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Hardware 101

"Note however that a scope with 100MHz bandwidth cannot
measure a 100MHz digital signal with adequate definition."
Click to expand...

Chuck(G) said:
....because waveforms can be viewed as consisting of combinations of
higher-frequency components ......

..... where an o-scope also gives you information about its form.
Click to expand...

Does it matter whether it is an Analog or a Digital O-Scope?

ziloo
 
ziloo said:
Does it matter whether it is an Analog or a Digital O-Scope?
Click to expand...

It does--DSO's are given to a somewhat different but related problem called aliasing. If the sampling rate is too low in relation to the signal being sampled, the results can be anything from inaccurate to (as with many things digital) downright bizarre.
 
DSO vs Analog

DSO vs Analog

I'd have to say the analog, all things being equal, and not knowing what the sample rate of the DSO is.
DSO's have a characteristic bandwidth, but they also have a spec for sampling rate, and the sampling rate is what will determine how well defined your displayed signal is. You can have a DSO with a wide bandwidth, but a fairly low sample rate, which means you will not see too much detail on your captured waveform. The higher the sample rate for a given bandwidth, the more resolution you get. If you really want to get into the nitty-gritty on picking a DSO, or to find out about the aliasing, you can google Nyquist.
Usually you'll get a smoother roll-off with the analog, so it'll be easier to interpret signals near the bandwidth (the -3dB point) than with the often rather sharp cut-off on some DSO's (this is to deal with the aliasing Chuck(G) mentioned)

patscc
 
ziloo said:
Hello Chuck,

Would you please explain a bit about "phase comparator", and
how it is done hardwarewise...

ziloo
Click to expand...

Hi
Sorry, I dropped out for a while. I had a kidney stone operation
and that got septic. Spent a few days in the hospital on anti-biotics.
I thought I'd jump in here.
The idea is that we want to control a VCO. This is a voltage controlled
oscilator. We need to take something about the tracking of frequency
and create a voltage change to represent that change. This is
where the phase comparitor comes in. It is usually just a simple
gating curcuit that uses the reference as a gating such that if
the edge of the actual VCO frequency comes early, it will produce
a pulse saying it was early. If the leading edge comes after the leading
edge of the reference, it will say it was late. The width of
these pulses will be the same as the time between the edges.
These two signal control what is called a charge pump. This is the
digital to analog part to get to the voltage for the VCO.
It takes the early/late signals and delivers a constant current for the
duration of the early or late signals that is propotional to the
error. This is captured onto a capacitor such that early pulse
will change the VCO input voltage to reduce frequency and
late pulse will change the VCO input voltage to increase frequency.
In between phase compare cycles, the capacitor just holds the voltage
steady.
Dwight
 
edge-sensitive

edge-sensitive

I can dig level-sensitive TTL as in a regular transistor where
the voltage level opens the gate for current to flow. But,
how does the edge-sensitivity work(I mean on the solid-state
level)? What "entity" is sensitive to edge behavior and why?

ziloo
 
oscilloscope probe, tips

oscilloscope probe, tips

Hello to all hardware gurus,

Back in college years and in my EE lab classes, I would notice
the effect of the OScope probe on performance of the digital
circuit. In placing the probe of a 100 MHz OScope on a given
circuit, what are the important issues that one must be aware of.

Thank you

ziloo
 
Make sure to (duh) hook the ground lead to the circuit.

Ensure the scope probe has the proper bandwidth (at least equal to the scope or greater)

If you have a 10MHz scope probe on a 100MHz scope and you go look at a high frequency signal you'll find that it will be badly distorted.

Same thing for looking at a 100MHz signal with a 100MHz scope. It will look distorted - rounded edges instead of square and it might not even show a good, accurate vertical rise time.
 
All probes, regardless of their frequency rating, are not created equal. Anything you add to the circuit will contribute its own inductive and capacitive loading.

Getting a probe to match the scope you're working with is equally important. You can't take just anyone's probe and expect to get the best results from your scope.

Probes can range in price from very cheap to astronomically expensive. Active probes (the ones with a tiny FET in the tip) tend to be on the very expensive side, but also are the best as they contribute very little capacitive loading (usually a pF or less). You can also destroy an active probe with much less effort than a passive one.

If your probe has a built-in attenuator (e.g. 10x), use it--the probe will load the circuit less.

My guess is that the probes you used in class were the least expensive that could be gotten away with.
 
Logic Analyzers

Logic Analyzers

Hello to all Hardware masters!

If we can have a general discussion about the use of Logic
Analyzers for circuit troubleshooting, that would be a great
addition to this thread and our wiki.

As a novice, how should I use the logic analyzer to find
the problem with a non-functioning board with a Z80
CPU?

Thank you

ziloo
 
Talk to yourself much, Ziloo? :)

A logic analyzer generally presupposes some level of detailed knowledge of the operation of the equipment being worked on. Otherwise, the question becomes akin to "what do you use a microscope for?"
 
Chuck(G) said:
Talk to yourself much, Ziloo? :)
Click to expand...

Thank you Chuck for joining in :pray: !
I knew if I would start talking to myself, then I would get more
attention. It worked :evilgrin4: !!!! (For threads like this, Terry would
have been hysterical :lolsign: ........where is Terry when you need him?)


Very recently, you ccontributed to troubleshooting on a Ferguson
Bigboard 1, and as always there were very valuable hints and
suggestions for using a logic analyzer:

http://www.vintage-computer.com/vcforum/showthread.php?19425-Ferguson-BigBoard-1&highlight=Analyzer

Would you please elaborate more as to a step by step guide on how you
would work on a faulty board like that? I am more interested in your
style/approach of working on such a problem.

ziloo :mrgreen:
 
That's a tough one. But here goes:
  • First off, is the "is it plugged in?" test. In other words is everything getting power; are there any capacitors or fuses that need replacing? How do the on-board voltages measure against what they should be?
  • Are any critical peripherals (e.g. terminal, monitor, disk drive) properly connected and operating?
  • Is there a heartbeat? In other words, does the CPU have a clock? Are the address and data lines flapping around to show some activity? If not, do any accessible status lines look normal? (A simple logic probe can determine most of this. As complex as most CPUs are, they're usually the last things to fail.
  • Is there any output at all? If so, and it's not right, what might cause it? For example, if there's a memory-mapped display and the characters are wrong, then looking at the associated memory might be in order.
  • If the CPU seems to be operating, but there's no life, then usually the simplest thing to do is to write a few small tests and burn them into the boot ROM. Code already existing in the regular boot ROM can be a big help in getting started.
  • If that fails, then it's time for the heavy artillery. Put a logic analyzer on the data and address (and status, if you have spare probes) of the CPU and see what things look like. Obviously, you first need to know what they should look like--and there the datasheet for the CPU can be very useful.
  • The most common thing to fail it seems, is a disk drive, then power supply (or associated decoupling capacitors). Memory would be next. Cold solder joints, broken traces (and these can be very hard to locate) seem to be next most common. Finally, you get down to plain old TTL "glue" and the CPU itself--but the possibilities there are pretty remote.
  • Obscure problems can require tools. An oscilloscope, logic probe, and a good DMM should be in anyone's toolkit if they're serious. EPROM burner/eraser and logic analyzers will come in handy less often, but they're still very nice to have when you need them.
  • The "hammer and screwdriver" -- a good soldering iron, desoldering tools, a magnifier, small pliers, screwdrivers and an assortment of small passive components (including wire) are important. Whenever I have to order a special part, I order extras and squirrel them away. You may never need the extras (I have a cache of 50W high-voltage zener diodes that I'll probably never use), but they can be quite handy if you happen to have a suspicion.
  • And of course, the two most important tools--the squishy stuff between your ears and a sense of humor.

This list is by no means exhaustive or even well-thought out, but maybe it'll be a start.
 
Digital Phosphor Oscilloscope (DPO)

Digital Phosphor Oscilloscope (DPO)

Hello Heardware 101 fans,

Would you please elaborate on the following:

Each time a waveform is captured it is mapped into the DPO memory.
Each cell represents a screen location. The more times data is stored into a location,
the greater the intensity attached to it. In this way intensity information builds up
in cells where the waveform passes most often. The overall result is that
the display reveals intensified waveform areas, in proportion to the frequency of
occurrence of the signal at each point. This has the same appearance as those
displayed on an analogue phosphor oscilloscope, and this gives rise to the name.

Has anybody had experience with these O-scopes?

ziloo :mrgreen:
 
Last edited:
How low can you go?!!!!

How low can you go?!!!!

Overclocking is very common in the world of modern CPUs, but
I have read that for troubleshooting it was common to "underclock" the
early 8-bit computers.

Aside from single-stepping the CPU, there were CPUs that would allow
you to step through the T-state within the machine cycle. Some people
would also use a debounced push button replacing the colock/crystal to
step through the cycles. There are also problems with slowing down
the 8-bit CPUs.

I would like to know more about these tinkerings.

ziloo :mrgreen:
 
ziloo said:
Hello Heardware 101 fans,

Would you please elaborate on the following:

Each time a waveform is captured it is mapped into the DPO memory.
Each cell represents a screen location. The more times data is stored into a location,
the greater the intensity attached to it. In this way intensity information builds up
in cells where the waveform passes most often. The overall result is that
the display reveals intensified waveform areas, in proportion to the frequency of
occurrence of the signal at each point. This has the same appearance as those
displayed on an analogue phosphor oscilloscope, and this gives rise to the name.

Has anybody had experience with these O-scopes?

ziloo :mrgreen:
Click to expand...

sure. pretty much every digital scope made does this. its a way to fake the old CRO scrope screen. did you have a specific question?
 
ziloo said:
Aside from single-stepping the CPU, there were CPUs that would allow
you to step through the T-state within the machine cycle. Some people
would also use a debounced push button replacing the colock/crystal to
step through the cycles. There are also problems with slowing down
the 8-bit CPUs.

I would like to know more about these tinkerings.

ziloo :mrgreen:
Click to expand...

what did you want to know? Often you had ICE (In-Circuit Emulators) that sat between the cpu and the system. You had complex and simple ones. Simple ones like ZAX Z80 ice (I have this one) was basically a giant box that really was a Serial port. You connected PC to it via rs232, and sent text commands like you would use dos debug.exe, it had a z80 cpu on it, and basically, stopped the clock between each instruction. you pulled the cpu off the test board and plugged the ice box to the board and used its cpu.


there were more complex ones like the fluke 9100/ 9000 series.

It was to do with how the cpu would work. some cpu were better at this than others for providing hardware interface for ICE to sit on.

see here http://www.tauntek.com/z80-in-circuit-emulator.htm for a z80 ice clone (a nicolet nice clone)
 
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