2 Oct
2008
2 Oct
'08
10:52 a.m.
On Wednesday 01 October 2008 16:59, Tony Duell wrote:
Waht does? The fact that HP desktop computer service manuals are anything
but, or the fact that I am trying to do soemthing about it?
The former of course, and I don't know why you'd ask me such a silly
question...
Well, sometimes I can be a pedant, and one thing that irritates _me_ is
the fact that English is not a precise language ...
[Graphics PCB RAM chip allocation]
Very few HP classic desktop computer service manuals
contain
scheamtics. You might, if you are lucky, get PSU schemaitcs, but not
for the processor, etc, sections. I am, of course, trying to remedy
this...
That stinks. At least it was possible to figure it out.
Yes. The circuitry is a little odd -- there are 16 4416 RAM chips, and
the data pins are not liked -- effectively there's a 64 bit databus on
that card. Each RAM chip can be written individually from the approprate
4 68K data lines -- there are a couple of '138s which select the
appropriate chip (or pair of chips for a word access -- 2 pixels at a
time) for wriing. I forget how the data is handles for reading, I think
there re 8 off 8 input multiplexer chips which bring the 64 data lines
down to 8, these then go to the appropriate 68K data lines again
('appropriate' here means the low nybble of each byte, so likes 0-3 and
8-11). The decoders and muxes are controlled by the low 3 bits (A1-A3) of
the 68K address bus.
The data also endes up in 4 16 bit shift registers (a total of 8 '166s I
think), which send it a a 4 bit serial pixel stream to the colour lookup
table and thence to the DACs.
I guess this was the easiest way to get the necessary pixel rate from
slow-ish DRAMs.
Having worked all that out, it was still not obvious what diagnostic
error was telling me. Since it occured at the first RAM location in the
graphics RAM (there's a pattial memory map in HP's CE handbook for this
machine), I first looked to see eich RAMs were being written to. When I
found it was only 4 of them, all on the high half of th word. I guessed
the diagnostic stopped as soon as it found an error, and that this was a
simple read/write test. Then, by carefully grounding RAM data pins
(rememebr everything was buffered, so this wouldn't crash the processor)
and seeing how the R: value changed when I re-ran the diagnostic, I
figured it out.
Then it was just a matter of desoldering the RAM chip and fitting a
replacement.
It's a nice feeling, isn't it? I miss that
lately.
Indeed yes!. I love it when a dead machine, or one with a reported fault,
works perfectly again.
[..]
I prefer to
use a 'scope or logic analyser whenever possible (and when
finding the memory problem I've just desicribed, I did look at the RAS/ CAS/
etc signals first...)
The scope was my most-used tool for that sort of thing, and I never had the
use of a logic analyzer, mostly working for myself I could never justify the I don't think I've seen many diodes in TO-3
packages. Plastic ones, yeah.
This is the modern equivalent of an EZ80 or whatever you call it :-). 2
diodes in one (To3) package with a common cathode connection (broung out
to tthe case) and separate anodes (to the 2 pins). It's used for the
classic biphase rectifier circuit with a centre-tapped transformer. I
think I've seen them in DEC machines too.
[...]
> I wonder
if it would be possible to get from some aftermarket
> suppliers? I
Als I doubt it. IT was not a common machine, so I doubt anyone bothered
to make a nrw flyback for it.
How certain can you be that they went and made one that was unique to
that machine? Different
strokes, etc. I suspect that with a little digging it might
be possible to find out who actually made that. They likely didn't make
it themselves. And an off-the-shelf product is much cheaper than one
that's custom-made for one specific product.
THis is HP we're talking about... Did you figure most of it out or come across it
somewhere else?
Now, given that I've had this machine in many bits over the last few
months what do you think?