MegaDebug interposer board

If only you could disconnect that one pin...

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Like many of you out there - I've been using Arduino CPU modules in test fixtures and some industrial control projects.

One problem that often comes up is the need to probe a signal - and more importantly - BREAK the connected between the Arduino and the "carrier board" (or a shield, or the rest of my project).

Hence, the MegaDebug "interposer board". Just slip this in-between your Arduino Mega (or an UNO, or any compatible footprint) and your shields. You now have the ability to disconnect any or all Arduino pins from your project. This board also adds a convenient (and stable) header pin for attaching your scope or LA probe.

You can also use this interposer board between different shields. No doubt you've come across a situation where two shields are "supposed" to work OK together, but don't. You can also temporarily re-wire a chip-select signal to a different Arduino pin.

When using this MegaDebug interposer board:

* Every Mega-2560 (or UNO, or compatible footprint) I/O pin can be disconnected, EXCEPT for Ground.

* Use standard shunts to make the "top" to "bottom" connections.

* Remove one or more shunts to effectively isolate Arduino I/O signals.

* Depending on your project's physical packaging, you may want to use the shorting-jumpers on one side or the other.  Either will work. Wherever you don't have a jumper installed - is where you can now attach a probe wire.

* This is a 4-layer board! Due to the density of signals at the 36-pin header area, getting everything routed as short and direct as possible meant a 4-layer scheme was preferable.

* Other than header connectors - there are no other active or passive components on this board. It should introduce no "problems" or "cures" when using it. However, certainly it does introduce some additional PCB track-length to signals (mostly the SPI port), so you should take that into consideration of you're running very fast, very critical signals (especially the SPI port).

I plan to offer this up on Tindie. Not sure if it will be just a blank board, a parts-kit, or fully assembled. Stay tuned...


3D-PDF view of the MegaDebug interposer board. NOTE: Best to download and open this PDF using Acrobat directly. The 3D-PDF's don't work for me when downloading inside a browser window.

Adobe Portable Document Format - 1.68 MB - 06/08/2020 at 17:32


  • MegaDebug Interposer: prototype build

    Ed Caceres06/26/2020 at 05:37 0 comments

    Status 7/4/2020: Partial build...

    Most of the Samtec headers arrived (courtesy Digikey). A few are still on back-order (probably built-to-order). However, I have enough parts to make a partial assembly and check the fit... So far, looks great!

    Top side (Arduino CPU plugs in on this side):

    Bottom side (Arduino shield(s) plug in on this side):

    Here's an Arduino Mega-2560 attached:

    You can see the 2x10 header there... that's where you install a little jumper across pin-pairs - thus connecting the CPU pins to the SHIELD pins. When you need to diagnose a problem and disconnect a signal - just remove the jumper!

    And the bottom side, all I had handy was an Ethernet shield, but you get the idea:

    Another view of the whole assembly:

    Status 6/28/2020: Ordered parts from Digikey!

    So pricey these $amtec headers! I've got to find a less expensive solution (but mechanically identical). For now, I just want to build a couple of boards to prove that everything fits together nicely when stacking together with a Mega-2560 and an appropriate Mega-sized shield board. One of my new industrial-control boards is arriving in a few days - and having this interposer board would really help with debugging (which is why I designed it!).

    Status 6/26/2020: Blank PCB fabs arrived!

    See below. All I need now are a handful of headers!

    Status 6/8/2020: Blank PCB fabs are on order!

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