[sdiy] Logic Analyzer Advice

[brianw]

Hi Terry,

There are several levels of quality with regard to logic analyzers.

0) The cheapest ones simply connect the probes directly to input pins, with little to no protection against over voltage or other issues that might exist in the circuit under test. You'll have absolutely no control over the logic thresholds, which are different for each class of logic circuits (CMOS, bipolar, etc.)

1) Mid-level quality will have some buffering, which might slow down the maximum sample rate, but will protect the analyzer from some circuit problems. Still, no logic threshold adjustments or settings.

2) The best logic analyzers can change the logic thresholds via easily-accessed settings so that you can measure the true logic value that the circuit sees, rather than getting different values for in-between levels.

3) In my experience with professional digital design engineers, they almost always use the analog 'scope probes for logic analysis, so that they can see the analog voltage between logic levels. I had an issue with a product under development where a certain stage in the protocol involved the logic level slowly rising from 0V, with a ramp shape, but then dropped to zero before the next stage. My digital-only logic analyzer did not register the input as 0-1-0, but read a constant 0-0-0. Meanwhile, the product was seeing a rising edge that I could not diagnose. Another engineer suggested using the analog probes, and then we saw the problem (the slowly rising ramp that changed and dropped back to zero). The issue here was that the logic threshold for the analyzer was much higher than for the product under test, so the logic values being reported by the analyzer did not match the product under test.

Unfortunately, the Tektronix 'scope that we were using for this - which has 16 logic inputs and 4 analog inputs - cost about $16,000.00, so it was beyond my budget for a home-based repair shop.

At the other extreme of the price range, you can certainly get a lot done with a cheaper logic analyzer, but you have to be mindful that some circuit problems can cause mid-scale voltages to appear on the logic signal lines, and these become difficult to diagnose without visibility. This explains why many engineers that I've worked with tend to start with analog 'scope inputs before relying on the pure-digital logic analyzer.

I've had good luck with the Dangerous Prototypes Logic Sniffer, with up to 32 inputs. This product may not be available currently.

http://dangerousprototypes.com/docs/Open_Bench_Logic_Sniffer

I've borrowed Saleae Logic Analyzers in the past, and they work quite well for the money, but this is the brand that had problems diagnosing an actual issue due to the differing logic thresholds. I think that never Saleae product revisions may have logic threshold settings, so it's worth investigating.

Anyway, just look at the data sheets for the chips used in the circuit that you want to analyze, and then make sure that your logic analyzer has compatible logic thresholds as well as logic levels. Some of the vintage gear that we work on uses CMOS logic gates running at voltages greater than 5V (like 10V or 12V or more), and these will not be compatible with most of the USB logic analyzers, unless the designers allowed for wider ranges of voltages and logic thresholds. You can potentially use voltage dividers to reduce the logic levels, but that will load the logic circuits and potentially slow the sample rate.

Brian


On May 16, 2025, at 10:24 AM, Terry Bowman wrote:
> I need to repair some old frequency counters and whatnot. It sounds like a job for a logic analyzer but I've never actually used one.  What would be wonderful is the ability to put a clip on the counter DIP, tell it what chip it is and then watch it count away in hexadecimal. Someone told me that this is "decoding" the data which makes sense. Anything that doesn't require me to do any mental gymnastics is ESSENTIAL. I'm not rich but I'll pay for what I need. Anything less is a waste of time.
> 
> What would be best for this? I'm sure there are factors that I'm not even aware of. PC based is an option as I'm going to put a 32" monitor on my bench. I have a line on a beautiful old HP DSO that's a dream to use and has one built in but there's no "decoding".
> 
> The counters in question have Nixie tubes in them. Some go down to 1Hz so they're not bad for fooling around with trans-AF. I have an LED model that I'll practice on first.
> 
> 
> Terry Bowman, KA4HJH
> "The Mac Doctor"
> 
> https://www.astarcloseup.com
> 
> #1: There is no substitute for good tools;
> 
> #2: Always use the right tool for the job.
> 
> Linn Westcott