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 The mystical 8th XT slot View next topic
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Pimped_Portable



Joined: 30 Aug 2005
Posts: 20
Location: Gurnee, IL, USA

PostPosted: Wed Aug 31, 2005 12:47 am Reply with quoteBack to top

Hey everyone...

I was just wondering if anyone knew, because I noticed a reference to it on here...is the 8th slot on a PC/XT a "special" slot? I ask for two reasons: One, in the post about monitors and graphic cards on XTs, someone mentioned the possibility that a Microsoft InPort bus mouse card is usable in that slot, and second, I could not get any card to work in that slot.

Can anyone shed some light on this?

Thanks

PP
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creepingnet



Joined: 19 Oct 2004
Posts: 137
Location: Lynnwood,WA

PostPosted: Wed Aug 31, 2005 12:59 am Reply with quoteBack to top

Pimped_Portable wrote:
Hey everyone...

I was just wondering if anyone knew, because I noticed a reference to it on here...is the 8th slot on a PC/XT a "special" slot? I ask for two reasons: One, in the post about monitors and graphic cards on XTs, someone mentioned the possibility that a Microsoft InPort bus mouse card is usable in that slot, and second, I could not get any card to work in that slot.

Can anyone shed some light on this?

Thanks

PP


The 8th slot (one closest to the power supply I assume you are talking about) was designed for a special interface card for use with an IBM 370 mainframe computer. It has some connections on it that are a little different, one I think has to do with the IRQ, and another has to do with the power connections I believe. I don't know for sure about the actual connections as I would have to look at a book I got at a computer museum a few years back (really cool, it has a ton of vintage calculators and XT clones in it and so on). It really explains the whole thing in detail, if I can find it when packing tonight, I'll post what I find up here.

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harshbarj



Joined: 01 Oct 2004
Posts: 163
Location: behind you!

PostPosted: Thu Sep 08, 2005 8:30 pm Reply with quoteBack to top

If I recall it also runs at a slightly higher speed as well.

found this on the net.
Quote:


after checking in my IBM XT reference manual, the fact is that
they added a buffer to the normal data and address bus and sent
buffered these signals into J8. Also, they added a 14.318mhz
oscillator on the expansion chassis side because they did not
want pass this high of a frequency over the cable. This does
mean that the manin 14mhz and the expansion chassis 14mhz are
not in phase.
The added buffers make the timning in slot 8 a littl emore tight
because of the added delays through the additional chips. Slot
8 also had a requirement that it supply a card selected signal
back to make sure the card was really ready to be accessed.

The buffering from an engineering standpoint was done to increase
the drive on the cable to the expansion chassis as well as help
isolate the rest of the system bus from any added capacitance
and noise from the outside world.



http://64.233.167.104/search?q=cache:WdzhOf1K_DkJ:www.classiccmp.org/pipermail/cctech/2003-April/012817.html+j8+slot+ibm+xt&hl=en

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xtguy



Joined: 02 Oct 2004
Posts: 28
Location: the mile high state, Colorado

PostPosted: Sat Sep 10, 2005 5:49 pm Reply with quoteBack to top

Here's what my copy of Mueller's "Upgrading and Repairing PCs" says:

"In the XT or Portable PC, the eigth slot, the one closest to the power supply, is a special slot. Only certain cards can be installed there. Any card installed in this eight slot must supply a special "card selected" signal to the motherboard on pin B8, which few cards have been designed to do. (The IBM Asynchronous Adapter Card and the Keyboard/Timer Card from a 3270 PC are two examples of cards that fit in that slot.) Additionally, the timing requirements for the eigth slot are stricter.

The reasoning for this strange slot in the XT is that IBM developed the system to support a special configuration called the 3270 PC, which is an XT with from three to six special adapter boards installed. The eight slot was specifically designed to accept the keyboard/timer adapter from the 3270 PC. This board needed special access to the motherboard because it replaced the motherboard keyboard circuitry. Special timing and the "card selected" signal made this access possible. (Contrary to what many users have believed, the eight slot has nothing to do with the IBM expansion chassis.) By the way, the IBM expansion chassis was a box developed by IBM that looked like another system unit. Because the IBM XT had eight slots, one full-height floppy drive and one full-height hard drive, the expansion gave room for more expansion slots and additional floppy and hard drives."

Note that what the book says about this slot only applies to the XT and the IBM Portable PC, not the original IBM 5150, the AT, or any other XT compatible or clone.

In practical terms, you really only need to be aware of it to prevent yourself from putting a "regular" card in the slot, only to discover it doesn't work. You'll likely run out of IRQ's before you try to fill up all 7 of the other slots in the XT.

I do have a "Video Seven" video card (I think it is VGA) with has a block of DIP switches, one of which is a 'slot 8' switch, to allow the card to be used in slot 8.

I've used the IBM Asynchronous Adapter Card (a fancy name for a serial port card) on slot 8 to run a standard serial mouse (a 25-pin to 9-pin plug adapter is needed to fit the mouse because the IBM card has a 25 pin serial connection on it's back plate, not a 9-pin serial connection.)
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wdegroot
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PostPosted: Wed Sep 14, 2005 1:52 pm Reply with quoteBack to top

Ibm said that the 8th slot had " fifferent timing characteristics.
it is intended for a serial card. this does NOT apply to the pc
which has 5 slots,
but the xt and portable which uses a xt board.
another board MAY work in that slot. nothing specific was said
anyway use an AT serial/parallel; card in another slot.
either an original card or an at i/o card in another card, the newer cards have multiple fuction on on card.
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