Cheap Logic Analyzer Examined

While experimenting with my AS3935 lightning detector, I needed my Saleae logic analyzer to view the SPI protocol to make sure I was accessing the sensor correctly.

I ended up at Saleae’s website to download new software, and, to make a long story short, I discovered that the current logical analyzer models are really expensive.

I purchased an original Saleae Logic 16 Analyzer for $299 and at the time that was kind of a stretch for my budget as I didn’t really expect to use it much (and honestly haven’t – maybe once or twice a year).

Of the current models, Saleae’s least expensive 8 line logic analyzer is $479 and the 16 line model is $1399. Yikes!

How can a hobbyist justify that price? If I were looking again, I would not be able to purchase a Saleae.

I wondered if there were any alternatives and quickly found logic analyzers for about $15 on ebay. OK, does a $15 device actually work? I wanted to know so I ordered one.

I’ve gotten into the habit of calling cheap stuff from China FC-cubed (F.C.C.C). Cheap and Chinese are 2 of the words in the acronym. I’ll let you guess the other two. Since there is no Mfg/Model info for this logical analyzer, I’ll refer to it as FC-cubed.

I ended up buying one of these on ebay for $9.89:

a week later I had it:

Review

My intention is not to test the heck out of this logical analyzer, but to see if it works at all and if so get an idea if it would work for the light duty logic analysis I do.

My Saleae Logic 16 has 16 lines and a sample rate from 500kS/sec to 50MS/sec (the new logic 16 will sample at 500MS/sec which I’m sure is part of why it is so expensive).

The FC-cubed logic analyzer has 8 data lines and a sample rate from 25kS/sec to 24MS/sec. OK, 1/2 the sample rate and 1/2 the data lines, but 1/30th the cost.

The FC-cubed is obviously very cheap. The connectors are just cheap crappy dupont connectors that were hard to use.

BUT, it does work!

Testing

When I first started looking at these logic analyzers, I saw mention that they would work with the Saleae software. So my plan was to test with their software first, then some open source software I heard of called sigrok.

My test is VERY simple. I connected my function generator to the logic analyzer and just examined one line on the logic analyzer.

First, here is the output using Saleae’s Logic 16 logic analyzer monitoring a 100kH signal on channel 0. Exactly what I would expect:

My function generator was reporting 100.5kHz, so this is right on.

Here is what I saw when I did the same test using the FC-cubed analyzer:

Channel 0 is what I expected, but what is channel 1? My guess is it is seeing a harmonic of Channel 0.

I moved the lead to channel 3 and now I see harmonics on channel 2 and 4:

Not impressive, but you can always hide unmonitored channels so you won’t see these bogus signals.

Next I increased the function generator from a 100kHz signal to a 1MHz signal. It saw this fine:

I increased my function generator to 2MHz signal which is as fast as it will go. The FC-cubed analyzer sees it just fine:

Now going in the reverse direction, I slowed the signal down to 1kHz:

Besides trying different signal frequencies, I also verified that the trigger worked correctly.

Using sigrok/PulseView

I assume that Saleae did not intend to just give their software away for free to be run on FC-cubed logic analyzers. This is really quite unfair to Saleae. The FC-cubed manufacturers can only sell their cheap products because someone else put the time into the software side. I wouldn’t put it past these manufacturers to have simply copied firmware as well. Not saying they did …

It turns out there is another group who has been writing software to take advantage of the cheap logic analyzers. That seems a little more fair.

I only learned of this software when researching the FC-cubed logic analyzer. It is open source software that can do logic analysis for many different logic analyzers, including these dirt cheap ones.

Their website is

https://sigrok.org/wiki/Main_Page

I downloaded and installed PulseView. Not quite as simple to use as Saleae’s software, but still it only took a few minutes to get it figured out.

Here is PulseView running against the FC-cubed logic analyzer monitoring the 1kHz signal:

Interestingly, the ghost signals do not show up in PulseView.

Protocol Decoders

The other extremely useful feature of a logic analyzer is it’s ability to decode signals like SPI. Can PulseView do that? Yep! In fact, there appear to be way more decoders for it than for Saleae’s software.

Bottom Line

For $10 you just can’t beat the FC-cubed logic analyzer. This is exactly what  is needed by hobbyists. Something cheap to get your foot in the door.

I should add that many new Oscilloscopes can decode some protocols like SPI, I2C, and serial.

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