9 pin Apple mini din connector at the end of the peripheral cable, from the front of the connector.
GeoPort is Apple’s extended serial interface introduced on the Macintosh Quadra 660AV and 840AV (1993) and carried forward on all pre-USB beige Power Macintosh models. It uses a 9-pin mini-DIN shell that retains the eight RS-422 balanced differential signals of the standard Macintosh serial port whilst adding a ninth pin supplying +5 V DC to power an external pod. The extra pin physically prevents standard mini-DIN-8 peripherals from being inserted, though GeoPort ports can drive ordinary RS-422 serial devices when pin 9 is not engaged. The high-speed DMA channel supported data rates up to 2 Mbit/s, enabling an external pod to emulate modems, fax machines, and other telephony devices using the Macintosh’s internal sound hardware.
| Pin | Signal | Description |
|---|---|---|
| 1 | SCLK (out) | Reset pod or get pod attention |
| 2 | Sync (in)/SCLK (in) | Serial clock from pod (up to 920 Kbit/sec.) |
| 3 | TxD- | Transmit - |
| 4 | Gnd/shield | Ground |
| 5 | RxD- | Receive - |
| 6 | TxD+ | Transmit + |
| 7 | Wake up/TxHS | Wake up CPU or do DMA handshake |
| 8 | RxD+ | Receive + |
| 9 | +5V | Power to pod (350 mA maximum) |
Signals
- SCLK (out) — Serial clock output used to reset an attached pod or gain its attention.
- Sync (in)/SCLK (in) — Serial clock received from the pod, supporting asynchronous data rates up to 920 Kbit/s.
- TxD− — Transmit Data negative leg (RS-422 differential pair with TxD+).
- Gnd/shield — Signal and chassis ground.
- RxD− — Receive Data negative leg (RS-422 differential pair with RxD+).
- TxD+ — Transmit Data positive leg of the RS-422 balanced pair.
- Wake up/TxHS — Wakes the host CPU from sleep or acts as a DMA handshake signal for high-speed transfers.
- RxD+ — Receive Data positive leg of the RS-422 balanced pair.
- +5V — Supplies +5 V DC power from the ADB power rail to the connected GeoPort pod; the combined current draw of all ADB and serial +5 V devices must not exceed 500 mA.
Notes
- The mini-DIN-9 shell is physically backwards-incompatible with standard mini-DIN-8 cables; the ninth pin prevents inadvertent insertion of conventional Macintosh RS-422 serial peripherals.
- Pin 9 draws from the ADB +5 V rail; Apple’s Power Macintosh 7500/8500 Developer Note states that the total current for all ADB and serial +5 V devices must not exceed 500 mA.
- GeoPort supports data rates up to 2 Mbit/s via an external DMA channel, far exceeding the 57,600 bps asynchronous limit of the standard mini-DIN-8 RS-422 port.
- The underlying RS-422 signal levels apply to pins 3, 5, 6, and 8: output high/low ±3.6 V, receiver threshold ±0.2 V.
- RS-232-C compatibility can be achieved by grounding the positive receiver legs (RxD+, pin 8) and leaving the transmitter positive leg (TxD+, pin 6) unconnected, creating an EIA RS-423 interface usable with RS-232-C devices over short cable runs.
References
- Apple IIgs Technical Note #30: Apple IIgs Hardware Reference Updates — Apple Computer, Inc.
- Inside Macintosh: Devices — Serial Driver