FPGA VAX update, now DIY TTL computers

[Allison]

>
>Subject: Re: FPGA VAX update, now DIY TTL computers
>   From: woodelf <bfranchuk at jetnet.ab.ca>
>   Date: Sat, 12 Nov 2005 10:49:26 -0700
>     To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
>
>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 :)

So throw away the excess bits.  It's still cheaper.  Also there is a 
Single full adder TTL part the 7480.

>>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.

Well the punch is ahrd but a HS reader is trivial to build AS it's 
been done many times.

as to putting it CPLD or FPGA, yes you could but I'm saying/staying in 
TTL with available SSI and MSI functions it's possible to be chip count
reasonable.

>>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 ).

I have nearly 30 IDE drives all under 500mb I can build around.  Its
easy to make them look like a RK05 or whatever to a PDP-8 databreak
interface.  What MS does next year or even lsat years does not impact 
me at all.

>>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?

The clock change for 7474 is postive side and 7473 is negative going side. 
That only a start, their logically differnt animals.  Also some types
of D ff can have a metastable state (both outputs high or low) under 
some cases! 

>>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.

Ah foo.


Allison