Re: Searching for broken PLAs

From: Gerrit Heitsch <>
Date: Sat, 07 Jul 2012 14:14:23 +0200
Message-ID: <>
On 07/07/2012 03:17 AM, Bil Herd wrote:
> Are you going to mention how we had the polaroid's of the Signetics part
> up in the chiplab?

When you did your own version of the 82S100, did you just convert the 
design into a mask-programmed version of the 82S100 or did you implement 
the logic terms with random logic?

I assume it was just a mask programmed 82S100 in the end...

> The PLA's were
> in some way not very boring as they were our last salvation to pull a
> problem out of the fire, and our chance to impart any coolness in logic.

 From what it does, I always thought that replicating the function would 
have taken a LOT of TTLs which would have increased space and power 

It's not without strangeness though... Watching the _CS signal for the 
Char-ROM of the C64 on a scope shows some very strange curves. Like
LOW, followed by a maybe 20ns HIGH, then another 120ns LOW and finally 
HIGH again (resulting in the CPU not getting data from it after all!). 
Or for the VIC: About 80ns LOW, then 120ns HIGH and only then finally 
LOW for VIC access...


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