286 -> 386sx Upgrades

[Dick Dunn]

kls30 at duts.ccc.amdahl.com (Kent L Shephard) writes:
> Upgrades via the "plug in an adapter to the 286" are expensive compared to
> what a new 386sx motherboard costs...

>...It is not that difficult to swap out a motherboard.  Takes a little more
> time than the upgrade board but is worth it...

A while back I wrote a note that agrees with Kent about the cost.  However,
it may not be as easy as you think to swap motherboards!  Contrary to
popular wisdom, the innards of clones are not all plug-compatible.  Most
are, but beware.  A tale of woe...

I tried several times to upgrade an old NEC APC-IV.  That's their basic AT
clone from some years ago.  The first attempts at upgrading were add-in 386
boards, either plugging directly into the CPU socket or plugging into the
bus with a cable to the CPU socket.  Either they didn't fit (given space
constraints on the board) or didn't work (somebody playing BIOS games, I
suspect; that always gets UNIX).

So I decided to swap motherboards.  Simple, right?  Just open it up, pull
the cards, disconnect everything, swap motherboard, reconnect and go.  Not
so.  The first thing I discovered was that the APC-IV power supply
connector was nonstandard.  No regular P8/P9; it has an 8-pin and a 4-pin
connector.  Fortunately I had the maintenance manual so I didn't just try
rewiring into new plugs, 'cause things ain't the same.  Not only is the
order different; the power-supply gozoutas differ:  There's no power-good
connection.  (Per the discussion a while back, that's important for reli-
ability.  I have no idea how the APC-IV does reset on power fail; I haven't
stared at the logics.)

OK, so replace the power supply, right?  Measure first; yeah, it'll fit
just fine.  Well, it almost fits, except that the power connector holes
have a slightly funny shape, and the switch doesn't quite line up with the
case hole.  A little "plastic surgery" fixes that, but by this time the
temper is getting a bit short...

Now let's do the motherboard.  Yank everything loose, pull the board.  The
replacement is a baby AT size, so it's gotta fit.  I've checked the mount-
ing holes; they line up.  Set the new motherboard in and...what the hell??
There's a brace for the center disk-drive bays in the way!  The APC-IV
motherboard is slightly shallower (front to back of machine) than standard,
so that it will clear the brace, but a normal motherboard won't fit.

(The solution was to get a new case *and* power supply!  Effectively the
upgrade has become a way to re-use existing peripherals.  But, *if* there
had been a usable CPU-only upgrade, it would have been more attractive.)

I'm sure these aren't the only things that can thwart a motherboard swap,
but they're certainly enough.
-- 
Dick Dunn     rcd at ico.isc.com -or- ico!rcd       Boulder, CO   (303)449-2870
   ...Never offend with style when you can offend with substance.