I'm wondering if the proper IBM card will work in Slot 8 in an XT? And, if it does, whether any boards that use DMA can be used in the expansion unit (for example, a Xebec HDD controller)?
Many thanks.
Maybe that's what they indirectly reffer to when they claim that cards in slot 8 requires "stricter timing" :roll: ...But the puzzling thing is... why DRQ/DACK signals are present on slot 8, or why the gate on U15 isn't or'd with CARDSLCTD.
I'm wondering if the proper IBM card will work in Slot 8 in an XT? And, if it does, whether any boards that use DMA can be used in the expansion unit (for example, a Xebec HDD controller)?
Many thanks.
Thanks very much. Even if it did control B8, my theory is that no DMA transfers can be done from slot 8 because XD[0..7] are interfaced to D[0..7] via U15, but U15 has a gate driven by AENBRD, itself driven high when HOLDA is asserted by the DMA controller. In other words when the 8257 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).
If that interpretation (of the tech ref logic diagrams) is correct, that does fit observed behaviour as I get NMI (parity error) when I try DMA from slot 8.
But the puzzling thing is... why DRQ/DACK signals are present on slot 8, or why the gate on U15 isn't or'd with CARDSLCTD.
From everything I've read regarding the expznsion unit, the extender and receiver cards SHOULD ALWAyS be installed in the last slot. So in an XT, that would be slot 8, and on the 5150 PC, slot 5.
Per is writing of the extender card, not the receiver card.This is from The IBM Personal Computer XT Hardware Reference Library ... Technical Reference. Page 1-77
Thanks very much for the input. I'm personally convinced that DMA transfers aren't possible from PC/XT slot 8 since U15 is gated by AENBRD (which = /AEN). But if someone with an expansion unit could test the extender card in slot 8 that would of course be very helpful
Per is writing of the extender card, not the receiver card.
If that is true, then why would IBM put its hard disk controller and drives in the Expansion Unit? The controller uses DMA 3 for transfers, doesn't it?
All of this asks "Why the special treatment of Slot 8?" I don't think I've ever heard a convincing explanation. It certainly didn't propagate onto the 286 and later systems.
An reason why such a number may have been choosed may have been to keep compability with older non-LS TTL chips on ISA cards. Their inputs puts more load on the outputs than the common LS-type TTL chips, and while a LS buffer may be able to drive 30 or so LS inputs, it may only be able to drive 15 older non-LS inputs.