assignment.
- The +5 V supply on pin 3 (Vcc) is switched off when the Series 3a enters standby mode; maximum current draw is 25 mA (30 mA per some variant documentation).
- On the Siena model, pin 3 is an input rather than an output: the Siena draws approximately 6 V from an external host via this pin rather than supplying 5 V to a peripheral.
- Pin 5 (SSD/INT) is an active-high interrupt line used by peripherals to signal the host processor.
- Pin 6 (SCK/EXON) is the external switch-on line; Psion’s own SIBO HDK documentation advises leaving this unconnected in standard peripheral designs.
- The SIBO bus uses 12-bit frames (8 data bits), LSB first, with clock timing of 195–260 ns per cycle (3.84–5.12 MHz).
- The 3-Link adapter for the Series 3a supports RS-232 at up to 19,200 baud; the adapter for the original Series 3 was limited to 9,600 baud.
- This interface carries 5 V SIBO logic levels; it is not electrically compatible with RS-232 (±12 V) at the connector itself.
Signals
- MSD / SDAT — Serial data line (half-duplex); maps to ASIC4/5 SDAT signal on the SIBO silicon.
- MCLK / SCLK — Serial clock output from host; nominal 3.84 MHz; maps to ASIC4/5 SCLK.
- Vcc / Vin — +5 V output on Series 3a/3c/3mx (switched, 25–30 mA max); 6 V input on Siena models.
- GND — Signal and power ground reference.
- SSD/INT — Peripheral interrupt, active high; used by expansion devices to request host attention.
- SCK/EXON — External switch-on line; do not connect in standard peripheral designs.
References
- The Psion SIBO Hardware Development Kit v1 — Psion PLC, re-published via doc.psion.info; authoritative hardware specification for the SIBO expansion connector pinout, electrical characteristics, and bus timing.