Re: 264/TED/Plus4 Story

On 08/20/2011 06:21 PM, Bil Herd wrote:


> So there was ONE RULE when I got the Ted project, it has to have xxx number
> of chips (I think it was nine chip) and no more, signed Jack Tramiel.  Well
> the TED’s reset circuit consisted of an RC circuit and a 7407.  I proved it
> wouldn’t work but I didn’t think about what this really meant as to who had
> designed the computer (the answer was no-one had designed the computer at
> that point)  It was an easy argument to make that the computer would fail en
> masse, we HAD to add a chip for reset, didn’t matter which chip, we simply
> could not do it with a transistor or a 7407 gate. (yes I tried feedback to
> create hysteresis  )  The answer came back from the mountain that it was
> okay to add a chip.  I remember that people were stunned that I had
> challenged this (some thought I would be fired) and won.

That was the famous 555 timer chip if I read the circuit diagram right. 
Same way as the Reset was generated in the C64 on the older revisions.

Thanks to whoever added the reset button to the 264. It always irked me 
that the C64 didn't have one.



> But here’s where I screwed up, I missed the opportunity to understand why
> the reset circuit sucked and consequently why the un-buffered data lines
> scanned the keyboard and ultimately the joystick.

This design, even with the 6529 added caused me some confusion back 
then. I couldn't understand at first why the 2 joystick ports were wired 
in parallel and how the system was able to distinguish between them 
until I noticed that instead of GND they used a bufferd data line (D1 
and D2 if I remember right). This had the sideeffect that joysticks with 
built in auto-fire never worked right on a 264.



> To a chip designer it would make perfect economic sense to scan the concept
> of a keyboard with the datalines…. Perfect sense.

Maybe they got the inspiration from the Sinclair Spectrum for this. 
There they used the address lines of the Z80 (more or less decoupled by 
diodes so the lines can only be pulled low) for one end of the keyboard 
matrix. The single register of the ULA (where the other end of the 
keyboard matrix is attached) appeared at every second I/O-address so to 
scan the keyboard you only have to read that register using different 
addresses. The spectrum didn't use that trick for a joystick though.



> So once I finally “saw” the dataline problem I knew I didn’t want a hard TTL
> part to isolate the lines, hard driven lines are RFI/FCC nightmares,
> especially since they would be above the metal keyboard plate, not
> below/inside.  TTL also doesn’t like to be shorted out, even via capacitive
> loads.  I remembered seeing that MOS had a single port I/O and in my mind
> this was perfect, it was ssssllloooowww on the output side (I/O speed
> instead of databus), It was NMOS which was more forgiving for RFI and shorts
> AND it was made by MOS meaning we could control how many are available
> (there was an LS shortage at that time that plagued pipleines for a couple
> of years).  In less than two hours the tech (amazing techs) had “dead
>   bugged” the 6529 to the underside of the board and also made me up a
> special joystick cable.

Sometimes I wonder what MOS did with all those I/O-chips... The only 
place I have ever seen a 6529 is a 264 system.

I also own a C16 with a TTL chip with a MOS logo on it labeled '7714' in 
U12. That should make it compatible to a 74LS02 according to the 
schematic. I have seen other MOS-TTLs... Did MOS make those to tide over 
shortages? But why not label them 74xxx?

The list of those I know of:

7707   7406
7708   74LS257
7709   74LS258
7711   74LS139
7712   74LS08
7714   74LS02
65245  74LS245 (found in VC-20)


> From there we did
> do some interesting things that the chip guys wouldn’t have known like
> creating a 1/4th duty cycle Phi2 so we could attach chips like the 6551
> UART. (Actually we created a Phi0 so we would have data hold time)

That was the time the PLA appeared in the 264? I do remember seeing a 
C116 prototype that didn't have it. How did you get them to give you 
permission to add even more chips to the design?


> The end of the story was that I went to a CES show and woman pulled me into
> her booth to show me the educational software she had written for the C64.
> I remember her display had nasty lines in it, she didn’t yet have access to
> the R7 chips.

The 6567 before R7 were that bad? Since I live in PAL land I have never 
seen them and the 6569R3 and later always produced a very good picture 
if the Monitor was hooked up by Chroma/Luma.

  Gerrit


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