Usefulness of a logic probe - Page 1
Forum section: electronics / repair

author: artag
author_rank: Super Contributor
timestamp: May 27, 2021, 06:04:44 pm
content: I used a logic probe in the '70s and may have occasionally found it helpful. They're still around now, and certain specific ones like the HP sets are quite sought-after as they're attractive historical pieces as well as being possibly useful.But I don't use one any more, even though I have them. They're small and quick to use but give so little information compared to a scope - which I'm going to be using anyway 99% of the time - that looking for activity with a logic probe is an unnecessary stage. The complementary HP current tracer and pulser may have have additional uses but I find the pulser, again, not useful on a microprocessor system. It's useful to kick some random combinational logic or flipflop, but because it can't do that in sync with the processor it's not going to do a lot.And I've also always wanted a Logic Dart. But they're not very common on ebay, and I often hear they're not all that useful : just a logic probe that displays a little more information ... I think I just want one because they're cute, have historical interest, and were things I lusted after back in the day. They used the HP48 calculator's LCD, which places them in the timeline alongside another desirable tool.My go-to piece of testgear is the scope. Quite often before the DVM, unless I have a reason to check the power supply carefully. Most of the time the scope's DC accuracy is good enough for digital work and the instant information it gives me about clock rate, logic levels, noise, glitches etc. is streets ahead of anything else : maybe I use a logic analyser or the scope's mixed-signal features if the problem is complex.I'm interested to know if other people's experience is different.Do you still find these tools valuable ?Is their low cost compared with even today's scopes an important feature ?If you use them, what is your strategy ?

author: ataradov
author_rank: Super Contributor
timestamp: May 27, 2021, 06:36:47 pm
content: I still use logic probes occasionally. I'm not a fan of commonly available probes - they are bulky and not that convenient. I've made my own version with clear indication in as slim of a form factor as I could manage -https://github.com/ataradov/logic(pictures herehttps://www.tindie.com/products/ataradov/programmable-logic-probe/)The advantage is that you can instantly see what is going on as you probe. With the scope you need to place a probe and then look away to see the result. Ability to see things instantly is great in some cases, but generally I do prefer the scope.And I see no value in more advanced tools like Logic Dart. If I have to use something bulky like that, I may as well use the scope. Generally anything that requires you to take you eyes from the probing point is better done with the scope.

author: dc101
author_rank: Regular Contributor
timestamp: May 27, 2021, 06:37:59 pm
content: I think it depends on what type of electronic/engineering work you do on a normal day. Some co-workers of mine routinely use a logic analyzer because they are trying to monitor a large high-speed parallel bus.At home, my oscilloscope is the first thing I power on and I usually never use a logic analyzer for anything. At home I'm mostly trying to search for debug interfaces on boards and for me an oscilloscope is much more useful for that. I have a cheap 8 channel usb logic analyzer, and I've only used it a handful of times. The last time I used it was to decode a 1-Wire serial bus and Sigrok (https://sigrok.org/) was the only solution I had access to that could decode 1-Wire.

author: artag
author_rank: Super Contributor
timestamp: May 27, 2021, 08:48:48 pm
content: I actually use the USB analyser quite a bit. Nowhere near as much as the scope, but far more than occasionally. It has far more depth than the scope, and far more tools for examining the content. My scope can handle RS232, CAN etc. but only a few seconds' worth. The USB analyser has somewhere like 32G to store it ..But yeah - a logic analyser is a tool for scope-like data with a different emphasis (more channels, more storage, sometimes even analog data too). Is nothing like as limited as a logic probe.The 'look away' aspect is interesting. I can see how on-probe data helps, but I guess I'm not accustomed to having that. Perhaps If I used it more, I'd find that more important.

author: ataradov
author_rank: Super Contributor
timestamp: May 27, 2021, 08:55:30 pm
content: Yes, logic analyzer for digital signals is a no-brainer. But LA is more useful when you have a fixed set of pins you want to observe.Logic probe comes in handy when you have 16-bit bus or something like this, and all you want to see if there is activity on the lines. You don't need to see it at the same time, so connecting 8 or 16 channels at the same time is either counter-productive or not even feasible. I often use logic probe when there is no way to connect a LA. Like debugging the bus between the MCU and SDRAM. There are no attachment points for the LA on QFP or QFN packages.That's why the sharp tip was critical to me - I can easily place it on a TQFP pin and it will not slip or short to the next pin. I spent months looking for a good source for the tip. And the thing that worked in the end was some new old stock machined tips from some obscure supplier of NOS parts.Looking for a tip is how I learned they still make gramophone needles. They ended up being too short, so I even considered asking manufacturers for a custom run with the same tip, but slightly longer. But the purpose made stock tip had perfect sharpness and hardness.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 27, 2021, 09:21:07 pm
content: The usefulness of a logic probe nowadays would be close to zero, IMHO.First, there's not many signals left to probe outside the ICs, since most of the digital part is encapsulated inside a microcontroller or inside an FPGA.Then, the signals that are still accessible from the outside of ICs are either very high speed signals that can not be visualized or even probed without very expensive high speed equipment, or low speed signals that can be easily probed and even decode with most of the digital oscilloscopes.For me a digital oscilloscope like Rigol DS1054z is more than enough.I also have a logic analyzer (400MHz sample rate, DSlogic), but never needed it or used it in practice.

author: james_s
author_rank: Super Contributor
timestamp: May 27, 2021, 09:41:13 pm
content: I use a logic probe frequently, when I work on vintage arcade games and pinball machines it is usually the first tool I reach for after my DMM when something is not booting or not running properly. A logic probe tells me quickly and easily if the clock is running, if the reset line is stuck or strobing, if there's activity on the bus or chip select lines for the RAM, ROM and other subsystems. They're useful with older stuff that is built with discrete TTL or CMOS parts, and they tell you the basics. You might then pull out a scope or logic analyzer but a logic probe is a quick initial test.

author: artag
author_rank: Super Contributor
timestamp: May 27, 2021, 11:37:21 pm
content: Is the portability an issue there ? My scope's power button is the nearest of all the power buttons around my bench. There's no pulling out involved - it's right there (and the logic probe is in an awkward cupboard!).But that's for bench work. With a pin or a vid you likely start by opening up the machine and poking around. Only once you've got an idea of the fault - and whether you can fix it in place - do you pull the board out and take it to a bench with the need to hook up power supplies etc.I know you can have a handheld scope but imho they're not as nice to use as a bench scope, and never stay balanced in the machine where you want them to. And auto setup only works some of the time, so you have to twiddle knobs too.

author: artag
author_rank: Super Contributor
timestamp: May 27, 2021, 11:43:26 pm
content: Quote from: ataradov on May 27, 2021, 08:55:30 pmDid you try a pogo pin / ATE test probe ? They're sharp, sprung, come in multiple head patterns, easily swapped etc.Agilent 116x probes include a 'browser tip' that seems to be exactly that. The spring, particularly, helps it not to slip as a slight drop in pressure is taken up by the spring rather than losing the grip.

author: EEVblog
author_rank: Unknown
timestamp: May 28, 2021, 12:34:08 am
content: Got to admit that I haven't used one for maybe 20 years now.

author: ataradov
author_rank: Super Contributor
timestamp: May 28, 2021, 02:15:56 am
content: Quote from: artag on May 27, 2021, 11:43:26 pmI tried the widely available ones. I did not like them - they are too wobbly for handheld use. I can apply a pretty measured amount of force with my hands, but the spring makes me loose the feedback. It is also harder to tell when you are slipping from the probing point.

author: james_s
author_rank: Super Contributor
timestamp: May 28, 2021, 02:24:29 am
content: I quite like that programmable logic probe ataradov made, I have a couple of perfectly good logic probes already but I'm still tempted to pick up one of those. In thinking about it I doubt a logic probe is much use for modern designs but I tend to work on a lot of old equipment where they're still pretty handy.

author: floobydust
author_rank: Super Contributor
timestamp: May 28, 2021, 03:50:53 am
content: I find a defective gate will have a weak output or leaky, shorted input and the signal swing is poor, not at the usual logic levels. For me a logic probe did not really assist finding a common hardware problem, such as an ESD zapped CMOS gate.I wonder if you could add that detection to the ATtiny, using the A/D but I don't know the probe's speed at all. Will it indicate a 16MHz clock. Even if it's aliasing, it should catch a pulse logic-level that is out of spec.Logic probes, they were all the rage in the 80's burt I never built one that I can recall. Usually a window-comparator and pulse stretcher for the LED, ability to choose TTL or CMOS and some operated up to 15V.

author: james_s
author_rank: Super Contributor
timestamp: May 28, 2021, 04:21:13 am
content: Most logic probes have a dead band so if a signal is at an illegal level it doesn't give you a high or low response. It won't find every problem but it has pointed me in the right direction more than a few times. Once I find something of interest with the logic probe, or if I don't find anything suspicious then I'll get out the scope.