Introduction to PC-based logic
How could a
PC-based logic analyser help me?
A logic analyser helps debug and
validate digital electronic circuits. It is a tool that allows numerous digital
waveforms to be acquired and viewed simultaneously. An oscilloscope can only capture two or four signals at once, the logic analysers we sell capture up to 8, 16 or 32 signals at the same time. A logic analyser processes
data across buses, something a scope can't do.
Why PC-based analysers? In the
past, logic analysers were large expensive "stand-alone" instruments only big
companies could afford. With universal access to PCs, a new type of instrument
been developed using the "common resources" of the PC, i.e. its display,
keyboard and processing power. An important example of these cheaper instruments
is the logic analyser.
The advent of USB ports has
improved these new instruments. Older PC based
logic analysers typically used the legacy parallel printer port. USB ports are now
more common, faster, can be physically more accessible, and provide automatic
recognition when the analyser hardware unit is connected. Perhaps most useful of
all they provide operating power, avoiding the need for a separate power source.
This all means that now small
companies and even individuals can afford to benefit from the diagnostic power
of the logic analyser.
FAQ: What to
look for in a PC-based logic analyser
This section is a collection of
some important aspects to consider while comparing the various PC based logic
analyzers available on the market today. (Text
© Janatek Electronic Designs, author E.J.Theron).
here to view and download a reference article containing this
information, use your browser back button to return)
1.What are the most important aspects to consider when buying a pc based logic
1.1 Quality and
1.2 Affordability and Cost Effectiveness
2. Some common pitfalls of buying a cheap logic analyzer.
2.1 Poor Quality and Unreliability
2.2 Necessary features being left out
2.3 Bad Design Practice
2.4 Essential parts not supplied
2.5 Expected things left out
2.6 Bad after sales service
2.7 Product Piracies
3. How fast should the sampling rate be?
4. Why is a deep sampling buffer important?
5. Would a small buffer with data compression be good enough?
6. How many channels do I need?
7. Other criteria to consider?
7.1 Sufficient input bandwidth
7.2 Input Impedance
7.3 Variable Threshold Voltage
7.4 Versatile Trigger Options
7.5 External Clock Input
7.6 Easy to use software, manual and help
7.7 Nice Extras
7.8 After sales support from dealer and manufacturer
7.9 CE and other compliance standard
7.10 Complete Package
1.The most important aspects to consider
1.1 Quality and Capability
Why is it that a logic analyzer
from one manufacturer costs $2000, while another manufacturer sells their
product, which at first glance seems to have the same specifications (or even
better), for just $400 ? Whether you buy apples, a logic analyzer or a car,
there is always a distinction between good and bad quality.
If anything, quality in electronic test instruments is more important than in
most other products. If an apple has a small bad spot you can cut it out and
enjoy eating the rest, but if your logic analyzer is the source of intermittent
glitches, you could spend days trying to sort out problems that actually
originate in your test instrument and not in the hardware you are trying to
A good logic analyzer is not a simple instrument to design and manufacture. It
connects to hardware by means of many channels using “lengthy” probe leads and
ground(s). It has to cope with fast switching busses which can generate
If “cheap” is a measuring instrument’s main design objective, you will pay
extra for it by solving bugs, which actually originate in the instrument. A
quality product gives you measurements you can trust.
Affordability and Cost Effectiveness
Cost effectiveness: What you
need is an instrument of good quality that offers the features that you actually
need at a reasonable price.
Some instruments offer features that are very sophisticated and expensive to
produce and support, but it may be something that you may never need. An example
of this is processor code assembly features, whereby you can capture data
synchronously with the processor read and write signals and assemble the code to
see exactly what the software code is that the processor has executed. This
function is generally provided for specific target processors, it is very handy
for the users that require this functionally, but most engineers would never use
it, so don’t pay $500 extra if you are never going to use it.
If you work on relatively low frequency circuits, you do not need to buy the
logic analyzer with the highest sampling rate on the market.
pitfalls of buying a cheap logic analyzer
2.1 Obsessive cost saving leads
to poor quality and unreliability
Poor quality leads to poor signal integrity. Poor signal integrity leads to
you having a hard time debugging your hardware.
Low quality PCBs, omitting PCB testing by PCB manufacturer, low quality
workmanship(!), poor quality connectors, leads, capacitors, no burn-in QA
A product that is suppose to measure high frequencies must be able to cope with
problems that come with high frequencies, e.g. poor quality ground leads, would
not be effective against e.g. high frequency skin effects. A very high quality
ground lead can easily cost 50 times more than a very poor quality lead, but as
good grounding is very important it is well worth paying those extra few
An instrument with many channels that specifies that it can handle high
frequencies must be able to handle high frequencies on many of its channels,
without being overwhelmed by the amount of noise generated by the switching
noise created by these inputs.
Poor connectors, probe leads:. Poor connectors corrode, whereas gold plated
connectors will give many years of excellent connectivity. Good quality probe
leads not only improve signal integrity, but are usually also very flexible,
making the experience of using the instrument just that more pleasant.
All this means that you may be debugging problems that arise from your test
instrument and not the hardware that you are actually trying to debug. It may
take you many hours before you realize that those glitches are generated by your
cheap instrument and not from your development hardware. This can quickly wipe
out any “savings” that you made by buying a cheap low cost measuring instrument.
In the end you normally get what you paid for. (top)
2.2. Obsessive cost saving
leads to more costly, but necessary, features being left out
Typically the product would not
have a large buffer size, but rather a very small one. Stay clear of those!
Some manufacturers would implement a small buffer from the available ram in
their low cost PLD chip and try to convince you that the 4K (or even less!)
buffer that they provide is enough and that you do not need the 1Meg buffer
supplied by other manufacturers. They may try to point out that they use
hardware compression that increases the size of the buffer. Don’t be fooled by
this. If hardware compression was the ultimate solution to buffer size problems,
all manufacturers would be using this. Reality is that hardware data compression
is limited in its use and can in fact have the result of reducing the buffer
size in some very common test circumstances! (more)
Note that low cost PLDs have many IO pins and can easily provide many channels,
if no external ram is used and inferior input buffering and threshold detection
is used. DO NOT be overly impressed by the number of channels provided. Buying
a good quality 8- channel logic analyzer is much better than buying a cheap
36-channel one. (top)
2.3 Obsessive cost saving leads
to bad design practice
design practice leads to bad signal integrity, reduced reliability, poor (more
expensive) repair support, etc.
Below are a few typical results if saving cost is put above all other design
Using double layer boards, despite
the chip manufacturer’s strong recommendation of using at least 4-layer PCBs
Insufficient power decoupling at
Insufficient or no bulk
Insufficient or no onboard power
regulation. There is for instance heavily relied on the power regulation of the
PC power supplied. A good PC power supplied may help a bit, but even the best PC
power supply cannot replace even basic power regulation onboard inside the
instrument. Proper power filtering is expensive, so the manufacturer of cheap
products simply don’t do it.
Self regulation/ calibration, self
diagnoses built in test (BIT) functions are all simply left out of the design.
These components have effect on accuracy and improved support, after and even
before any hardware failure.
Protection circuitry such as over
Voltage/Current protection is simply left out.
2.4 Obsessive cost saving leads
to essential parts not being supplied
Check what is supplied in the
packaging before you buy.
Test clips are expensive. After you bought your logic analyzer you may find
that it does not have test clips included! So now you will have to shop around
for test clips and buy a small quantity at excessive cost. Going back to the
manufacturer, they will try to convince you of your own stupidity for not adding
test pins to your design/pcb onto which their test leads that end in crimp
contacts can fit. They will of course supply test clips to you – not so cheaply
Good test clips are important, even if you use a BGA on your board. Be sure that
they are included in the package and add their cost. (top)
2.5 Obsessive cost saving leads
to parts that actually cost very little not being supplied
Don’t be surprised if your
cheap logic analyzer comes without any packaging material, not even in an outer
Some don’t even have the software on CD, manual, etc. True, you can download the
software from the internet and print your own manual, and your logic analyzer
may still work if some heavy objects were shipped on top of it in the cargo
carrier, but is this really what you wanted and how you wanted it? If you see
yourself as a professional person, then buy professional tools of your trade.
2.6 Obsessive cost saving leads
to bad after sales service
A product that is suppose to have
sophisticated features, has a reasonable chance of creating technical queries
for the manufacturer.
If the manufacturer makes little profit out of his product, his profit can be
wiped out by having to deal with problems or simply general questions from the
Such manufacturers would normally simply refuse to accept that there could be
anything wrong with their product and would simply blame the user for “not using
the product correctly”. Do not expect any help if you buy an inferior product.
2.7 Obsessive cost saving
sometimes leads to product piracies
Hardware and software developments
are expensive. If the hardware protection of a product is insufficient, it is
possible for fraudulent companies to copy the hardware and use the original
manufacturer’s software on their illegal hardware.
Some cheap products are actually illegal copies made of products of authentic
developers. If you get problems with such a product, you can forget about
getting support. They may mostly in fact not be able to help you, since they
have no detail knowledge of the product.
Some manufacturers build in sophisticated hardware protection, which can in fact
be activated as soon as they become aware of clones appearing on the market.
You may find yourself stuck with a product for which the security hardware
has been triggered, disabling the hardware. (top)
This is required for high capture
The higher the sampling rate the more accurate the representation of the
captured signals on the screen.
With higher sampling rates, more accurate measurements can be made between edges
on different channels.
In analogue electronics people may often refer to the Niquest theorem that
states that to recreate any captured signal you need to capture the data at
least twice the frequency of the maximum frequency component present in the
A logic analyzer of course captures square waves. The highest frequency
component in a square wave is infinite as required to create the sharp edges of
the square wave. So at what frequency should a signal be captured relative to
the base frequency of the square wave?
A logic analyzer simply distinguishes between a high (signal higher than the
input threshold) and a low (signal lower than the input threshold). Say a 1kHz
square wave is captured at 4 kHz sampling rate. This means that the incoming
signal could be sampled twice while the signal is high and twice while it is
low. This will result in the signal being displayed with a 50-50 mark-space
ratio. If there is distortion on the signal and say the threshold is not set
correctly for the incoming signal, you could easily end up sampling the signal
say once while high and three times while low and the signal will be displayed
with a 1-3 mark-space. This means you need to push up the sampling rate. At 5kHz
sampling rate, with everything set correctly it is easy to see that you will
most likely capture the signal say 3 times on high and 2 times on low and later
on 3 times on low and 2 times on high, resulting that a nicely regular incoming
signal may be displayed with irregular mark-space features. In short you should
push up the sampling rate even further. We recommend that you should try to
sample as high as possible as allowed by your buffer depth. A deep sampling
buffer is important (more on that in the next section). In general a sampling
rate of 10 times the frequency of the incoming signal is sufficient. This also
explains why logic analyzer manufacturers would not limit the sampling rate to
say 2 times the input bandwidth. (top)
4. Why is a deep sampling buffer important?
A large buffer allows longer
captures without lowering the sampling frequency.
To capture high and relatively low frequency signals simultaneously, both a high
sampling rate and a large buffer are needed for a meaningful measurement. A
high sampling rate without a large buffer is of little practical value if your
signals have both low and high frequency components. For example, say you
need to measure a very high frequency serial data stream which is accompanied
with a low frequency strobe that denotes a frame of 64 bytes. To be able to
capture the high frequency data meaningfully you need to sample at a
sufficiently high frequency. If your data buffer is too small the low frequency
strobe would not be completely captured before the buffer is full. To capture
the low frequency strobe you need to bring down your sampling rate, but now the
sampling rate is too low to get a meaningful capture on the high frequency data.
So now it becomes difficult to capture and view your data. With a large buffer
you would have enough depth to set a high sampling rate and capture both the
high frequency data and the low frequency strobe. Now, to view the bigger
picture, you would simply zoom out and to see the high frequency details, zoom
5. Would a small buffer with
data compression solve the problem above? (top)
If a logic analyzer line remains
low and never changes state, it is easy to see that the data can easily be
compressed to a few bytes, which simply indicates that the channel never changed
state. This requires very little memory depth.
If the signal changed state once, all you need to record is the initial state
and the sample number where it changes stage. This takes a few more bytes, but
you are still saving a lot of memory.
If the signal you capture has a relatively high frequency, you would still need
a few bytes per transition and if the frequency becomes comparable with the
sampling rate you would soon move into the situation where the compression
requires much more memory than when the signal was simply captured without any
compression and where every sample requires only one bit per channel. This means
that in the presence of high frequency signals the compression may indeed
decrease memory depth.
Another point on hardware data compression is that the compression circuitry is
situated between the inputs and the memory buffer and causes propagation skew
between channels. This skew is difficult to remove. When capturing straight into
memory the data path is straightforward, resulting in little channel to channel
The conclusion is that hardware data compression has advantages when
capturing slow signals, but has severe limitations in the presence of high
frequencies and cannot replace “real” deep memory.
6. Number of channels
The number of channels determines how many signals can be captured at the same
When buying a logic analyzer, do not be overly impressed if a large number of
channels are offered at low price. The quality of the channels, whether they are
backed up by an adequate buffer and have a quality input stage is more
important. It is better to buy a logic analyzer with a few good channels than
one with many poor channels.
Many channels combined with a very small buffer is of value only to view short
data sequences, such as a single read/write to a ram chip, but if you wish to
zoom out to get the bigger picture, you will be disappointed wishing you bought
a more professional instrument.
With the many IO lines available on today’s PLDs, it is quite easy to provide
many channels at low cost. All you need is a DAC to create a threshold reference
for all channels, and then a few small surface mount resistors and capacitors
per channel. The PLD provides a small amount of ram which can be used as a data
buffer. In this way you can make a “logic analyzer” that costs very little.
Whether you provide 16 or 128 channels does not add much to the price. To create
a top quality logic analyzer the cost per channel is much more than just adding
a few capacitors and resistors and putting on a bigger external connector. This
is the reason why a good 8 channel logic analyzer would most likely cost more
than a 64 channel product as described above and why it makes sense to rather
buy a quality 8-channel product.
It is surprising how far you can go with just a few logic analyzer channels,
once you became “wise” in the usage of logic analyzers. Personally I think that
16 channels are more or less the optimum required for debugging most circuits.
This includes debugging boards with processors with many address and data
lines. If you have gone through the pains of connecting 128 test clips of a
large logic analyzer to your 32-bit processor with many address lines, you would
most likely never do it again and rather start using your logic analyzer in a
“smart way”. You will be surprised what you can do with a 8-channel logic
Criteria to Consider
7.1 Sufficient Input Bandwidth
This specification indicates the maximum frequency that can be measured.
A logic analyzer input acts as a low-pass filter and the “bandwidth” normally
indicates the -3dB point where the input signal size has been reduced to half of
its low frequency amplitude. The logic analyzer thresholds can be adjusted to
cope with the signals getting smaller. Independent threshold voltages on
different input ports, allow different thresholds for different signals.
The maximum sampling rate should always be at least four times higher than the
maximum input bandwidth. This factor of four is needed for a reasonable
representation of the signals after capture.
If the input bandwidth is too high compared to the sampling rate, external
switching noise may be introduced to the logic analyzer circuitry, by signal
frequency components, that are anyway too high for effective capturing. Such
noise only serves in degrading the capture integrity.
7.2 Input Impedance
The ideal measuring instrument can pick up information from the unit under test
without influencing the functioning of the unit under test at all.
The input impedance should be as high as possible (high resistance and low
capacitance), such that the instrument would not add excessive load to the unit
under test. (top)
7.3 Variable Threshold Voltage
The variable input threshold is needed to measure signals of different
amplitudes. An independently variable threshold voltage is preferable for every
eight inputs. Independent thresholds allow measuring different technology types
at the same time.
It is becoming increasingly common to have components operating on 5V, 3.3V,
2.5V, etc, all on the same PCB. If you buy a logic analyzer with many channels
and only one threshold for all of these channels, you are likely to have
problems, measuring on mixed technology boards, especially when the signal
frequencies becomes high.
When input signal frequencies are relatively low, the actual threshold voltage
is not all that important, because a perfect 2.5V logic signal will be a square
wave varying between 0V and 2.5V and a 5V signal will vary between 0V and 5V.
This means that a threshold of say 2V will display both logic types correctly.
In the real world though the square waves will look more like sine waves for
relatively high frequencies, will have a DC offset and will diminish in size as
a result of input bandwidth limitations. This means that especially for “high”
frequencies independent threshold adjustments become more important.
7.4 Versatile Trigger Options
This enables you to capture the exact data you want to see.
You need to trigger on edges, patterns, edges and sequences, e.g. edge
condition, then pattern. Deep sequencing is seldom necessary and is limited to
2 to 3 stages by some manufacturers to keep the user interface easy to use.
7.5 External Clock Input
Synchronous capture is usually used to capture clocked data, using a clock
signal from the hardware under test.
For example you could capture data read by a processor by using the processor
read signal as clock input to the logic analyzer. As every sample is of a
distinctly clocked moment it is usually best displayed as a text listing.
7.6 Easy to use
software, manual, help
Easy-to-use software and manual: Of course!
A logic analyzer is an instrument that you may use intensively for a while and
then you may not need it again for 6 months or a year. It is therefore very
important that the software is easy to use and you would not need to go through
a heavy learning curve each time you use it.
It is easy for manufacturer’s to pack a lot of features into the onboard
programmable chips, but the problem is that this could lead to overcomplicated
user interfaces, in which the user has trouble finding the commonly used
features between the many often completely unnecessary features that clutter the
setup dialog boxes.
For manufacturers the secret of balance is to provide the important features
clearly and in an easily understandable way and not to allow features that users
would never use to overcomplicate the user interface.
7.7 Nice Extras
How about an onboard pattern generator to provide signals to your unit under
test, while the logic analyzer simultaneously captures the response
You can use this to set up communication protocols to send to your unit under
test, or to simply create a controlled clock to your circuit. (Have a look at
the Janatek La-Gold-36!)
Some manufacturers provide assembler features. This may add quite a bit to the
price of the logic analyzer and should only be considered if you really need it.
7.8 After sales support from dealer and manufacturer
Both the manufacturer and dealer need to provide good before and especially
after-sales service. Capable sales personnel should know the basics of working
with a logic analyzer.
Difficult technical question regarding the usage and details of the specific
instrument should be referred to the manufacturer, which should be answered
Most PC-based logic analyzer manufacturers supply the latest software updates on
their websites for free. (top)
7.9 CE or other compliance standard
This indicates that the product
meets certain electromagnetic emissions and susceptibility and safety standards.
7.10 Complete package
If everything you need is not included in the package, e.g. test clips, you will
waste time finding it and pay more for it. Quality test clips are expensive. Not
including them into the package is a way in which some products appear much
cheaper than what they really are. Good test clips are important, without which
the usability of the instrument is severely limited. (top)