The expansion port
A 50-pin header for adding cards: input/output registers, extra ROM or RAM, sound. Deliberately, it is not a PCIe connector.
The principle
The expansion port is a keyed 2 × 25 header, and never a physical PCIe connector. An expansion card finds there:
- the data bus, buffered by two 74LVC245: their direction follows R/W, and they are enabled only if the card is selected (/IOSEL);
- the 16 low address bits (A0 to A15);
- one select, /IOSEL, for a page of registers at
$00:9F00–$00:9FFF. The former memory window (/MEMSEL) has been removed: the 16 MB of RAM fill the whole space; - the PHI2 clock, /RESET, the /IRQ and /NMI interrupts, RDY and /BUSREQ;
- I²C, and an audio input coming from the expansion card;
- 3.3 V, 5 V and plenty of grounds.
Pinout
| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1 | GND | 2 | GND |
| 3 | +3.3 V | 4 | +3.3 V |
| 5 | +5 V | 6 | +5 V |
| 7 | D0 | 8 | D1 |
| 9 | D2 | 10 | D3 |
| 11 | D4 | 12 | D5 |
| 13 | D6 | 14 | D7 |
| 15 | GND | 16 | GND |
| 17 | A0 | 18 | A1 |
| 19 | A2 | 20 | A3 |
| 21 | A4 | 22 | A5 |
| 23 | A6 | 24 | A7 |
| 25 | A8 | 26 | A9 |
| 27 | A10 | 28 | A11 |
| 29 | A12 | 30 | A13 |
| 31 | A14 | 32 | A15 |
| 33 | GND | 34 | PHI2 |
| 35 | R/W | 36 | /IOSEL |
| 37 | reserved (ex-/MEMSEL) | 38 | /RESET |
| 39 | /IRQ | 40 | /NMI |
| 41 | RDY | 42 | /BUSREQ |
| 43 | I²C SDA | 44 | I²C SCL |
| 45 | card audio input | 46 | audio ground |
| 47 | reserved | 48 | reserved |
| 49 | GND | 50 | GND |
On the Mini-ITX board
The expansion port is planned on the left edge of the board, with its buffers. Its exact position depends on the expansion slot of the Mini-ITX form factor, to be checked against an official template before laying out the board. See The Mini-ITX board.