FPGA VAX update, now DIY TTL computers

woodelf bfranchuk at jetnet.ab.ca
Sat Nov 12 11:49:26 CST 2005


Allison wrote:

>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  
trigger).

>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.

>Allison
>  
>




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