12 Apr
2002
12 Apr
'02
10:14 p.m.
Richard Erlacher wrote:
I hope you're not suggesting that devices that use
core memories are in any
way BETTER than more modern machines that use semiconductor memory.
Core memory had it's day. However it had the advantage of non-volitile
that is hard to duplicate in modern memory. The read/write memory cycle
was well used by computer architecture of the 1960's.
A PAL can generate any combinatorial function of its
inputs. A prom can do
that too, but the PAL does it with fewer fuses.
Did not the early PAL's burn on PROM programers? (512x8 fuse prom)
Sadly this not true today.
Before people draw those schematics, they first
manipulate the concepts on a
big dry-mark-board (whiteboard), waving their arms and arguing vehemently that
their take is the one. Once the blocks have been mapped into the
requirements, low-level design meetings are held where they do the same thing,
only at more detail. That's where the block that later becomes a PLD is born.
Block 6B divides the clock by 1-1/2 to generate the clock you need and then
propagates the appropriately divided output to block 5D which demodulates the
data and passes it to the framing logic in block 2G. Three different guys
design those blocks, and they're later implemented in separate PALs, or in a
single PLD.
And marketing droids sit on the design 'Go ahead' for months and then
want it yesterday.
They're state machines, but nobody uses them
except for repair parts nowadays,
since it's more efficient and cheaper to have 100 GALs costing well under a
dollar and not having to track stock and worry availability. For simple
logic, I always have some 22V10's, 20RA10's, 16V8's and a few 20V8's
around
just for the case where I need some function I don't have in TTL. I don't
usually buy the TTL any longer unless it's for a repair. I do scrap old
hardware and save the parts, since that saves me driving around. I do have a
tester, after all.
I use TTL because 1) I don't have programer ( got any schematic for one
that does not use PAL's and easy to find TTL) 2) Pals never are the
right size to replace simple but messy logic -- say a D-F/F followed by
a 4/1 multiplexer followed a xor gate and another D-F/F.
I know that wooden ships were a beautiful and quiet solution to the problem of
how to get from England to Central America, but there are lots of them on the
bottom of the ocean, and I'd rather take my chances with a 747. Does that
mean that flying is better than sailing, well, maybe not, but it is the method
of choice, for most of us, nowadays. If I have two weeks and a lot of budget,
I like cruising, but if I have to be at a meeting tomorrow, and don't want to
put on ten pounds, then I fly.
Old sailing ships used a lot of man-power. While OIL is cheap wind
powered modern ships will not be developed.
--
Ben Franchuk - Dawn * 12/24 bit cpu *
www.jetnet.ab.ca/users/bfranchuk/index.html