25 Jul
2013
25 Jul
'13
1:19 p.m.
Except it's not really that simple. What
you're really saying is:
- I'm wasting my time RE'ing the 9830 because you've already done
it.
- My time would be better spent RE'ing something else because
you've already done the 9830.
- I should use your schematic of the 9830 instead of doing it
myself.
Not at all. I was just curious as to why you'd spend considerable time
working o na machine that was already docuemtned (not just my schematics,
nut also the HP patent (which I discovered _after_ figuring most of it
out...)
I have no interest in your advice or suggestion about how I should
spend my time, or where my efforts would best be expended.
Here are some reasons I did not use your schematic:
- The 9830 was one of the first two computers I used. When I was
using it as a kid I never had the opportunity to examine it
internally. When, thanks to Rob, I recently acquired one, I was quite
interested in going through it thoroughly, and - as tedious and
labour-intensive as it is - I take some enjoyment in RE'ing it.
Exceellent. They certainly are nice machines and an ineresting design.
And I will cerdtainly agree that reverse-engineering something is a good
way to learn about that design (I will admit to doing so even when the
full service manual is available for this very reason).
- I have seen some of your schematics. I expect they are
competently done and reasonably accurate, and may be useful to others
in the absence of anything else. They fall far short, however, of
meeting my standards in areas such as presentation, layout and
functional organisation.
Oh well... We can't all be perfect.
I downloaded your 9830 schematic the other night and took a look at
it. I would have had little interest in wading through 89 pages of
your hand-drawn scratch for a machine as complex as the 9830. I'm not
telling you you should move to CAD or computerised drawing - if you
prefer to hand-draw them that is certainly your preregotive. (I like
hand-drawing schematics for a simple item every now and then, just
for the memory and tactile sense of it). But there are real,
FWIW< I have yet to see a modern CAD-produced schematic that I can easily
read. Most are plain ridiculous. This must be a personal thing, what I am
used to, etc, but I much prefer the draughtsman-produced diagrams of
30-40 years ago.
I cna't really explain it. Certainly multi-colour schematics drive me
mad. But it's more than that. If a CAD system has automatically routed
the 'wires' they never look pleasing to me.
functional, disadvantages to hand-drawing, in addition
to other
objective shortcomings of your schematics (absence of IC and
connector pin numbers, for instance), and you have ZERO basis from
Well, I have to disagree with those commonets...
Firstly, I do not number the connector pins becuase I draw out every
connecotr with the orientation lavelled on the diagram. I find this a lot
easier to follow than a single connector being scattered over several
shets with pins labelled 'J1/5 or J2/F or whatever. With the conenctor
drwan out I can say things like 'OK, that pin should be ground -- it is,
now the one I want is 3 down o nthe other side'. I find I make many fewer
errors that way.
Secondly, IC pin numbewrs. Every IC has all the connecitons unambiguously
described. If the pins all have unique names (like a RAM, or a counter or
a '154 decodoe) then I simply use those names or something obviously
equivalent to them That is why tyou see things like signal BRA(2) going
to a pin on an 11/03 RAM labeleld A0. BRA(2) is 'Buffered RAM Address'
bit 2. The 'bit 2' meanming it comes from address line 2 of the processor
eventially. A0 is the pin on the RAM that Intel called A0 in the databook.
Nwo, for simpler TTL ICs, I have a convention. For things like AOI gates
I put the pins in alphabetical order from the TTL databook going dwon the
page. And for simple gates I draw them so that the 'upper' pin on the
schemtic is towards the pin 1 end of the IC. So that, for example
---------|\
| >o---------
---------|/
U5c
'00
Means
10
---------|\ 8
| >o---------
---------|/
9 U5c
'00
THis may not be obviouus, but all you had to do was ask me...
which to bitch when somebody else looks at them and
says: 'Sorry ..
not interested'.
-
And BTW: there is what appears to be an error in your 9830 schematic.
In the tape drive optical sensor circuit, you have the 18.2K resistor
drawn as a feedback resistor from the output of the 741 back to the
LDR and negative input. In all 4 instances of the circuit I have
seen, the resistor goes to ground rather than the 741 output - half
of a voltage divider with the LDR, not a feedback resistor. The 741
runs open-loop as a comparator.
YEs, you sre correct. I had already spotted that one and corrected my
master copy of the diagram. Alas this has not made it to the electronic
version yet.
-tony