Re: 7854 vs 7D20

Harvey White

On Wed, 13 Dec 2017 07:37:27 -0000, you wrote:

Unfortunately the 7854 did not have floating point math, a fast microprocessor, or lots of storage
things you need to do an FFT) so it is not practical except as a proof of what the 7854 was
capable of.

Dennis Tillman W7PF
The 7854 uses a TMS9900 processor, with a maximum clock speed of 3MHz. Takes me back to 1981 in my
first paid job, designing video switching networks driven by that processor. Massive ceramic package
that needed a great amount of force to push into a turned pin socket.

Because it was so sluggish, that processor design worked right off. I put all the bodge logic into a
PAL. The one gotcha is that Texas had different division for processors and memories. And each of
them labelled their buses the opposite way. Bastards. One has A0/D0 as LSB and the other had them as
MSB - so there was a massive bit scramble on the buses. While I was moaning about my not noticing it
and beating my chest, and just about to run at the circuit board with a knife and wirewrip, our
brain-on-a-stick software guy said "Don't do anything - I'll just do a bit scramble in software.
It'll be a security feature!". I could have kissed him; well not really, but you get the idea that I
was a relieved guy.
I ran into the same thing for the TMS9914 in a very early 488 bus
control design. The hardware engineer "guessed" wrong. I was the one
doing the software, so it was easy enough to redefine the commands and
flip the data in software.

That software fella was of course a bit weird. You could never catch him doing any work. He was
either drinking coffee, was at the pub, or eating smelly food at his desk. But magically 100 lines
of bug-free and beautifully commented code appeared at the end of each day. Blow the compiled
finished code into a PROM and it would work right off. Damndest thing. I only once accused him that
his code didn't work. He asked what the symptom was, and immediately without noticeable pause and
looking over his pint of beer said "bit 4 of the third BCD switch from the left - the wire has
fallen off". He was of course absolutely right.
There are people like that. Not sure how he managed it.



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