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IBM XT 8th Slot Video Card

H

Holmes

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Marietta, GA
I was browsing eBay today, looking at ISA video cards, and found this:

Vintage G2 Deico EGA-S800 Acer-EGA Card 8-Bit IBM Clone
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=370448708791&ssPageName=STRK:MEWAX:IT

In the description, they included the switch settings, which has a "slot configuration" switch for "slot 8". It says "slot 8 for IBM PC" which is obviously wrong, so I presume it means XT (maybe it got cut off).

This is interesting. I thought the only things that could go into the 8th slot was an IBM Asynchronous Card or a 3270 emulator card. I thought I heard someone had used a "Video Seven" card in the 8th slot once.

Anyway, if anyone's interested, there's the link.
 
Holmes said:
It says "slot 8 for IBM PC" which is obviously wrong, so I presume it means XT (maybe it got cut off).
Click to expand...

What makes this obivous wrong?

From the schematics, it seems like slot 8 is similar to all the other solts with the exception of the /Cardselect line. Pulling /Cardselect low can override the direction of the local-to-global data-bus buffer. If it is left high, the slot should theoretically work just like any other slot.
 
I seem to remember that you had to install the 5162 expander card in slot 8 on a 5160.

There were cards with jumpers to set if you used them Slot 8 and others who specifically directed you not to use them in Slot 8.

All in all, a bad idea.
 
Chuck(G) said:
I seem to remember that you had to install the 5162 expander card in slot 8 on a 5160.

There were cards with jumpers to set if you used them Slot 8 and others who specifically directed you not to use them in Slot 8.

All in all, a bad idea.
Click to expand...

In fact, the 5161 extender card is one of the cards that don't relay on slot 8 at all. The receiver card is suggested to be placed in slot 8 of the 5161, but there it doesn't matter since all slots in the unit are connected together.

But I just saw another difference between regular slots and slot 8, which is the one that makes it problematic to use regular cards there.

As I slightly mentioned earlier, the XT has two busses, a local and global one. The local bus is controlled by the global bus, and is used for most the components on the motherboard itself. This local bus consist of the memory address lines, data lines and the most basic bus controll signals (MemR/W and I/O).

The thing is that slot 8 is directly connected to the local bus, while all of the other slots are connected to the global bus. Because of this, the /cardselect line must be activated for the card every time data is going from the card to the global bus.

Why IBM did this is peoblably related to the fact that you can only have a limited number of TTL inputs connected to each output. Due to this, the technical reference clearly states that no expansion card should use the system driven bus lines more than twice.

---

All in all, it may well be that the video card in question properly drives the /cardselect line when the given switch is set. If it does; it will work in slot 8.
 
I have a Video-7 Vega EGA card....it has a jumper setting for operation in the XT Slot 8..

From the manual:

SLOT 8 SETTING

Setting
JP2
Enable
Pins 1 & 2 closed

Disable
Pins 2 & 3 closed

Note:Slot 8 on the IBM XT had a different timing than the other 7 slots, this board provides for the option of changing it nature to fit this difference
 
per said:
What makes this obivous wrong?

From the schematics, it seems like slot 8 is similar to all the other solts with the exception of the /Cardselect line. Pulling /Cardselect low can override the direction of the local-to-global data-bus buffer. If it is left high, the slot should theoretically work just like any other slot.
Click to expand...

Er, because the IBM PC (model 5150) doesn't have 8 slots?

Sorry, am I missing something? It's very likely.
 
Originally Posted by Holmes:
It says "slot 8 for IBM PC" which is obviously wrong, so I presume it means XT (maybe it got cut off).

per said:
What makes this obivous wrong?
Click to expand...
How many slots did PCs have in Western Norway? Over here they only had 5... ;-)

Oops; Holmes beat me to it...
 
Holmes said:
Er, because the IBM PC (model 5150) doesn't have 8 slots?

Sorry, am I missing something? It's very likely.
Click to expand...

Ah. Such things happens during skimm-reading.

I agree that this must obivously be a typo in TH99, and I'll assure you it's not the only one. TH99 was supposed to be a database as complete as possible. This means that they problably prioritized quantity before quantity when they made it, and thus you get such statements.
 
modem7 said:
Slot 8 on the IBM 5160 has been discussed a few times on these forums already.
For example: post #10 at http://www.vintage-computer.com/vcforum/showthread.php?9461
Click to expand...

That description quoted there is reasoned wrongly as the author bases it on the hypothesis that there is a relation between slot 8 and the 5161 expansion unit interface. This is wrong as neither the receiver or extender card actually generates any /Cardselect signal. Without that signal, it's impossible to receive data from slot 8 in the XT.
 
As per mentioned the 8-th slot (J8) is connected to so called "X-Bus" (for some unknown reasons IBM calls it External Data bus, and the J8 is the only external connection on it), which is a buffered version of ISA. Various controllers on the motheboard (PICs, PIT, DMA, etc) are connected to the X-Bus. The additional buffering of the X-Bus is required because buffers / transceivers on the ISA bus (between CPU and ISA) won't be able to pull all ISA slots and I/O devices on the system board together.
The /CARDSLCTD signal controls data transfer direction, when it's active (low) the X-Bus data transceiver will send data from X-Bus to the ISA bus. IBM manual mentions "much stricter" timing for J8, and I guess the only reason for that is the latency added by additional buffers and transceivers.

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. BTW, IBM XT technical reference has a diagram showing "Receiver Card from Extender Card" in J8. It seems Receiver/Extender cards don't generate /CARDSLCTD signal, but forward it from the extention unit to the main unit.
 
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