Logic Analyser on the Cheap

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Icky
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Logic Analyser on the Cheap

Post by Icky » Sat Nov 10, 2018 10:47 pm

I had noticed a few people on in this community using Logic Analysers (LA) to trace digital signals.

Not wanting to spend a lot of money on one such as the Saleae but wanting some nice software like Saleae I had a bit of a search on the interweb.

I ended up getting the following AZDelivery Logic Analyzer USB 24M 8CH 24MHz



Now here is the interesting bit - although this isn't as good as the Saleae you can download the software from them and it will work with this LA.

So for just over a tenner I have a working LA.

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Re: Logic Analyser on the Cheap

Post by sandord » Sat Nov 10, 2018 10:50 pm

Icky wrote:
Sat Nov 10, 2018 10:47 pm
I had noticed a few people on in this community using Logic Analysers (LA) to trace digital signals.

Not wanting to spend a lot of money on one such as the Saleae but wanting some nice software like Saleae I had a bit of a search on the interweb.

I ended up getting the following AZDelivery Logic Analyzer USB 24M 8CH 24MHz



Now here is the interesting bit - although this isn't as good as the Saleae you can download the software from them and it will work with this LA.

So for just over a tenner I have a working LA.
Wow, that's very interesting! I was actually browsing the webs for a logic analyzer a few hours ago but I was put off by the high prices.

So do you already have it on your hands?

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Re: Logic Analyser on the Cheap

Post by sandord » Sat Nov 10, 2018 10:51 pm

By the way, if there's something similar to be found that does function generation, I'd be interested as well.

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Re: Logic Analyser on the Cheap

Post by JezC » Sat Nov 10, 2018 10:59 pm

I'm not sure if it's the same model as this, but a colleague at work bought a very cheap logic analyser from China a few weeks ago...I'll ask him how he is finding it.

I don't think these cheap ones have much protection against over-voltage input (so it could fry the USB port on your PC if you slip & touch a high voltage point) but you can get USB isolators for a little more than the price of the L.A. so then the L.A> itself becomes a disposable item.

I've been looking at some of the USB-based 'scopes but haven't taken the plunge yet - anybody got any recommendations for those? Cheapness is a prerequisite. :lol:

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Re: Logic Analyser on the Cheap

Post by Icky » Sat Nov 10, 2018 11:06 pm

sandord wrote:
Sat Nov 10, 2018 10:50 pm
Wow, that's very interesting! I was actually browsing the webs for a logic analyzer a few hours ago but I was put off by the high prices.

So do you already have it on your hands?
Yep - Have my hands on it and it's working well as a disposable :)

Here is a thread where I have been using it with screenshot

viewtopic.php?f=19&t=1234&start=40#p13393

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Re: Logic Analyser on the Cheap

Post by czietz » Sun Nov 11, 2018 9:44 am

Differences to a genuine Saleae logic analyzer:

- Really shoddy input protection. They just use a 74HC245 buffer powered from 3.3V. When connecting to 5 V logic signals, you're already using it out of specs. They put series resistors on the buffer inputs so that it doesn't get destroyed, though. (Side note: For a few cents more they could have used a 5-V-tolerant buffer.)
- Only 24 MS/s sampling rate. May or may not suffice, depending on your application.
- No internal FIFO buffer. Depending on your PC, OS, USB drivers, other connected USB devices, etc. sometimes it might drop capture data.
- The test clips that come with those cheap LAs are also obviously very cheap and prone to slip off the IC's pin in just the wrong moment. I really recommend getting a proper set of test clips (ca. 20 € for a set of 10) unless you want to damage an expensive IC by an accidental short circuit sooner or later.

Note that you don't have to (illegally) use the Saleae software, these cheap FX2LP-based analyzers are also supported by the open source Sigrok/Pulseview software: https://sigrok.org/

In the end I found using a cheap LA so cumbersome that I bought a Saleae Logic 8, which isn't that expensive for professionally made test equipment.

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Re: Logic Analyser on the Cheap

Post by Icky » Sun Nov 11, 2018 10:19 am

czietz wrote:
Sun Nov 11, 2018 9:44 am
Differences to a genuine Saleae logic analyzer:

- Really shoddy input protection. They just use a 74HC245 buffer powered from 3.3V. When connecting to 5 V logic signals, you're already using it out of specs. They put series resistors on the buffer inputs so that it doesn't get destroyed, though. (Side note: For a few cents more they could have used a 5-V-tolerant buffer.)
- Only 24 MS/s sampling rate. May or may not suffice, depending on your application.
- No internal FIFO buffer. Depending on your PC, OS, USB drivers, other connected USB devices, etc. sometimes it might drop capture data.
- The test clips that come with those cheap LAs are also obviously very cheap and prone to slip off the IC's pin in just the wrong moment. I really recommend getting a proper set of test clips (ca. 20 € for a set of 10) unless you want to damage an expensive IC by an accidental short circuit sooner or later.
Good to know the differences. This is partly what I am looking for as in the end I would like a decent LA but have never tried one. Hence going for this cheap version before taking the bigger plunge.
czietz wrote:
Sun Nov 11, 2018 9:44 am
Note that you don't have to (illegally) use the Saleae software, these cheap FX2LP-based analyzers are also supported by the open source Sigrok/Pulseview software: https://sigrok.org/

In the end I found using a cheap LA so cumbersome that I bought a Saleae Logic 8, which isn't that expensive for professionally made test equipment.
That is my now intended route (Logic 8) as I now see that these things work and I can use them. I guess gone are the days of going down to your high street electronics store to try these things out before buying on the web. RIP - Maplin

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Re: Logic Analyser on the Cheap

Post by rubber_jonnie » Sun Nov 11, 2018 10:34 am

I just downloaded the Saleae and Pulseview software to try and they are both quite nice and offer demo modes so you can (sort of) try before you buy.

I probably will try one of these cheap clone LAs, because I don't do enough to justify the £150+ price tag for something like a Saleae Logic 4.

I also have a Hantek 2ch USB oscilloscope for the same reason, but it does serve a purpose.

I started off with one of these as a scope, https://www.ebay.co.uk/itm/2-4-DIY-Meas ... rk:54:pf:0 partly as a soldering exercise, and partly because "I want a 'scope". It also served it's purpose.

But it is a well made point by czietz, it's good to have input like this so we can make informed choices.
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Re: Logic Analyser on the Cheap

Post by exxos » Sun Nov 11, 2018 10:56 am

I had the open logic sniffer a while ago. Probes were always pinging off pins and just got frustrated with it. The software has a few versions, which are hit and miss if they work. I had to use a old laptop as couldn't get my PC working with it at all. Software was pretty terrible. Later the LA died, I got another one and that died first use. IIRC that had no protection. I gave up with it all in the end.

I got a 32chan LA after that, but just couldn't be bothered to make breakout boards for things, also just couldn't be bothered to keep putting the wires on everything, I mean like 32chans and a wire pops off, then trying to work out where it came from...aarrgghh..

For me a LA isn't that useful. Unless you get one which has good protection and very high sampling rates. The cheaper ones have a hard time in MHZ ranges and cause jitters in the captures. So if you are trying to find a few ns in the wrong place with a LA, basically forget it. You can end up chasing timing errors which are not actually there.

Also LA are basically useless for finding ringing on signals which is one if the main issues I have. They basically lienand make signals look perfectly fine, when they could be pretty trashed in reality.

This is all why I gave up with LA. If you want to do serious debugging then you need some expensive kit , otherwise you are just buying a toy.

I use my scope to check on signals. A bit of a limit as only 2 channels and things can take a lot longer than if I had 4 chan's for example. But I can't justify buying a 4 chan scope for what it would get used for.

So in terms of a LA, they can be very useful tool, terrible fire proved that. But if you want to do serious debugging, then you need proper kit. If you only want to get a rough idea to what's going in, then the cheaper LA may well be fine. Me personally however, just endless bad experiences with LA, granted I only got cheaper ones, but I don't have time to mess about with iffy have hardware and software. One reason I like my scope, power on and away you go .
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Re: Logic Analyser on the Cheap

Post by czietz » Sun Nov 11, 2018 11:15 am

Only while writing about this, I realized that relying on input protection diodes and series resistors means that the cheap logic analyzers draw current from the DUT when some signal is at 5V. So I've measured the input current into one channel of the cheap LA:
chinese-la-input-currents.png
chinese-la-input-currents.png (14.61 KiB) Viewed 55 times
Multiplying this, it means that if you probe eight channels of a 5V IC with that LA and all those channels happen to be high at the same time, the IC will power 72(!) mA into the logic analyzer. Keep this in mind.

Obviously, one has to select the right tool for the right task. For me, logic analyzers shine when it comes to decoding protocols such as SPI or USB. Sure, protocol decoders are also available on scopes, but only on higher-end ones or as an optional (=expensive) extra. Also, on the scope they are limited in terms of capture/decode time etc. I'd probably never have found some nasty issues with the USB chip used in the Lightning VME without the Saleae logic analyzer.

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