Re: 264 Series and their chips

Hi!,


On 2011-08-11 18:32, Gerrit Heitsch wrote:
> 
> So I noticed when I tried it yesterday evening... Oh well, another
> pretty theory shot to hell...
> 
> The reason I came up with this idea was that I read a posting on this
> list where someone stated that the 6510 and the 8500 have the full 8 bit
> port on die and that you could use some clever coding to distinguish
> between a 6510 and an 8500. Go look for the thread called '6510 and 8500
> differences'.

Ah, Nicolas Welte's findings about the unimplemented portbits of the
6510/8500 (...one kind of stuff I like cbm-hackers for :-) ).

Thinking it over, even that one seem to contradict with the idea of a
full 8-bit port that was supposed to be implemented on the 6510/8500's
die. Bits of the output register don't fade out by the corresponding I/O
pins absence. That might rather suggest that MOS already had a 8-bit
port equipped CPU design, maybe more than one CPU design with several
features incorporated into the 6510 at the time they actually needed the
6510, and pasted/stripped parts as needed, with no care for leftovers
(ie. those that didn't hurt them).

It's interesting though, that they didn't change the design once they
switched to HMOS-II. I thought that move needed a more or less complete
redesign of the chip. Still, the artifact has been kept (it's only the
manufacturing process that results in different time until the fadeout
happens), which suggest they didn't change the layout. ...I'm really
looking forward for this thing to be demistified.

...Hmmm, as I can see, visual6502.org don't list either the 6510, or the
8500 (...and not even the 8502) in their collection yet.

> Looks like Commodore really did make 3 different CPUs instead of one and
> then just using differend bonding options.

Seems so...

>> Contacted them, will see what happens (also seem to have a 6529B and
>> probably a 8551 for that purpose... who knows...).
> 
> The 8551 is just an HMOS version of the 6551(A), so there shouldn't be
> much of a difference. Maybe a tad smaller.

Possibly (they also don't seem to list the 6551 yet).

> The 6529B is an odd chip, no reset, no PHI2, no DDR... And I've only
> seen it in the 264 series and I've never head about one going bad.
> Replacing it with TTL chips would still need a handful though.

Definitely. The single broken one that I have (and ever encountered)
just died because of some Chuck Norris descendant who seem to have
removed most chips from this Plus/4 motherboard, and put them back by,
err, applying some force. So that a couple of pins of the chips just
broke off / some broke out of the packaging :-/.

Did design a partial replacement myself last year, actually, as part of
a control board design of an internal 256K memory expansion hack for the
Plus/4. It indeed took some TTLs to keep the thing fully compatible to
the original design.

> Even the original VIC did produce S-Video (not called that back then),
> but the VC20 output only gave you composite.

Yeah.

Actual (PAL) color signal quality still slightly suffered, IMHO. ...But
that's one thing we're still loving the VIC-20 for, hehe... :-)

> There is one thing I have to do soon, find out why the video output of
> all C16 I have seen so far contains enough chroma signal on the luma
> signal that most monitors will only need the luma signal for a color
> image. Could be the routing of the traces between TED and modulator or
> the modulator itself that's 'leaking' since it's the part that produces
> the composite signal.

That's the RF modulator. The luma signal doesn't look contaminated until
it enters the RF modulator (checked with an o'scope). And it looks
terrible, right after it exits. So I suppose it's the RF modulator.

It's interesting to note that the RF modulator used in the 264 series
differs from those in any C64 motherboard versions.

I couldn't find how/why this all gets wrong in the RF modulator yet.
Zimmers has schematics diags for at least two different RF modulator
versions in the Plus/4 service manual,
http://zimmers.net/anonftp/pub/cbm/schematics/computers/plus4/manual/
(both NTSC unfortunately; from the other hand, I tested it and as far as
I can see the problem also affects NTSC machines, so the schematics
should still be relevant, after all). They definitely looked suspicious
in the way the composite signal was generated (the circuit looks pretty
"simplified"; at least I couldn't see how the luma signal should be free
of chroma interference after the point the composite mixing was done).
...Unfortunately, it's a pretty tough process to mess with the rf
modulator in practice. Even after removing the top of the shielding box,
one founds the RF PCB mounted upside down. Did attempt to measure things
with little to no success yet :-/.

> I have a little cicuit (from a german forum for
> the C64) that allows you to remove the modulator in a C64 and still have
> S-Video output. The picture looks much better but you no longer have
> composite video, only S-Video. This should also work with TED, the
> output drivers on the chip seem to be the same as in the VIC-II. The
> circuit is just a 1N4148,  2 transistors BC547 plus a few resistors and
> 2 capacitors.

Yeah, if you replace circuitry that interfaces the thing to the outside
world (ie. video signal buffers) that should greatly help.

Fixing the rf modulator (learning how ie. the one found in late C64s
work and modifying the Plus/4 modulator accordingly) might be another
option. ...Possibly, it just won't worth the effort. Removing and
disassembling the unit is already a pretty risky and tough job :-/.
Incorporating an RF modulator from a C64C might be another option,
though, it possibly won't fit (...well, at least not easily), and not a
bit simpler to do than fixing the original unit.


Levente

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