FPGA VAX update, now DIY TTL computers
bfranchuk at jetnet.ab.ca
Sat Nov 12 11:49:26 CST 2005
>Must only be using 7400 and 7404s doing it the hard way. As far back
>as '68 ALU blocks were availble, sure they cost $4 each then but the
>chip savings was there.
I was thinking 7401's ( 2 input OC Nands) and 7414's ( hex schmitt
>Likely you'll never build it.
True, but the chalenge is there. PS if I add as many displays as you say
I need this
will at least look impresive.
> Moving up just one step integreation wise
>is the 7483 (it equivilent is PDP8 era) full adder. That alone will cut
>the ALU chip count.
This is 18 bits ... 2 or 3 or 6 or 9 is needed here, not 2 or 4 or 8
or blah! 16 :)
>My fun was not replicating the archetecture in an acient way but using the
>most MSI TTL parts I could get to replicate it in a moden way. IE: can I
>build a TTL PDP-8 with 1kx4 MOS ram in minimum TTL count for everything
>else. One can simplify that to working replica rather than exact replica.
The 8 reduces well for modern chips but the lack of a TTY or a high
speed punch/reader and a
dumb terminal is what is preventing me from build a 8 in 3 CPLD's.
>The difference is 2 'ls273 for a 16 bit register or 8 LS74s. But it goes
>further with fewer sockets, pins and wires, power and debug time. That
>also relects reliability once working as used parts are going to be
>a bit shakey untill (re)infant mortaility is again worked out from
>removal stresses. That may be minor but with a 200-400 peice TTL system
>you would be surprized unpleasently with how bad bring up can be if there
>is any uncertanty in the parts used.
I plan to get new parts, but this TTL design will be slower version the
CPLD cpu I am building,
I am going backwards here since other the IDE drive and moden ( 90's )
memory don't want
to have rely on M$ if I want to make a hardware change. ( Ok not quite
true as I'll be using
windows for PCB and CAD work ).
>When I did the 200 peice TTL system it was 1972 into 73 and we were using
>the then common silicone plastic TTL aka the gray plastic and those were
>not reliable. That system took over a month to debug between wiring
>errors, timing errors (races and spikes) and new but partially dead chips.
>and after it was working for about three months after it was cranky when
>hot till we weeded out a few more bad actors.
This is making me think twice to goto LS. Now what I need help is with
the J/K flip flops (TTL).
7473 -- "Do not change J/K while the clock is high." But I want to
STOBE J/K when the clock is
high for a D F/F. Will this work in practice?
>One thing I'd suggest for those building something greater 50 TTL is lots
>of LEDs to indicate the state or status of a block of logic plus the ability
>to slow the clock if possible to near DC so you can watch it do stuff
>without a fast logic analyser. It's a great fault finding tool.
PS. I got a $5 brick for this years project. :)
>Theses days free PC power supplies (usually free scrap AT form factor)
>and cheap brick form factor switchers make 20A at 5 volts a trivial deal.
No wait the brick err door stop is the old PC.
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