Bus expansion (one way or another); HwNote08

[John B. Milton]

In article <733 at bacchus.UUCP> darren at bacchus.UUCP (Darren Friedlein) writes:
>OK - here's another question.  What's involved in building a card cage for
>the UNIXpc if it was possible to get these connectors?  Could it be done
>with like ribbon cable?  Since this machine IS based on a back-plane, this
>would appear to be possible.  Any comments?

It is not strictly a bus like S-100. 100 pins come off the mother board, and
99 pin Eurocard-ish connectors are used in the bus. Each slot has 3 pins for
slot ID, which are of course different for each slot. Other than that, the
rest of the signals (as far as I know) are bused. The little 3 slot bus board
in the UNIXpc is an 8 layer board.

There is an "Expansion Unit" out there, Comcode 405176785, I've seen a price
of $1695 for it. It adds 5 slots and has a board that plugs into one of the
slots. You can't put memory in it. I have thought about building a bigger bus.
My thought was to take the little three slot bus out and use a new stacked 8
slot bus. Turn the whole mess into a vertical "tower" arrangement:

     +------ mother board
     v
 [==]|   <-- floppy
 $$$$|   <-- hard drive(s)
 $$$$|   <
 ----|   \
 ----|   |
 ----|   |
 ----|   |-- 8 slots
 ----|   |
 ----|   |
 ----|   |
 ----|   /
 %%%%    <-- Power supply
 %%%%

The problem is, who would buy it? A new bus PC would have to layed out. Probably
even re-designed to be properly balance for all 8 slots. The power supply would
have to be out-and-out changed. New cables: hard disk (3), floppy (2), video,
keyboard. You would need some kind of base for the monitor. Few people would
even look into it without a proper case, and on and on. How many people out
there can afford 8 cards for this beastie anyway?

The better idea seems to be a big project looming in the future. What everyone
wants is lots of cheap expansion. Nothing for this machine is cheap compared to
the PClones. You guessed it, why not build an IBM-PC bus bridge? A SCSI card
for the PCs only costs $45. I think they're putting serial cards in Cracker
Jack boxes these days. There are lots of other tantelizing boards out there:
Hard and floppy disk, xGA, serial, parallel, IEEE-488, gaggles of co-processors.
I don't even want to think about DMA just yet. The part that scares me most is
noise trouble. My friend James Nugen (jcn at uncle) has had an IBM hard disk
controller working on an Amiga, so this is close along those lines.

The software for this idea would be a nightmare. The IBM-PC BIOS pieces parts
are on cards, a lot of good that does us. I don't think it would be very fun
to run an 8086 interpreter in the device driver. This all leads to the same
problems the MINIX folks are having with all these boards. It also means we
have a good source for drivers, hard disk in particular.

I would envision a new twist to get this attached to the UNIXpc. A lot of people
are already maxed out to 3 cards, so they don't have a slot to spare. For
something like a bridge, which needs everything on the bus, I am wondering if
a flexible PC edge connector tap might work. I thought this one up talking to
Gary Sanders (gws at n8emr) on the phone. The idea is to design a flexible PC
board which matches the fingers on the mother board. You detach the mother
board from the bus connector, fold the flexible PC up the right way, wrap it
around the mother board fingers, and then carefully plug the whole mess back
into the bus edge connector. The flexible PC could be layed out with interleaved
grounds and heavy traces for power. It could even be folded between the UNIXpc
and a PC box for easier Advanced Cable Management. The problems would be cost
and mechanical repeatability of the connector end. Has anyone ever seen this
done before?

Just thought I'd put one MORE iron in the fire.

John
-- 
John Bly Milton IV, jbm at uncle.UUCP, n8emr!uncle!jbm at osu-cis.cis.ohio-state.edu
home (614) 294-4823, work (614) 764-4272;  Send vi tricks, I'm making a manual