What would it take...

[Peter C. Wallace]

On Thu, 7 Dec 2006, Chris M wrote:

> Date: Thu, 7 Dec 2006 17:13:59 -0800 (PST)
> From: Chris M <chrism3667 at yahoo.com>
> Reply-To: "General Discussion: On-Topic and Off-Topic Posts"
>     <cctalk at classiccmp.org>
> To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
> Subject: Re: What would it take...
> 
> that's all relevant of course, if it's a big
> production house, producing thousand or millions of
> boards. But the outifits that make these retro-fits
> (LOL LOL no pun intended) are done in a garage more
> then likely. So that being said, just use individual
> ic's. Or perhaps it's alot about protecting their
> investment. But I would also have to say a
> considerable amount of development time goes into
> designing something that way.
> Why doesn't someone just draw up PLANS to build this
> stuff, and sell that? I'd buy it for sure (well, if it
> was for something groovy I owned...).

Well even if you only make 50, a GAL may save a lot of after the fact PCB 
editing with an X-acto knife...

Also a CPLD (maybe a 9536XL) is about as cheap as a GAL now ($1.00 or so), 
much more capable, and programmable with nothing but a parallel port (JEDEC 
ISP)


>
> --- Ethan Dicks <ethan.dicks at gmail.com> wrote:
>
>> On 12/8/06, Chris M <chrism3667 at yahoo.com> wrote:
>>> --- Lee Davison <lee at geekdot.com> wrote:
>>>> Two 8 bit buffers, two 8 bit latches and one
>> GAL20V8
>>>> or similar.
>>>> The GAL isn't essential but saves four or more
>>>> packages to do the address decoding.
>>
>> Note the "or more" here... it's not hard to imagine
>> reasonable
>> situations where one GAL saves 6 or 8 popcorn TTL
>> parts, especially
>> where you start using latched outputs.
>>
>>>  So 10 discrete ic's. Why all the fuss then? And
>> why
>>> bother with the GAL at all if it only saves 4
>> chips?
>>>  I really have to wonder why these developers go
>> to
>>> all the trouble they do if that's all it takes (I
>>> would have guessed at least 20).
>>
>> There are two major reasons why a hardware developer
>> would use a GAL...
>>
>> 1) board space/interconnections are "expensive"
>>
>> When one is hand-wiring a project, replacing 4 16-20
>> pin parts with a
>> single 18 or 20 pin part, that's a significant
>> savings of effort.
>> Equally if one is trying to fit in a small space, 4
>> DIPs is a huge
>> amount of room these days (and there are PLCC GALs,
>> allowing a lot of
>> DIP circuity to fit in one cm^2).
>>
>> 2) GALs are easy to change
>>
>> Practically speaking, folks don't want to rewire a
>> board to change an
>> address setting, so you end up with address
>> comparators, jumpers, etc.
>>  With a GAL, for infrequent changes (once or twice
>> over the life of
>> the product, not once or twice per year), you can
>> arrange the logic so
>> that there are no jumpers, but the GAL still does
>> internal address
>> compares, etc.  The Spare Time Gizmos Elf2000 does
>> this - if you don't
>> like what devices are selected by which combination
>> of N-lines, etc.,
>> take the (provided) GAL source, change it, recompile
>> it with WinCUPL,
>> then burn a GAL to your liking.  They are re-usable,
>> so you don't even
>> have to buy a spare unless you'd like to do a quick
>> swap back.
>>
>> The downside, of course, is that as the
>> hobbyist-end-user, you are
>> somewhat out in the cold if you don't own a GAL
>> programmer.  They can
>> easily run to hundreds of dollars for basic ones,
>> and, unlike an old
>> 4K EPROM, they are not trivial to make programmers
>> for from scratch.
>>
>> I have an older GAL programmer, so I don't mind
>> GAL-based designs.
>> The only thing that's eluded me lately is
>> programming "C rev" Lattice
>> GAL22V10s.  I've had no problems with burning older
>> Lattice GALs or
>> other sizes of new Lattice GALs, so it's been an
>> inconvenience, but
>> not a show stopper.  OTOH, I'm willing to look
>> around for a newer
>> programmer and consider an upgrade.  I'm not going
>> to give up GALs
>> entirely, so I'll find some solution when the need
>> arises.  Others
>> (those not already posessing GAL programmers) might
>> find the jump to
>> be too expensive to justify.
>>
>>
>> -ethan
>>
>
>
>
>
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Peter Wallace
Mesa Electronics

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