IBM 5155/5160 - Expansion Slot 8
Slot 8 is different to the other slots.
From IBM's March 1986 edition of the 5155/5160 Technical Reference:
Page 1-3: "Slot J8 is slightly different from the others in that any card placed in it is expected to respond with a 'card selected' signal whenever the card is selected."
Page 1-16: "Timing requirements on slot J8 are much stricter than those on slots J1 through J7."
Cards placed in slot 8 must be slot 8 compatible in order to guarantee proper operation. Some cards will not work at all.
Some user manuals for particular cards indicate that the card is not compatible with slot 8. An example: In the EGA Wonder 800+ manual is, "... install the EGAWONDER8OO+ into any available slot except slot 8 in an IBM XT."
Cards that are slot 8 compatible usually have a 'slot 8' jumper, fitted when the card is in slot 8, and removed when the card is in a different slot.
Example slot 8 compatible cards
1. IBM Asynchronous Communications Adapter (later version of the card)
Jumper: J13
See IBM Asynchronous Communications Adapter
2. Video-7 VEGA video card
Jumper: P1
Described on pages 48 and 52 of the card's user manual
3. Video-7 VEGA VGA video card
Jumper: P3
Photo here
4. Microsoft InPort
Jumper: J2
This is a card for Microsoft bus mice.
Photo here
What is the purpose of slot 8 ?
I used to think that slot 8 was a special slot created for the 'extender card', a card required to connect an IBM 5161 expansion unit to the IBM 5160. However, one piece of evidence that it is not, is on page XVIII of the March 1986 edition of the 5155/5160 Technical Reference. That page is for the IBM 5155 (IBM Portable Computer), a computer that contains a 5160 motherboard, and shows the extender card in slot 2. There is evidence elsewhere. For example, the extender card is compatible with the IBM 5150 (later models), a machine that does not have an equivalent of the 5160's slot 8.
So, what then is the purpose of slot 8 ?
Sergey on the VCFF wrote, "Not sure why J8 was designed this way. One of the possibilities is to use it for adding "system" devices that will use 0h - 0FFh I/O addresses."
The user's manual for the Breakthru 286 includes, "If you have a board in your system that can only go in slot 8 of an XT, such as the IBM 3270 board ..."
Technical
Refer to the diagrams at here and here. Unlike slots 1 to 7, slot 8 sits on the 'external' address and data buses. Accordingly, when the CPU reads data from a card in slot 8, that card needs to generate a CARDSELECT signal in order for the data to be gated through chip U15 to the CPU.
Not all cards have the ability to generate a CARDSELECT signal. Those that do, have a 'slot 8' jumper for that purpose.
Accelerator cards in slot 8
If an accelerator card is fitted into slot 8, an 1801 error may be seen at power-on of the IBM 5160.
One example: Discussed on page 2 of the Update Bulletin that Orchid released for their TinyTurbo 286 card. The bulletin is at here.
Some other examples: See here.