LaPOD: Low cost Logic Analyzer probe for MSO5k, DHO900 and more!

[oliv3r]

After seeing a couple of great pod designs, I decided I wasn't quite happy with the designs overal.

What I wanted was to use (mini) HDMI cables, as they are cheap, super common and shielded. Also 4 channels per pod had my preference, to make them nice and small.

The design is based on a breakout board, that plugs into the scope, and offers 4 (mini) HDMI connectors. The other end consists of a POD, featuring either a LMH7322 (like the original), LMH7324 (cheaper quad channel part) or SNV65LVDS (much cheaper, less flexible) comparator. Mix-n-match is the keyword here, feel unrestricted in what pod to connect, get 4 LMH7322 based pods, or one of each. In the future, more pods may be added if it makes sense.

As for the breakout board, there's currently two models, both with 50pin IDC connectors, matching the MSO5000 series and DHO900. The mini-HDMI variant was made to offer a super small/narrow profile, which is nice for the DHO900, but either work. In the future, given we have some testers/interest, we can very easily add a 68pin IDC variant for the DS1074 series, and very likely also for the PCIe based scopes such as the DSO7k and MSO8k series.

Some words on the other great designs.
Gandalf_Sr's Budget probe set design.
https://www.eevblog.com/forum/testgear/ms05000-budget-logic-analyzer-probe-set-design/msg2938442/#msg2938442
At first, it seemed like a good idea, but I didn't like that it wasn't open source so couldn't even respin it. Secondly, I liked the idea and even was planning on doing a pod for it, but the SNV65LVDS seemed more sensible at the time.

S. Petrukhin's easy eda low cost probe
https://www.eevblog.com/forum/testgear/low-cost-logic-analyzer-probe-for-rigol-mso5000-easyeda-project/
I really liked this design, but the many small comparators I didn't quite like, nor the flat cable design. But it was the thread that inspired me (after 3 years? to finally make my own design) after the suggestion of the SNV65LVDS. It was open-source though, but easy-eda

Nikki Smith's variant
https://climbers.net/sbc/clone-pla2216-logic-probe-analyzer/
Was what really maybe got me moving, as he did the work in KiCad and used the LMH7322 and LMH7324. I really wanted to have balanced lines (lapod all traces are length-matched). But, still wrong connector I was still hung up on the HDMI bit. Also Nikki went silent after september

dren.dk's first foss unit
https://gitlab.com/dren.dk/mso5k-la-pod/tree/master
The name was actually inspired from this post https://www.eevblog.com/forum/testgear/rpl1116-active-logic-probe-pod-for-1000z-series-teardown/msg2125477/#msg2125477 LA pod. But 8 pins in a pod was not my liking, and flat cables.

thmjpr's USB-C variant
https://gitlab.com/thmjpr/stm32f03_la_monitor
I do like the idea of using USB-C cables, but USB-C cables are a huge pain. are all wires routed, does it have an E chip, will that be in our way. Should we do USB-PD on the breakout board? Also, reversability is cool, but not ideal for the breakout.

Others I have missed? Regardless. Here's my variant

All schematics and stuffs come from the pipeline when released https://gitlab.com/riglol/lapod/-/releases, and I've attached a whole bunch of some WIP stuff here https://www.eevblog.com/forum/testgear/another-low-cost-la-probe-for-rigol-mso5000-by-oliv3r/msg5311123/#msg5311123

Note, that I haven't ordered any PCB's yet, and nothing is tested. It just looks really sexy in the renderings :p


EDIT 2024-02-03: Changed the starting post as we are now our own thread. Find below the remainder of the starting post
(left-over of the original first post)
I am too working on a respin of this

mostly, because I wanted to use HDMI cables and 4 channels per pod. Here's my WIP screenshot; but it's far from finished. The pinout of the connector is crazy, causing all sorts of weirdness to be needed in routing bits, and to have at least the easiest routing, the connectors will end up having a weird order, J2, J4, J1, J3, which will translate to port1, will have pins 5 - 8, port2, 13 - 16, port1 1 - 4, port3, 9 - 12, which I admit is a bit odd, but keeps routing sane, or the size down, whichever you prefer.

I'll route the other bits next (which is just power and gnd) and then route the pods.

For those curious, I'll also attach a screenshot of the HDMI pinout to shout and shame

[ebastler]

The pinout of the connector is crazy, causing all sorts of weirdness to be needed in routing bits, and to have at least the easiest routing, the connectors will end up having a weird order, J2, J4, J1, J3, which will translate to port1, will have pins 5 - 8, port2, 13 - 16, port1 1 - 4, port3, 9 - 12, which I admit is a bit odd, but keeps routing sane, or the size down, whichever you prefer.
[...]
For those curious, I'll also attach a screenshot of the HDMI pinout to shout and shame

Not sure I understand how the pinout of the HDMI connector could drive the need to swap the sequence of the pods. I would expect the HDMI pinout to potentially force an awkward order of the four channels within each pod group, but the pod sequence to follow the order on the scope's connector?

[UK]

I wanted to use HDMI cables and 4 channels per pod.

Why HDMI exactly? Why not RJ-45 LAN cable? Moreover, Cat-7 / 8 cables have superior shielding for each pair, the connector with fixation also may be a very handy feature when a lot of stuff is going on on your table. Also, RJ-45 also has more robust contacts than an HDMI.

[oliv3r]

I wanted to use HDMI cables and 4 channels per pod.

Why HDMI exactly? Why not RJ-45 LAN cable? Moreover, Cat-7 / 8 cables have superior shielding for each pair, the connector with fixation also may be a very handy feature when a lot of stuff is going on on your table. Also, RJ-45 also has more robust contacts than an HDMI.

All true, but RJ45 doesn't have enough pins. You can do 4 channels, but then have no power or anything. If you do all power pins, you end up with just 2 channels, which is usable, but leaves a lot to be desired ...

HDMI cables are cheap and easy to get, and give you differential pairs too. The 5V pin is gonna be a challange, as cheap cables will not allow much current to go through, but must let 50ma go through at the very least to pass certification.

The pinout of the connector is crazy, causing all sorts of weirdness to be needed in routing bits, and to have at least the easiest routing, the connectors will end up having a weird order, J2, J4, J1, J3, which will translate to port1, will have pins 5 - 8, port2, 13 - 16, port1 1 - 4, port3, 9 - 12, which I admit is a bit odd, but keeps routing sane, or the size down, whichever you prefer.
[...]
For those curious, I'll also attach a screenshot of the HDMI pinout to shout and shame

Not sure I understand how the pinout of the HDMI connector could drive the need to swap the sequence of the pods. I would expect the HDMI pinout to potentially force an awkward order of the four channels within each pod group, but the pod sequence to follow the order on the scope's connector?

because the connector's pin-order isn't in sequence So i took the effort to make sure that the 4 pins within the HDMI connector are in order, so that took a lot of swapping. But swapping all pins for all connectors just takes way to much PCB space to keep the connector within reasonable size. Also, I wanted to keep the inner layers for power and ground, if I'd put signals on the inner layers, i could have swapped about a little bit more ...

[UK]

Okay, now it makes sense. But why not a Mini DisplayPort, it's smaller but has all the same.
Or even a full-size DisplayPort, since it has a fixing feature.

[oliv3r]

Those thoughts crossed my mind too, but mini-hdmi connectors are much harder to source; but what's worse, mini-hdmi cables are much harder to source

also, as you can see, from a size point of view, it doesn't matter much.

same goes for displayport. HDMI you can get for a couple of € from action or poundshop or what not. They are 'ubiquitous'. Even when the HDMI consortium is evil

[UK]

By the way, how is it going with pin swapping to make all channels in order?

[oliv3r]

for now 'not happening' :p

so each pod will have properly ordered pins, just the main breakout won't (for now, but re-doing that one is simple now, only had redone it 5 times, due to size, manufacturing, differential matching mistakes); but if you look at the pinout you'll see why, they are just to spread out, but maybe i'll give it one more go later (or someone else :p) I'll post my WIP kicad files soon so that others can tinker with it.

The problem, is that one channel will end up going from completely left, to completely right, meaning each trace will need to be this long, requiring a lot of space to snake around, thus the PCB will become bigger (sticking out the front more), which i'd rather avoid ...

[oliv3r]

The first pod, based on the LM7324 is ready, and boy was it a party :p

I'll do an LM7322 next, followed by some cheaper simpler chips as found in these threads.

The current pod PCB size is 4.5 cm X 3.0 cm using all 2012 (metric) hand solderable bits. Using 0402 (metric) would be much more fun :p but that's an exercise left for actually producing these units with automated pick-and-place solutions.

[UK]

It looks sleek, but it would be great to add a small led on it.
When you have a mess on the table, especially when you use all four pods it will be nice to have some illumination as to what pods are connected to MSO. 

[oliv3r]

It looks sleek, but it would be great to add a small led on it.
When you have a mess on the table, especially when you use all four pods it will be nice to have some illumination as to what pods are connected to MSO. 

Fair enough, I did add leds to the pod, to indicate which 'type' is connected (e.g. the LM7324 one will be blue, the LM7322 will be red + green). Having matching leds on the pod would be a nice idea, I'm a little worried with regards to power consumption.

[oliv3r]

So the LM3222 was a lot easier to do, but (due to double the chips) is slightly larger. I've added an LED, though not sure how to make it visible later. I guess heatshrink instead of a case is certainly an option

Once I've done all the pods, I'll size them up to be all the same size with the mounting holes in the same location, and the led as well

Here as before, big 2012 (metric) caps and resistors except for the differential resistors, everything should be length matched. Impedance matching is probably not the the case ...

[oliv3r]

Finished the sn65lvds variant, which was the easiest and most pleasurable to route; also because it had the most space available It is basically Nikki's design, but with trace length matching, and using 4 channel pods of course.

Gonna use this as the size of the PCB and cleanup the previous editions next, making sure the LED is on the same spot etc ...

[oliv3r]

Read back this thread, and saw discussion about the need, or lack thereof, matching the traces for delay's. I know the scope doesn't match traces either, if we look at the X-Ray ...

I never put the following pic on https://gitlab.com/riglol/rigolee/firmware/-/wikis/MSO5000%20teardown (cropped due to size restriction of the forum). I'll see if I can trace it out in kicad to determine an estimated trace length ...

but again, rigol doens't care either

Rigol does care, I just found a pretty good shot which shows the serpentines clearly (bastardized to jpeg cause of forum)

[ebastler]

I'm sure I have overlooked it somewhere in the thread, but could you please (re-)state the specifications of the LM7322 and LM7324 versions? Do they have configurable thresholds (in which range)? What's the range of input voltages and what is the speed? Thanks!

[oliv3r]

I'm sure I have overlooked it somewhere in the thread, but could you please (re-)state the specifications of the LM7322 and LM7324 versions? Do they have configurable thresholds (in which range)? What's the range of input voltages and what is the speed? Thanks!

I have no idea, I just took other people's design , or rather, the LM7322 is the same chip used in the offical probe, which was reverse engineered. The LM7324 is a quad driver, hoped to be cheaper ...

[ebastler]

Could you point to those designs you used as a starting point? I had come across a thread once which aimed to do a full-spec replica of the Rigol probe, but it is not this thread here, right?

Also, could you restate the comparator part numbers please? Are they LMH732x?

[UK]

Hey folks, just finished a shell for oliv3r's pods in Rigol's triangle style. The shell should print vertically as two halves without any supports. And as you can see it has a small oblong cutout and a light channel under it to make the LED shine in the center. It also has a flat lowering in the bottom for the label with channel numbers, which should prevent its easy pealing.

Hope you like it!

When oliv3r finalizes all PCB dimensions I'll do final refinements and upload STLs.

[ebastler]

Nice! Looks very "Darth Vader" to me, matching the MSO5000. 

[oliv3r]

By the way, how is it going with pin swapping to make all channels in order?

for now 'not happening' :p

So it happened. I took some measurements and realized my current design was super small. Which is great, and would have fit, but I was under the impression, the traces aren't length matched within the unit, might as well just fix the ordering with different length traces. But turns out, it is probably length matched within the scope (lets just hope they didn't take the mismatch of the bods into account).

Anyway, with the new measurements in hand, looking at the probe design, I figured, I'm so zoomed in, I can easly spare 3 - 5 mm. Also, I've learned how to do _proper_ differential track length matching in kicad (did it manually per track before, which was of course not good), figuring that would save me quite some space too. It did, but also using the 4 layers gave me plenty of beef to work with. So without further ado, correctly ordered breakout.

Also attached is a screenshot of what it looks like on the various layers.

Next steps, adding kicad automation to generate all the outputs from the pipeline, addinga pipeline, putting it all in git and pushing it to gitlab.

Also, some silkscreen fixups, attributions etc are deff. also in order. I really should fork this to its own topic too ...

[ebastler]

Congratulations on solving that puzzle -- nice job!

EDIT: Looking again at the layout, I think there is a bit more puzzling to be done. The orange traces in general don't look length-matched to me; the traces for IN9 in particular are much shorter than any of the other pairs?

On the silkscreen, and also when you create a new thread about this, I would not call it an "MSO5000" LA board. The new DHO900 series (low-cost 12 bit scopes) uses the exact same pinout for the LA probes, and will certainy get a lot of attention. So be sure to let those users know about your probe too! 

If you could take a moment to address my earlier question? Thanks!

Could you point to those designs you used as a starting point? I had come across a thread once which aimed to do a full-spec replica of the Rigol probe, but it is not this thread here, right?

Also, could you restate the comparator part numbers please? Are they LMH732x?

[tooki]

Okay, now it makes sense. But why not a Mini DisplayPort, it's smaller but has all the same.
Or even a full-size DisplayPort, since it has a fixing feature.

The full-size DisplayPort is possibly the least-reliable connector I have ever encountered. Because of how the plugs are implemented (a PCB with the contacts soldered to each side, and the wires from the cable to that PCB, but not overmolded because of the latching release button), they are extremely susceptible to damage from bending. Literally every DP cable I’ve encountered in the wild has had to be replaced at least once. Mini-DP is a far more robust connector.

Those thoughts crossed my mind too, but mini-hdmi connectors are much harder to source; but what's worse, mini-hdmi cables are much harder to source

also, as you can see, from a size point of view, it doesn't matter much.

same goes for displayport. HDMI you can get for a couple of € from action or poundshop or what not. They are 'ubiquitous'. Even when the HDMI consortium is evil

They didn’t say mini-HDMI, they said mini-DisplayPort. They are available.

The biggest risk with using popular connectors like HDMI for some other signal is what happens if someone connects a cable with an actual HDMI signal to it. (Not entirely crazy, given how modern scopes have HDMI outputs.) Will it damage your device? Will it damage the HDMI device?

In this regard, a more obscure connector — or one which was not designed specifically for a common consumer use — is a better choice.

[ebastler]

The biggest risk with using popular connectors like HDMI for some other signal is what happens if someone connects a cable with an actual HDMI signal to it. (Not entirely crazy, given how modern scopes have HDMI outputs.) Will it damage your device? Will it damage the HDMI device?

At least oliv3r is in good company there: Siglent use an "HDMI" connector for their proprietary S-BUS interface to an external logic analyzer module. It's right on the front panel of various entry-level scopes. 

Looking at the renderings above again, I have a different concern about the full-size HDMI connectors though: @oliv3r, is there enough room for the HDMI plugs' plastic bodies? Looks like the jacks are spaced very closely together.

[UK]

And I think so too, there isn't enough space between the ports... especially for standard cheap cables.

As a possible option to solve the problem with the PCB dimensions is to swap the current HDMI sockets to the upright/vertical version.

[ebastler]

Could you point to those designs you used as a starting point? I had come across a thread once which aimed to do a full-spec replica of the Rigol probe, but it is not this thread here, right?

Also, could you restate the comparator part numbers please? Are they LMH732x?

Answering my own question, since oliv3r seems to drop in only sporadically:

The thread on analyzing and replicating the original MSO5000 logic probe (PLA2216) is here: https://www.eevblog.com/forum/testgear/rpl1116-active-logic-probe-pod-for-1000z-series-teardown/. And the comparators are indeed the LMH7322 (dual comparator) and LMH7324 (quad). Both are available from Mouser etc., and both are on the expensive side.

Regarding the form factor of a logic probe, I am not sure whether I prefer Rigol's approach (all 16 channels routed to a large external pod) or oliv3r's (4 pods with 4 channels each). Rigol's design clearly limits flexibility, requiring all 16 probed channels to be relatively close to each other on the PCB under test; and it has the bulky, broad flat cable connection. But four HDMI cables can get a bit messy too...

And it's worth mentioning that Rigol tells you not to hot-plug the logic probe connector! Having those individual HDMI-style jacks on the front may just be too tempting?

The other thread has one version with 4x4 channels, but using USB-C connectors -- debatable, since inserting the plugs the wrong way round will swap the channels, it seems.  It also includes a design for 2 pods with 8 channels each, connected via narrower flat cables. That one might actually hit the sweet spot for me -- but I am not sure whether it was fully polished and tested.