Open Source Modular Test Bench

Description

One major hurdle in making hardware is verifying that it works correctly. Sure, an LED circuit is pretty easy to debug but once it gets more complicated, testing and verification becomes difficult. You may need a low frequency signal source, or a network analyzer, or a switch, or any number of things.

In the professional world, these are solved problems. However, not many of us in the amateur/hobby/startup world can afford the many thousands of dollars for a suite of test gear to characterize and verify our systems.

The aim of this project is to create a framework around which instruments can be built and integrated to form a single piece of test gear with almost unlimited capability. The modules will be of standard size and interface. If you can't find one that fits your needs, make it, and make it available for everyone else's benefit!

Details

To start with, this project would require an enclosure to hold the main control board, backplane, and all associated daughter boards/modules. This could be done like the PXI chassis or it could be set up where every module is in it's own case (for instance a Hammond extruded aluminium case) and is then slid into a rack or shelf system for neat storage.

The interface to each module is extremely simple, having power and SPI connections. A six-pin connector with pins for +12V, GND, SCLK, MOSI, MISO, and slave select pins could handle all communications between the daughter boards and the control board.

The control board would have connections to the backplane of +12V, GND, MOSI, MISO, and eight slave select pins (one going to each of the potential daughter boards). These slave select pins could be driven directly from microcontroller pins on the control board or through a combo of 3-to-8 multiplexer followed by an eight channel inverter to limit pins required to three. As SPI can be run at many MHz, there is plenty of bandwidth available on the backplane to get commands and data back a forth between the control board and the modules.

The control board would interface to a computer via USB. A program running on the host computer would be where the user interacts with the test bench and where data is displayed.

Board form factor would be 10x10 cm. These boards are big enough to pack in a lot of circuitry yet compact enough to fit in reasonably priced enclosures. Also, they can be had cheaply from many manufactures. For example, Elecrow produces 10x10 cm 2-layer boards for about $25 (which gets you ten copies) or 4-layer boards of the same size for about $45 (also for ten copies).

Here are some examples for daughter boards that I have thought of:

General purpose signal/instrumentation amplifiers
RF Frequency synthesizers
Signal/waveform generators
Small power supply
RF Switch
Network Analyzers
Spectrum Analyzer
General purpose ADC
GPS disciplined oscillator
Distribution amplifier

One great thing about this project is that anyone can contribute a module. If a library of modules could be designed and published, then getting a suite of test gear is obtainable by many more people, not just professionals. It also serves as quite a fun project!

Discussions

Myself wrote 06/15/2018 at 20:41

Seems to me that you could just assign SPI signals to the unassigned pins in a Tektronix TM5xx rack, and be done with it. The chassis are all over the used market, and plugins like power supplies and multimeters are cheap and already exist. You could add your own plugins and run alongside them with no need to reinvent certain wheels.

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ivan003003 wrote 04/24/2018 at 20:05

Any kind of progress ?

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loldavid6 wrote 08/16/2016 at 12:50

Would it be worth adding high speed communication pins as well for daughter boards that need to send data at a very high rate, like a logic analyse or oscilloscope? Also, do you intend to use card edge connectors for the back plane connection? Really great concept by the way, can't wait to see how it develops!

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Brett Killion wrote 08/17/2016 at 15:00

SPI can run at several 10s of Mbps (20...?) so that might be quick enough to get samples from a daughterboard in reasonable amounts of time if you did a burst capture and then sent everything in bulk. If you had a microcontroller with DMA, you could implement a circular buffer to basically stream samples as you got them in. For instance the STM32F4s I know have this capability.

As a logic analyzer, you could have 16 inputs, condition them, and wire them to the inputs. You can initiate memory to peripheral DMA transfers and read the pins and then directly transfer them out of the SPI bus. This could all be clocked from a timer interrupt or through a delay loop in main.
As far as an oscillocope, they always seem harder to me but I guess you could use an external ADC, read its conversion in parallel, and send the bits over the same way, although the bandwidth wouldn't be great and honestly I'd recommend a Rigol.
Once the control board had the samples from the SPI bus, it could USB them back to the host computer for processing. I think this is essentially what the Saleae series does. It's pin inputs direct to computer through USB.

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loldavid6 wrote 08/17/2016 at 18:31

Nice, I can't wait to see more!

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K.C. Lee wrote 08/17/2016 at 18:51

Signal integrity with multidrop bus in a backplane will post an upper limit on your real life connection speed. Each of the bus connection to the SPI will act a stub - the extra parasitic capacitance of those stubs will cause reflection.

Ideally, you want a much higher ground to signal ratio to prevent cross talks. A point-to-point connection with source termination will have much better signal quality.

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Open Source Modular Test Bench

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Open Source Modular Test Bench

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