FPGA VAX update, now DIY TTL computers
ajp166 at bellatlantic.net
Sat Nov 12 09:34:33 CST 2005
>Subject: Re: FPGA VAX update, now DIY TTL computers
> From: woodelf <bfranchuk at jetnet.ab.ca>
> Date: Fri, 11 Nov 2005 23:12:46 -0700
> To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
>The quick logic alu design is about 20 TTL per bit and
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 am planning 18 bits wide... say 50 ents each for a 74XX gate that
>is $10 per bit.
>Rounding that to 20 bits that is $200 for the logic. Good sockets are
>about $200. Not knowing PCB layout say a square inch per chip ... 20
>@ 75 cents a inch ... $300 for the PCB's ... That is where my money
>OH well I have all next year for that project, but I have two goals
>for the ALU
>and Control so I can have feel for what the 60's Computers were like
>1) 14 pin DIPS. 2) 7400 logic.
Likely you'll never build it. 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.
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 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.
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.
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.
>I can get a 200 watt PC power supply ... 5 volts @ 20 amps that I
>hope is ample PS. As for getting 7400 off scrap boards, if I had
>the scrap boards I would most likely try to get the scrap item working
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.
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