Date   : Wed, 15 Mar 2006 13:36:36 +0000
From   : Fragula <fragula@...>
Subject: Re: 1MHZ SCSI/ATA board

Hi Jules/All!

Jules Richardson wrote:

> Ok, DRAMs on mine are HM50256 at 150ns.
Errrm.. I know its a 256Kx1, but beyond that (pinout, supply, drive
chars etc.) I'm not at all sure, and would rather nobody roasted a rare
old bird finding out.

Later : Lawdie! Pretty purple ceramic ones, available for purchase at a
heady £6.29 *each*+p+p+VAT :-O

Later still: These do appear to be pin-compatible with most generic
41256 in most circuits. <phew>

> Your mention of the 4MHz clock speed's got me wondering - there's sign
> of extra soldering around the 12MHz crystal on my board, suggesting that
> it's been changed for some reason post-production.
IDD. Phil's 4MHz chip did check out. it does appear to be a kosher 4MHz
part, an especially slow and rare version of the 80286. The "C" at the
beginning signifying the ceramic case, not CMOS as I'd mis-assumed and
then compounded my confusion with a Harris or Weitek part number.

> Makes me wonder now if that's the root of the initialisation troubles
> with my board-  I'm speculating that someone may have tried upping the
> clock speed (the CPU's a 12MHz one too) and something on the board won't
> cope or there are all sorts of nasty timing issues going on.
Probably the same thing, or some non timing related problem. Try just
popping an 8MHz xtal on if the current xtal is 24Mhz, or a 4 if the
current is 12. Normal 74xx TTL of that era is usually ok within itself
up to about 14MHz, though obviously propagation delays can occur.

The (genuine Intel) 80286 is often very fussy regarding clock rate (but
still rock solid when clocked in-spec!) and external buffer propagation
(depending on circuit) tho.

> I'll have to see if I have a lower speed 286 CPU kicking about (unless
> of course they can happily be underclocked without timing problems).
Yeh.. Underclocking, within reason, should be fine. I had a 12 on a (PC
server) board that used to switch it down to 10,8,6,5 and 4.77MHz, which
of course I duly tested. In fact, for obscure "real-time" obsolete and
closed(probably lost!)source ISA hardware-bashing software compatibility
reasons I wish I still had that one. :-/

<thinks hard>
One problem you /may/ find is that the DRAM refresh might be to
slow/late though. leading to memory errors. but WTF. risk it!

Cheers!

M.