IBM Personal Computer XT System Board Slot 8

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The IBM Personal Computer XT System Board featured 8 ISA slots connected across two buses:

Slots Purpose IBM Data Bus Designation Limitations
1-7 Standard ISA slots D[0..7] None
8 (closest to CPU) Special IBM cards XD[0..7] Expansion card must drive CARD SLCTD line
Cannot be used with DMA

Original Motivation

The original motivation for the 8th slot remains unclear. Whilst the IBM Personal Computer System Board was known to be limited by its 5 slots (multi-function adapters being uncommon at the time), because an adapter in slot 8 needs additional logic to function at all, its use was limited - mostly to specific IBM adapters such as IBM's special asynchronous serial card.

IBM Description of CARD SLCTD

Any card in slot 8 must drive the CARD SLCTD line (ISA B8). The IBM 5155/5160 Technical Reference Manual (pg 1-21) provides a description:

CARD SLCTD (I) Card Selected: This line is activated by cards in expansion slot J8. It signals the system board that the card has been selected and that appropriate drivers on the system board should be directed to either read from, or write to, expansion slot J8. Connectors J1 through J8 are tied together at this pin, but the system board does not use their signal. This line should be driven by an open collector device.

By observation though:

  • It appears that CARD SLCTD should be driven only to read from the card
  • Whilst expansion slots J1 to J7 may indeed be tied together at B8, it appears that slot J8 is separately connected, since driving line B8 in slots J1 to J7 has no effect

Achieving Slot 8 Compatibility

  • Cards must respond to /IOR or /MEMR by asserting line /CARD SLCTD (ISA B8)
  • Cards must not assert /CARD SLCTD in response to /IOW or /MEMW
  • A card in slot 8 cannot be used for DMA transfers, because the XD[0..7] bus is interfaced to the D[0..7] bus by U15, itself gated by AENBRD (driven high when HOLDA is asserted by the DMA controller). In other words, when the 8237 DMA Controller has control of the bus, D[0..7] must be driven by something in an ISA slot other than 8 (or system RAM via U9) - see PC/XT System Board Schematics from the 5155/5160 Technical Reference Manual.

ISA B8 History

See Also