31 Dec
2008
31 Dec
'08
7:58 p.m.
On Wed, Dec 31, 2008 at 1:31 PM, Dave McGuire <mcguire at neurotica.com> wrote:
Most other people scrapped theirs then, too. Those SRAMs are not
pennies-per-pound as they were when the world migrated from 486s to
Pentium boards with a pair of SMT cache RAMs.
Real cache
memory 32k*8 is about 15ns access time in a skinny 28 pin
package.
Just don't use "PC motherboard SRAMs" as someone suggested earlier.
They're nowhere near as easy to find as some people think they are, at
least for some of us...I trashed all of my 486 boards a decade ago. Standard, currently-available SRAMs are cheap and
easy to work with, and
are just as fast. I have a few 512KB SRAMs in SOJ-36 packages sitting on
the bench right now; I think they were about $7/ea in small quantities from
a distributor.
I think 32Kx8s faster than 70ns are also about $7 each.
Fortunately, we are not talking about anything that fast - the DRAMs
on the boards in a real MK11 are most likely 150ns, and 70ns-80ns
SRAMs aren't all that expensive, even in +5V DIP varieties.
I did notice that trying to replace cache _and_ main memory would have
to sit in many slots, it might not be so expensive if paddlecards
could be employed. The question then becomes how many. If it's a
dual-height card for each of 3 slots, then one hex-height card for the
the slot with the RAM, that's not so bad. If you need to tap more
spots in the cache card slots, it could get pricey quickly.
I agree that a 4-layer board is vastly preferable over a 2-layer board
for something like this. We did 4 and 6-layer boards back in the day,
and I'm pretty sure we didn't have noise problems running an 8MHz
68000 with its own local memory bus.
Oh, and point taken about the lack of Unibus interface ICs not driving
up the price of this. I did kinda skirt that issue by leaving the
chips out of my target price since I have quantities of many things
that one might find on such a board and really just need the PCB.
-ethan