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

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oliv3r:

--- Quote from: tooki on October 28, 2023, 01:45:45 pm ---
--- Quote from: UK on October 07, 2023, 11:34:09 pm ---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.

--- End quote ---
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.


--- Quote from: oliv3r on October 15, 2023, 09:02:03 pm ---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 ;)

--- End quote ---
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.

--- End quote ---

Sure, but this is a built for experts isn't it? I'm not gonna run production and sell millions of them to unsuspecting users :p

So if you connect your PC or laptop to your scope, the laptop's HDMI port will probably be damaged, as it's outputting power on the I2C pins at the least. Don't think the scope will care much, as all those extra used pins, are either to drive a led on the adapter, or to feed power from the adapter.

Sure, people do dumb things, but again, this isn't a consumer dumb device, and while I agree, safety first, and using a common consumer connector is a dumb idea for a commercial product, this isn't that :) But getting consumer HDMI cables, is easy and cheap, and they are differential pairs to boot. I just got 60cm HDMI cables for 25cents each (the shop was closing down, but still). I think even normal HDMI cables can be had for 2€ nowadays. But yes, the resistance of the voltage carrying wires will have an impact ...

oliv3r:

--- Quote from: ebastler on October 28, 2023, 02:14:02 pm ---
--- Quote from: tooki on October 28, 2023, 01:45:45 pm ---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?

--- End quote ---

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.

--- End quote ---

crap! I Didn't even think about that one ... i'll measure my cables and adjust accordingly. Only bit that's frustrating, is that'll probably mean I have to do my differential lines, again, and the longest one will probably be ending up even longer :S

Oh well!!


--- Quote from: ebastler on October 31, 2023, 08:40:08 am ---
--- Quote from: ebastler on October 26, 2023, 12:38:56 pm ---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?

--- End quote ---

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

--- End quote ---
true, but often threads are silent for days :D, and ... life happens.


--- Quote from: ebastler on October 26, 2023, 12:38:56 pm ---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.

--- End quote ---
that's the one! the third pod is the one from this Nikki Smith's remix, he called it 'v3.1' or something.


--- Quote from: ebastler on October 26, 2023, 12:38:56 pm ---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...

--- End quote ---
Yeah, but cables end up always being messy, 8 cables is still too much for most of my tasks and still a big-ish cable. Also, those flat cables aren't differential cables, which is why I dislike them. 4 was the sweetspot for me, not just because hdmi has exactly 4 differential channels, but also because 4 is what you'd use for i2c + irq, or SPI + irq etc. And it does scale to use both :p one can always even do a pod with dual hdmi sockets :p


--- Quote from: ebastler on October 26, 2023, 12:38:56 pm ---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.

--- End quote ---

I think this thread, and nikki's v3 are then just what you are after. I didn't like the USB-C for the same reason, reversability AND the actual problem of more easily plugging something incorrect in. HDMI just felt 'less likely'. Initially I really wanted to do mini or micro hdmi even, and with the rasp-pi they seem slightly easier to source now ...

ebastler:

--- Quote from: oliv3r on November 01, 2023, 10:22:31 pm ---
--- Quote from: ebastler on October 28, 2023, 02:14:02 pm ---@oliv3r, is there enough room for the HDMI plugs' plastic bodies? Looks like the jacks are spaced very closely together.

--- End quote ---
crap! I Didn't even think about that one ... i'll measure my cables and adjust accordingly. Only bit that's frustrating, is that'll probably mean I have to do my differential lines, again, and the longest one will probably be ending up even longer :S

--- End quote ---

Making the PCB wider might also run into collisions with the connector on its left -- even more so on the DHO900, which has a USB jack quite close to the logic analyzer port. Maybe look into mini-HDMI or mini-DVI again?


--- Quote ---8 cables is still too much for most of my tasks and still a big-ish cable. Also, those flat cables aren't differential cables, which is why I dislike them. 4 was the sweetspot for me, not just because hdmi has exactly 4 differential channels, but also because 4 is what you'd use for i2c + irq, or SPI + irq etc.

--- End quote ---

Agree that 8 signals plus 8 GND wires is a lot already. I am not sure why all the individual GNDs would be needed though. My preference for an 8-signal pod would be a 2*5 pin connector (8 signals, 2 GND). That also works well with an IDC connector and colored ribbon cable, with the colors nicely matching the channel numbers.


--- Quote ---I think [...] nikki's v3 are then just what you are after.

--- End quote ---

Nikki Smith did an updated version of the low-cost (TTL/CMOS) probe only, right -- or did I overlook something? I definitely want a probe with the large voltage range and configurable threshold provided by the LMH7324, since one of my use cases is work on vintage (tube) computers with 20V logic levels.

The design by @dren.dk is the configuration I personally like best, https://www.eevblog.com/forum/testgear/rpl1116-active-logic-probe-pod-for-1000z-series-teardown/msg2316423/#msg2316423. It needs some work though: Some signal polarities are flipped due to early mis-information about the 50 pin connector's pinout, and he did not bother with length matching of the traces.

But I am in no rush -- will wait how your design pans out, and probably borrow (and use in duplicate) your very nice LMH7324 pod layout.  :-+

UK:

--- Quote from: ebastler on November 02, 2023, 08:43:23 am ---Making the PCB wider might also run into collisions with the connector on its left -- even more so on the DHO900, which has a USB jack quite close to the logic analyzer port. Maybe look into mini-HDMI or mini-DVI again?

--- End quote ---
Why not just use vertical ones?!
Previously, I also suggested mini-HDMI and mini-DP, but after looking at the range of cables available I agreed that the ordinary HDMI is the best solution... since slim connectors, flat or soft versions of cables are not even available for mini-HDMI / DP / DVI ...unless you're a fan of cables that look like a garden hose on your table  ;)


--- Quote from: oliv3r on November 01, 2023, 10:22:31 pm ---... that'll probably mean I have to do my differential lines, again, and the longest one will probably be ending up even longer :S

--- End quote ---
If you going to redo all differential lines again )) pls look closely at the upright HDMI sockets I've already mentioned before. Then you won't have to make a board that looks like a palm tree. And all four HDMI sockets can fit on a pcb in the width of a 50-pin header.

ebastler:

--- Quote from: UK on November 02, 2023, 09:30:35 am ---
--- Quote from: ebastler on November 02, 2023, 08:43:23 am ---Making the PCB wider might also run into collisions with the connector on its left -- even more so on the DHO900, which has a USB jack quite close to the logic analyzer port. Maybe look into mini-HDMI or mini-DVI again?

--- End quote ---
Why not just use vertical ones?!

--- End quote ---

They are certainly worth a try in the layout! But it seems to me that you will have to add more depth to the PCB: The connectors themselves are longer than the standard horizontal ones, and on the MSO5000 you might also need to give them extra clearance so they don't collide with the protruding scope housing above the connector.

If that PCB becomes to deep, and then has four full-size HDMI plugs and cables dangling from it, I would be worried about mechanical strain -- and about bumping into when you want to use the touch screen. Also because the assembly would come closer to the touch screen since it is taller. And it might just look a bit awkward, with the HDMI connectors sitting much higher than the row of BNC jacks and USB.

Having written the above -- maybe the vertical HDMI connectors should sit below the PCB? It might still look awkward, but you avoid the collision with the scope housing, and the plugs and cables would be further away from the touch screen.

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