1 Jun
2006
1 Jun
'06
2:56 p.m.
An interesting question.
If you have a module of the same age, I don't see any problem with a
module swap.
My main objection, actually, is to _random_ module-swapping, as
suggested
by certain HP calculator service manuals. How you can _know_ the fault
has been cured if you don't know what/where it is is beyond me. That's what I do, but my modules are probably
logically rather
smaller. One printed
circuit board holds one 3 input not-J not-K flip-flop. Another type
has four And gates.
But why not repair them? I can't believe they're that complicated
to test
or debug. And I would have thought the components were a lot easier to
find than complete modules. When I eventually run low on spare modules I will
have to start
component level
repair, hopefully being able to make 3 good modules out of 4 bad
ones.
That is one reason why you should never throw away a defective PCB. It
might well be repairable later, or you might be able to use parts
from it
to fix another board. OK, 'almost never' -- ISA cards with one ASIC
and
not much else don't count :-)
No, not much hope there.
Roger Holmes.
1962 ICT 1301 mainframe nut (and Apple software author)
3 Jun
3 Jun
5:40 p.m.
An interesting question.
If you have a module of the same age, I don't see any problem with a
module swap.
My main objection, actually, is to _random_ module-swapping, as
suggested
by certain HP calculator service manuals. How you can _know_ the fault
has been cured if you don't know what/where it is is beyond me. I have scrapped an almost complete machine, so I have
many of the
commonest
modules. I understand digital electronics but I find it hard to get
my head around
the analogue electronics on the boards. Testing bare components is
'Digital circuits are built from Analogue parts' (one of Vonada's lawas
IIRC).
I haev never understood how you can understand digitial electronics
properly and not understand analogue electronics. I certaimly couldn't
understnad digital stuff until I understood things like transmision
lines, termination, etc.
hard enough
but testing them in circuit is tricky, especially if they require
half a dozen different
supply rails at weird voltages such as -18.0 -17.1, -12.6, -6.3,
-4.6, -2 and +12.6.
I think if I had a machine like that to maintain, I'd make up a test rig,
with a power supply giving those voltages.
1960s components which look right are not easy to get new. The
To be honest, I don't care too much about the appearance of the
components, provided they are electrically correct. There are certainly a
few obvious modern replacements in my 1968 HP9100B.
-tony
10:53 p.m.
On Saturday 03 June 2006 05:40 pm, Tony Duell wrote:
'Digital circuits are built from Analogue
parts' (one of Vonada's lawas
IIRC).
I haev never understood how you can understand digitial electronics
properly and not understand analogue electronics. I certaimly couldn't
understnad digital stuff until I understood things like transmision
lines, termination, etc.
I think it's the way that it's too often taught these days -- people come out
of that *thinking* that they understand it, when they don't, really.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
3 Jun
3 Jun
10:56 p.m.
On 6/4/06, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
'Digital circuits are built from Analogue
parts' (one of Vonada's lawas
IIRC).
I haev never understood how you can understand digitial electronics
properly and not understand analogue electronics. I certaimly couldn't
understnad digital stuff until I understood things like transmision
lines, termination, etc.
Well... as someone who considers his own knowledge of analog circuits
to be inferior to his own knowledge of digital circuits, let me say
this - while I _know_ that at the lowest level, all digital circuits
are really just collections of analog parts, in practice, as long as
you aren't working with a design at the edge of the envelope, you can
ignore 90% of analog theory and still have an understanding that works
on the circuit in front of you.
For example - if you are building a clock out of TTL, if you follow
the recommendations from the various chips data sheets about fan-out,
etc., it's not that difficult to come up with a design that requires
nothing more elaborate than a logic probe to debug. In fact, if you
build an electronic debounced momentary switch, you can use a DVM to
"scope" the circuit, pulsing it as needed to step through the states.
OTOH, I've seen mysterious problems take a long time to solve when
*completely* overlooking the analog aspects of a TTL circuit - one in
particular was undershoot and ringing with a 74S409 DRAM controller
hooked up to a field of 32 4164s. The eventual solution was to insert
33 Ohm resistors in-line on the CAS and RAS lines to dampen the
undershoot to where the ringing didn't cause a problem - it was still
there, but at an amplitude that didn't have a negative effect. I was
amused to see the identical solution 2 years later in a Commodore
"slap-on-the-front" RAM expansion for the Amiga 1000 - the card was
literally the connector, one capacitor per DRAM, the DRAM, and a
couple of 33 Ohm (38 Ohm?) resistor packs.
So there are certainly many cases where it _is_ important to have a
good grounding (pun intended) in analog theory (transmission lines,
parasitic capacitance, hidden inductance, RF effects, etc), but for a
lot of TTL work, pretending that everything is exactly +5 and ground
will still get you a working circuit. When it _doesn't_ work, though,
be prepared to pull out all the conceptual tools you can. Obviously,
the faster the clocks and the more complex the design, the analog side
does start to rear its head earlier.
-ethan
4 Jun
4 Jun
5:04 p.m.
On 6/4/06, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
The problem comes when you end up with a circuit that works 90% _of the
time_ becuase you've got a reflection somewhere that you shouldn't, or
coupling between gates (either due to stray capacitance between traces,
or becasue of inadequate power rail decoupling, or...) that you don't
want.
One thing that's _very_ important to rememebr is that the frequency you
need to consider when designing is not determined by the clock frequency
you're using, but by the sharpness of the edges of the logic signals
(that is, the rise/fall times). Most modern chips (things like CPLDs,
etc) have quite fast rise/fall times. You may only have a 1kHz master
clock on your board (or whatever), you still need to do the design
assuming there are multi-hundered-MHz signals around.
'Digital circuits are built from Analogue
parts' (one of Vonada's lawas
IIRC).
I haev never understood how you can understand digitial electronics
properly and not understand analogue electronics. I certaimly couldn't
understnad digital stuff until I understood things like transmision
lines, termination, etc.
Well... as someone who considers his own knowledge of analog circuits
to be inferior to his own knowledge of digital circuits, let me say
this - while I _know_ that at the lowest level, all digital circuits
are really just collections of analog parts, in practice, as long as
you aren't working with a design at the edge of the envelope, you can
ignore 90% of analog theory and still have an understanding that works
on the circuit in front of you. OTOH, I've seen mysterious problems take a long
time to solve when
*completely* overlooking the analog aspects of a TTL circuit - one in
particular was undershoot and ringing with a 74S409 DRAM controller
hooked up to a field of 32 4164s. The eventual solution was to insert
33 Ohm resistors in-line on the CAS and RAS lines to dampen the
Yes, series termination. Very common in such circuits. And that's another
thing I recoemnd. Read -- and understand -- as many complete circuit
diagrams as yoy can. Not the cut-down, ideallised ones you find in some
so-called text books. But real ones, from actual working products. You'll
soon see where termination is considered essential.
No, I am not suggesting copying complete commercial designs and claiming
them as yor own. But you can pick up tricks from such circuits. The list
of references in my Ph.D. thesis includes several to PDP11 printsets and
technical manauls. Not because I copied the PDP11 (I wasn't even building
a processor), but because some of the tricks pulled in the PDP11/45 to
save a few ns were, when translated into F TTL and ECL, were very useful.
Alas most students never get to see a complete, working circuit diagram
of anything. Once I met a chap who'd just graduated in engineering, and
had specialised in RF work. I put the scheamtic of a discrete-transistor
AM radio (nothing special, a superhet of course) and asked him to explain
what each of the components was for. He was totally lost. I don't think
he even spotted the IF transformers.
undershoot to where the ringing didn't cause a
problem - it was still
there, but at an amplitude that didn't have a negative effect. I was
amused to see the identical solution 2 years later in a Commodore
"slap-on-the-front" RAM expansion for the Amiga 1000 - the card was
literally the connector, one capacitor per DRAM, the DRAM, and a
couple of 33 Ohm (38 Ohm?) resistor packs.
I am trying to think of a DRAM board using 4116ws, 4164s or 41256s that
doesn't sereis-terminate the RAS/ and CAS/ lines and probably the address
lines and WE/ line as well. 33 Ohm or 68 Ohm resistors are common for
this. I rememebr soldering in dozens of the darn things when I upgraded a
part-filled Unibus RAM board.
So there are certainly many cases where it _is_
important to have a
good grounding (pun intended) in analog theory (transmission lines,
parasitic capacitance, hidden inductance, RF effects, etc), but for a
I learnt years ago that the most difficult part of a digital circuit to
understand is the wire (OK, PCB traces :-)) linking the gates together.
-tony
6:47 p.m.
Alas most students never get to see a complete,
working circuit diagram
of anything. Once I met a chap who'd just graduated in engineering, and
Sol Libes just reminded us at this years VCF/east that DEC
printsets were so open that a poor student with a summer free
would just order the printset and build themselves a PDP-8 from
scratch if they wanted one bad enough.
John A.
11:05 a.m.
Tony wrote:
I haev never understood how you can understand
digitial electronics
properly and not understand analogue electronics. I certaimly couldn't
understnad digital stuff until I understood things like transmision
lines, termination, etc.
You can understand it well enough to use for many
practical purposes by just
knowing a few "rules of thumb"*. A better, but still not necessarily
detailed, understanding of the analogue background can help considerably in
some of the more tricky situations - especially if you are
"stretching"/abusing the "rules". Too detailed a consideration of the
background can actually get in the way [the best example I have of this is
the realisation that one has much more tolerance in termination than
analogue theorists tend to expect - half or double the normal termination
resistance does produce reflections ... but they are small enough that
digital inputs will ignore them even though they would be a disaster in,
say, an audio or RF amplifier].
* eg, "fanout";
"use lots of decoupling - then add some more";
"keep connections short - and be very cautious of long adjacent runs";
"If mixing LSTTL and CMOS don't connect both types of input to the same
output";
and so on.
Andy
11:20 a.m.
From: "Andy Holt" <andyh at
andyh-rayleigh.freeserve.co.uk>
Tony wrote:
---snip---
Hi
Still, Tony has a point. In order to do intellegent trouble shooting, one
has to be able to understand how the analog elements of real circuits
effect how a circuit will function incorrectly. Sure, one can hunt and
swap parts until it works but to know the part you are unsoldering is the
cause requires true understanding of what that part is failing to do
I haev never understood how you can understand
digitial electronics
properly and not understand analogue electronics. I certaimly couldn't
understnad digital stuff until I understood things like transmision
lines, termination, etc.
You can understand it well enough to use for many
practical purposes by
just
knowing a few "rules of thumb"*. A better, but still not necessarily
detailed, understanding of the analogue background can help considerably in
some of the more tricky situations - especially if you are
"stretching"/abusing the "rules". from an analog sense.
Of course, one is
hopelessly lost fixing power supplies without some
analog knowledge. Understanding feedback and how each part effects
the other is the key to fixing these parts. I see so many times that
some fellow will say, I've replaced all the capacitors and it still doesn't
work. They ask for help but I often don't know where to start. Should
I begin with elementary electronics or just use them as a remote VOM and
oscilloscope.
Dwight
5:17 p.m.
Of course, one is hopelessly lost fixing power
supplies without some
analog knowledge. Understanding feedback and how each part effects
I would love to see anyone understand an SMPSU without understanding
analogue electronics...
the other is the key to fixing these parts. I see so
many times that
some fellow will say, I've replaced all the capacitors and it still doesn't
work. They ask for help but I often don't know where to start. Should
I begin with elementary electronics or just use them as a remote VOM and
oscilloscope.
That depends, IMHO, on who it is, and what they are trying to do. If they
just need to get the machine up and running again, then ask them to do
some tests, and tell them what component to change. If they want to learn
about repairs, then explain what they are doing.
Anyone who askes me how to fix their classic computer gets the latter
approach. I think it's important that you should understands what you are
doing. I remember talking to a chap in Germany about repairing a
(digital, but the principle is the same) fault on an HP9810. I had a
pretty good idea where the fault was at the start, but I wasn't prepared
to tell him -- that is to make a guess. I got him to do a number of tests
(including teacing bits of the CPU microcode) to fidn out where the fault
really was. He ended up fixing his machine _and_ understanding the processor.
-tony
5 Jun
5 Jun
12:59 a.m.
On 6/5/06, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
That is, I think, why I have so much problem trying to fix them - my
understanding
of analog is inadequate.
-ethan
Of course, one
is hopelessly lost fixing power supplies without some
analog knowledge. Understanding feedback and how each part effects
I would love to see anyone understand an SMPSU without understanding
analogue electronics...
6:24 p.m.
I would love
to see anyone understand an SMPSU without understanding
analogue electronics...
That is, I think, why I have so much problem trying to fix them - my
understanding
of analog is inadequate. 
8:07 p.m.
Tony Duell wrote:
The main problem I have with fixing SMPSUs is missing some seemingly
minor fault (like a diode that breaks down under load only) which causes
all the nice, expensive, new components that I've fitted to blow at
switch-on. A series light bulb helps (if only to reduce the shock to _me_
when the thing tries to put a dead short across the mains), but won't
always save the expensive chopper transsistor.
We designed a 2KV, 2KW switcher many (MANY!) years ago.
Getting drive transistors was a *bitch* (expensive and
hard to come by!).
There was damn little you could do to "prevent" losing
the entire set in an ohnosecond. Then, spend the better
half of the day disassembling, looking for possible
faults in the design, soldering in a new set of Q's
and doing the same thing over again the NEXT morning.
Advice to others trying this: do it some MONTH when your
boss is on vacation!! :-/
I would love to see anyone understand an SMPSU without
understanding
analogue electronics...
That is, I think, why I have so much problem trying to fix
them - my
understanding
of analog is inadequate.
4 Jun
4 Jun
5:13 p.m.
Tony wrote:
True enough. I don't set up a TDR every time I wire up a simple logic
circuit.. (I have been known to assume that a twisted pair of wire-wrap
wire with 3 twists per inch has a chracteristic impedance of 120 Ohms,
though. That is near enough for most circuits!).
I haev never understood how you can understand
digitial electronics
properly and not understand analogue electronics. I certaimly couldn't
understnad digital stuff until I understood things like transmision
lines, termination, etc.
You can understand it well enough to use for many
practical purposes by just
knowing a few "rules of thumb"*. A better, but still not necessarily detailed, understanding of the analogue background can
help considerably in
some of the more tricky situations - especially if you are
"stretching"/abusing the "rules". Too detailed a consideration of
the
background can actually get in the way [the best example I have of this is
the realisation that one has much more tolerance in termination than
analogue theorists tend to expect - half or double the normal termination
A good understanding includes an understanding of tolerances IMHO. I have
no time at all for so-called engineers (and I've met plenty of them) who
can calculate component values to 10 significant figures but have no idea
how accurate they actually need to be (hint, for pull-up resistors a
factor of 10 may make little difference).
A good undersanding of the analogue side of 'digital eleectronics'
includes, therefore, knowing when you need to terminate a signal, and how
accurate that teminator needs to be.
-tony
6:16 p.m.
On 6/4/2006 at 10:13 PM ard at p850ug1.demon.co.uk wrote:
A good undersanding of the analogue side of
'digital eleectronics'
includes, therefore, knowing when you need to terminate a signal, and how
accurate that teminator needs to be.
In a sense, hobbyists have been spoiled by the simplicity of connecting up
TTL (and before that, RTL and DTL). Had the industry standardized on ECL,
things would be quite a bit more interesting on the design end.
Cheers,
Chuck
10:34 p.m.
In a sense, hobbyists have been spoiled by the
simplicity of connecting up
TTL (and before that, RTL and DTL). Had the industry standardized on ECL,
things would be quite a bit more interesting on the design end.
Yes, like programmiung in Pascal, engineers would need to follow all the
rules. Some might say that would be a good thing...
Anyway, some years ago I did some 100K design, and pretty much if you
follow all of the rules in the Fairchild book, nearly all of the weirdo
analog (radio?) problems drop from sight. It actually was not too hard to
get away with doing a good digital design while not knowing much analog
theory. You just had to pull out the ruler quite a few times - now how
long is that trace?
100G scared me, so I never did anything with that except get some parts
and datasheets.
William Donzelli
aw288 at osfn.org
11:04 p.m.
William Donzelli wrote:
Blech. It's awfully had to do anything *interesting* in Pascal...
If you want to force folks to carry around lots of excess baggage,
why not pick Ada? :-(
In a sense,
hobbyists have been spoiled by the simplicity of connecting up
TTL (and before that, RTL and DTL). Had the industry standardized on ECL,
things would be quite a bit more interesting on the design end.
Yes, like programmiung in Pascal, engineers would need to follow all the
rules. Some might say that would be a good thing... Anyway, some years ago I did some 100K design, and
pretty much if you
follow all of the rules in the Fairchild book, nearly all of the weirdo
analog (radio?) problems drop from sight. It actually was not too hard to
get away with doing a good digital design while not knowing much analog
theory. You just had to pull out the ruler quite a few times - now how
long is that trace?
I worked on a 100MHz (doesn't sound like much, 30 years later :> )
CPU in the mid 70's. The problems I found were all the *different*
ECL families (10K, 100K, MECL III, etc.) plus all the other
cruft to interface the real world to them (4000 series CMOS
for the JTAG stuff, other level translators for the "fast"
stuff). And, the colossal *power* requirements (>500W for
that CPU alone!).
Of course, with more integration, that wouldn't be as big an issue
(since a good deal of power is expended in the pin drivers).
But, the idea of having to debug with nothing metallic on
one's person (for fear of serious injury) was not something
I looked forward to...
100G scared me, so I never did anything with that
except get some parts
and datasheets.
11:26 p.m.
I worked on a 100MHz (doesn't sound like much, 30
years later :> )
CPU in the mid 70's.
Damn impressive for the 1970s. What CPU?
The problems I found were all the *different*
ECL families (10K, 100K, MECL III, etc.) plus all the other
cruft to interface the real world to them (4000 series CMOS
for the JTAG stuff, other level translators for the "fast"
stuff).
10K sucked. MECL III sucked. 100K was the way to go, except for the
relatively skimpy selection. And never mix...
And, the colossal *power* requirements (>500W for
that CPU alone!).
Real computers used real power supplies.
William Donzelli
aw288 at osfn.org
11:57 p.m.
William Donzelli wrote:
It was a custom device. I.e. the CPU was fabricated entirely
of discrete ECL chips. The size of a medium sized briefcase.
But you couldn't get "everything" in any given family.
Hence the problems...
Yeah, I always enjoyed "adjusting" the -Vbb supply -- a
*shunt* regulator fabricated with a whopping big Lambda
power supply driving *BIG* diodes to ground... "select
at test". <shrug> Crude but it *worked*! Power supplies
that big can't descriminate between their load and a dead
short! :-(
I worked on a
100MHz (doesn't sound like much, 30 years later :> )
CPU in the mid 70's.
Damn impressive for the 1970s. What CPU? The problems
I found were all the *different*
ECL families (10K, 100K, MECL III, etc.) plus all the other
cruft to interface the real world to them (4000 series CMOS
for the JTAG stuff, other level translators for the "fast"
stuff).
10K sucked. MECL III sucked. 100K was the way to go, except for the
relatively skimpy selection. And never mix... And, the
colossal *power* requirements (>500W for
that CPU alone!).
Real computers used real power supplies.
12:03 a.m.
On 6/4/2006 at 10:34 PM William Donzelli wrote:
Yes, like programmiung in Pascal, engineers would need
to follow all the
rules. Some might say that would be a good thing...
A little anachronistic, don't you think? Pascal didn't come until about
1970. Maybe ALGOL-60?
Cheers,
Chuck
4:31 p.m.
In article <m1Fmdr5-000IxvC at p850ug1>,
ard at p850ug1.demon.co.uk (Tony Duell) writes:
I haev never understood how you can understand
digitial electronics
properly and not understand analogue electronics. [...]
Simple. Digital electronics implement logic and for the most part
you can ignore any analog effects within reason (I don't care about
rise time when I'm concerned with the correct logic, for instance.)
Start out by learning programming, then have someone teach you
digital electronics reasonably competently.
Then have someone awful try to teach you analog electronics from a
crappy book using antiquated terminology. (When was the last time
you heard someone refer to a capacitor as a condenser?)
Further, make sure that you limit meaningful lab time or give lab
assignments that only embody trivial analog and digital electronics.
End result?
You have a good handle on boolean logic and digital circuitry but
don't know f*ck all when it comes to analog crap.
Welcome to the digital option for the Electical Engineer curricula at
the University of Delaware from 1982-1986.
I didn't realize how ripped off I was at the time until I came to the
University of Utah in 1988 and learned that the undergraduate CS
students were building more digital hardware in 1 year than we had
built in 4...
--
"The Direct3D Graphics Pipeline"-- code samples, sample chapter, FAQ:
<http://www.xmission.com/~legalize/book/>
Pilgrimage: Utah's annual demoparty
<http://pilgrimage.scene.org>
10 Jun
10 Jun
4:28 p.m.
On Mon, 2006-06-05 at 14:31 -0600, Richard wrote:
Then have someone awful try to teach you analog
electronics from a
crappy book using antiquated terminology. (When was the last time
you heard someone refer to a capacitor as a condenser?)
When I looked in the workshop manual for my car? ;-)
Gordon.
11 Jun
11 Jun
5:14 p.m.
On Mon, 2006-06-05 at 14:31 -0600, Richard wrote:
YEs, for some odd reason motor car manufacturers refered to said
component as a 'condenser' long after the rest of the world had stopped
doing so.
Although the workshop manual for the Austin A60 (the nearest one to hand
:-)) referes to it as a '.2 microfarad metallized capacitor' (and it is
identified as such on the exploded diagram of the ignition distributor).
-tony
Then have someone awful try to teach you analog
electronics from a
crappy book using antiquated terminology. (When was the last time
you heard someone refer to a capacitor as a condenser?)
When I looked in the workshop manual for my car? ;-)
6594
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23 comments
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participants (12)
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allain＠panix.com -
andyh＠andyh-rayleigh.freeserve.co.uk -
ard＠p850ug1.demon.co.uk -
aw288＠osfn.org -
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legalize＠xmission.com -
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rtellason＠blazenet.net