View Full Version : Any recommendation for a logic probe?

October 7th, 2014, 02:51 PM
I have recently gotten a 4MHz Z80 computer (a Tiki-100) that has certain problems. The CPU seems to start and access the floppy drive just as during a normal boot, but the video output gives no response whatsoever. This system uses video hardware made from the ground up entirely by TTL, so it should technically not be too great a problem fixing the issue. If I had the right tools.

My multimeter obviously does not hold up to the task. I need some kind of logic probe, that can be used to verify if a signal reaches its destination or not. The probe must have several inputs, and must be able to trigger and record for some time when a given change occurs. The reading and displaying of video RAM seems to work, so the problem is problably somewhere in the interface between the CPU and video hardware. There is quite a bit going on there, especially lots of multiplexing and waitstate shenanigans.

Is my best bet some custom code on an Arduino, or a comercially available logic probe? Price is a major factor at the moment.

October 7th, 2014, 02:55 PM
Open Bench Logic Sniffer (OBLS) is a great tool on a budget.

October 7th, 2014, 03:39 PM
For everyday use, one of the made-in-China probes should be sufficient, as long as it has the "pulse catcher" on it.

If you want to go overboard and can find a deceased EE you can try to pry an old HP Logic Dart from his cold fingers...

What's interesting is that for about the same amount of cash, you can pick up a Chinese 8 ch USB logic analyzer.

October 7th, 2014, 06:33 PM
If you want to go overboard and can find a deceased EE you can try to pry an old HP Logic Dart from his cold fingers...

I've never been that excited about the HP Logic Dart that I have. For the price they seem to sell for on eBay you could pick up an HP 16500B or 16500C system, as long as you didn't have to ship it (and sometimes even if you did), although storing an HP Logic Dart when not in use is a lot easier.

October 7th, 2014, 06:42 PM
Yeah, my impression of the Logic Dart was that it was pretty expensive but nifty--a handheld multifunction logic device. It must not have sold very well, because HP did very little to promote it. Back in the day, it seemed pretty fantastic, but now we have handheld multicolor DSOs.

Time marches on. HP had so many really cool products that they simple let go. My HP16C is still cranking right along--I'm surprised that they didn't carry the design forward, like they did with the 15C.

October 18th, 2014, 03:56 AM
For basic stuff I use a Logic Probe that looks like this (the place I bought it no longer stocks them, so here is an ebay one that 'looks' identical other than the brand) http://www.ebay.com.au/itm/EISTAR-Digital-Logic-Probe-DTL-TTL-Circuit-Tester-Pen-DC-18V-Max-gi-/361046654307?pt=AU_Gadgets&hash=item54100ebd63
I have 2 of them and they work far above their rated frequency. I think they are rated to 20MHz, but I can see pulses of 66MHz.

October 18th, 2014, 04:12 AM
I have a Salae logic-16

They have a new model which is also a combined oscilloscope.


I find it indispensable because of the ability to record 16 channels for minutes at a time on vintage systems. It is way better than any of the Chinese versions out there and I always find myself needing more than 8 channels. It also can translate serial and binary into the actual data when watching a channel. For example checking a UART you can watch all the strobes and data channels and the actual serial in and out. It's very useful. Turns tasks that could take hours with a probe into minutes since you can see everything at once. It's not cheap but I have never regretted the decision.


October 18th, 2014, 04:32 AM
Yeah, at work there's a Logic Analyser but it takes quite a bit of effort to setup all the leads (lots of channels) and configure all the triggering compared to grabbing the logic probe and having a quick poke around. I also have a HP Logic Probe (and various HP gear), but find the cheap Chinese Logic Probe I have much better surprisingly.

October 18th, 2014, 05:10 AM
With the Salae it's pretty quick. Especially if you have an IC test clip. Takes only a few minutes to setup but once it is, then you are set. I find it easier than poking around because you can capture so much at once and make educated measurements and less guess work and luck to find the pin where there is a problem.


October 18th, 2014, 08:53 AM
I was loaned a Salae logic-8 halfway through my first PET repair projects, and while more channels would have been nice (like, just one more, the regular eight plus a ninth to look for a chip select or r/w strobe...) it made checking out hunches amazingly easy. They are indeed not cheap but the hardware and software combination is probably worth what they're charging.

October 18th, 2014, 05:11 PM
The probe must have several inputs, and must be able to trigger and record for some time when a given change occurs.

I spent a lot of time staring at an HP 1651A logic analyzer for my VCFMW project (CP/M on a Credit Card Terminal) (http://randomvariations.com/cpm-on-a-credit-card-terminal/) this past summer.

The IC test clips Corey mentioned are invaluable. They make two kinds, one with heads and one without. The ones without make it easy to connect headers directly to the clips. The ones with heads are better for connecting the test hooks that normally ship with the logic analyzers.

For my longer term project, I took an old ribbon cable with an existing 34-pin header and stripped and soldered one wire to each of the test points I was monitoring. I could then easily connect and disconnect the ribbon cable from the logic analyzer using a header between the two cables.

I will note that I was monitoring an 8-bit data bus, 17-bit address, and assorted other signals (R/W, chip select, etc.) Having 32 inputs and two triggers was extremely useful. I wish that I had a larger bit-depth, but didn't need any more speed for what I was doing. I could not have reverse engineered the credit card terminal without it.

Some of these older logic analyzers can be had for cheap, but be careful about the test hooks, pods, and cables. They can get expensive if what you obtain second-hand doesn't come with them.

If you can make do with the Arduino and get the timing you need, then that may be your best option given a tight budget. Otherwise this is probably a tool that is worth spending the money purchasing if you see the need for it in the future.

November 28th, 2014, 11:19 AM
..... as long as it has the "pulse catcher" on it...

Hello Chuck,

Would you please elaborate more about this "pulse catcher" feature?



November 28th, 2014, 11:27 AM
Sure--it's basically a monostable multivibrator ("one shot") that triggers on pulses of either polarity and "stretches" them enough to be seen (maybe 0.5 second). Nothing elaborate--I suspect you could do it with an NE555 timer IC. Lets you see microsecond pulses that you would otherwise miss.

A related feature is a pulse "memory feature" that will light an LED if a pulse comes by and leave it lit until reset. Not as useful, IMOHO.

November 28th, 2014, 11:36 AM
Sure--it's basically a monostable multivibrator ......

On a regular logic analyzer, do you manually enter which channel you want to set as
a "pulse caught" one...?


November 28th, 2014, 11:44 AM
On a regular logic analyzer, do you manually enter which channel you want to set as
a "pulse caught" one...?

If you mean as a trigger, yes--and trigger conditions can be quite complex. In fact, most logic analyzers continuously record logic states and allow you to see logic states on either side of a trigger event (i.e. you can see what happened before the trigger, as well as after). That, in itself, can make a logic analyzer infinitely more valuable than a simple logic probe when looking for the "rare" events.

However, one usually takes some time to set up the probes on a logic analyzer, while one can just "poke around" with logic probe.

November 28th, 2014, 02:30 PM
On a regular logic analyzer, do you manually enter which channel you want to set as
a "pulse caught" one...?

If by "regular logic analyzer" you mean something like a classic HP 16500B/C with something like a 16555A state/timing module you can set up all sorts of complicated trigger schemes based on sequences of various events.

Timing Trigger Macro Library
Basic Macros
1. Find anystate n times
2. Find pattern present/absent for > duration
3. Find pattern present/absent for < duration
4. Find edge
5. Find Nth occurrence of an edge
Pattern/Edge Combinations
1. Find edge within a valid pattern
2. Find pattern occurring too soon after edge
3. Find pattern occurring too late after edge
Time Violations
1. Find 2 edges too close together
2. Find 2 edges too far apart
3. Find width violations on a pattern/pulse
1. Wait t sec

State Trigger Macro Library
Basic Macros
1. Find anystate n times
2. Find event n times
3. Find event n consecutive times
4. Find event2 immediately following event1
Sequence Dependent Macros
1. Find event2 n times after event1, before event3 occurs
2. Find too few states between event1 and event2
3. Find too many states between event1 and event2
4. Find n-bit serial pattern
Time Violations
1. Find event2 occurring too soon after event1
2. Find event2 occurring too late after event1
1. Wait n external clock states

(See the HP 16554A, HP 16555A, and HP 16555D State/Timing Logic Analyzer User’s Reference manual http://literature.cdn.keysight.com/litweb/pdf/16555-97015.pdf if you are really curious)

Iteratively refining the trigger scheme to capture the events of interest can be a large portion of the effort involved in investigating something with a logic analyzer.

November 28th, 2014, 10:49 PM
Thank you Chuck and gslick for brilliant comments!


November 30th, 2014, 10:39 AM
I managed to pickup a TechTools DV1-100 18-ch analyzer at a swap meet for $35. Not sure I would have paid $499 for it new, but it's been very handy to have the few times I've needed it. They sell a more contemporary and smaller analyzer (9-ch, I think) for $300-something.

November 30th, 2014, 12:03 PM
If anyone not too far from the Seattle area is interested in an HP / Agilent 16500B or 16700A logic analyzer system I have more of them than I need and could make better than eBay deals. They can be shipped, but last time I shipped one it was at least $50 to do it right, maybe more now.