On 2017-03-17, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...
[ ... ]
factory assembled or in kit form.) They had others to fix the
Altair
8800 (Intel 8080 CPU), but I was the only one for the Altair 680b
(Motorola MC6800 CPU).
I had to scratch through the solder mask and the short to fix
it.
The semiconductor test equipment maker I worked for pushed the state
of the art and went through a lot of circuit board revisions as a
result. They used layout design rules too tight for the board vendor
to consistently meet so internal shorts were an issue.
Oops!
20A at 1V usually identified and cleared the short,
It could also clear a buried lead for multi-layer boards,
depending on cross-sectional area. :-)
literally in a
flash. If not, the voltage drops could be followed to locate it, even
in a solid plane. I cleared one by slowly milling down until a copper
chip appeared.. They would complete and test that one board to check
for other problems before sending out another rev.
Fun! Was the board populated before you went short hunting, or
was it checked out first to determine whether there were shorts or
opens?
That was in the first half of the 1980's. By the early 1990's the PC
board vendors had substantially improved their process control and I
had little trouble with shorts despite substantially tighter spacing
rules. They gave me enough detail on the processes to know what I
could likely get away with at standard or higher cost, ie more labor
and lower yield. I've seen a circuit board that cost the Air Force
$30,000 and heard of yields of 1 in 20 for high layer counts.
Ouch! $30K populated, or just the bare board?
For example the fiberglass threads deflect a fine drill bit and
determine the amount a pad has to be larger than the hole. They don't
stack the boards as high on a small prototype order so I could use
smaller via pads than would be acceptable in production.
Solid carbide drills, or HSS? I know that the HHS ones in the
#70 size range were quite flexible, but the solid ones you even bounce
too much light off one side and "plink". :-)
The size of
the board also affects misalignment between the drill and the pads due
to uncertain thermal expansion when they press the layers together.
Making power planes a lattice instead of solid and filling unused
spaces helped keep my multilayer boards flat, unlike some other
people's.
I've seen the latticed power planes and wondered why. One
thought was to minimize weight in avionics. I've even seen them on
plain double-sided boards -- no buried power planes..
BTW -- the MC6800 had an undocumented instruction dubbed "HCF" (Hang
and
Catch Fire) which tri-stated the data buss, and cycled the
address bus through all 64k addresses. Made it easy to spot
problems in address decoding. :-)
I built mine around an 8080 chipset that the company gave me after I
finished the lessons on a microprocessor trainer. The were very
supportive of home study as long as we weren't caught using their
parts to build pirate HBO decoders.
:-)
They generated a lot of useful scrap like the 9" square wirewrap cards
I built it on. I had to make a manual wirewrap tool from brass tubing
for their 0.062" square high-current backplane pins.
0.062" hmm ... the size for the phone company relay pins which
used wire-wrap. I've got some bits for that size as well as the more
common logic circuit size.
I used counters for the front panel address registers so it would auto
increment when toggling in bootstrap code.
That would certainly save time compared to the Altair 680b's
front panel simply with toggles for both the address and data input. I
got pretty good at incrementing binary numbers with the switches. :-)
The 8080's Wait signal
could trigger a variable one-shot controlling Ready that decreased the
bus speed to 1/2 - 200 cycles per second as well as single step. It
was very helpful to fast-forward through subroutines, using the
address jump to detect the Return.
Hmm ... the MC6800 could be halted by the clock to stretch the
cycle for slower chips -- but the Altair 680b did not implement that, so
the whole thing ran at 500 KHz instead of the proper 1 MHz to allow the
1702A EPROMs used for the monitor.
The CPU could be halted as long as you wished by stopping the
clock -- no dynamic memory in it, and if you used static RAM as well, no
problems.
I learned enough from it that later I was able to design a
special-purpose DRAM controller IC.
Nice!
[ ... ]
Now Mitre could have been a more convenient place for me to
work. There was at least the octagonal mushroom building belonging
to
them a lot closer to my home than Ft. Belvoir was. :-)
Mitre was a real country club, which is great for the golfers, not so
much for the caddies attending to them. You weren't squat without at
least an MSEE. I got by on the unusual breadth of my practical
engineering experience rather than its rather shallow theoretical
depth.
Hmm ... I would have had to do the same, then -- if I could.
Enjoy,
DoN.
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