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DIY test equipment part 1

Open Source Logic Analyzers

I ran across a reference to an open source/open hardware logic analyzer at comp.arch.fpga. (Idea for future reader survey: how many internet users still use usenet? How many even know what usenet is?). So it got me thinking about open source test equipment. What’s out there, and how usable is it? Can a hobbyist outfit a workbench with open source/hardware test equipment and get decent performance and a decent price? Here are a few things I found:

Over at the gadget factory I found an open source FPGA based logic analyzer. It uses an FPGA platform called Butterfly, which is an expandable FPGA based prototyping system. This logic analyzer supports 4K of sample memory, 32 channels sampling at 100 MHz or 16 channels sampling at 200 MHz. Samples are RLE encoded to make the most of the limited sample memory. It also has complex triggering with four levels of trigger states. It offers externally clocked state analysis, including I2C, SPI, and UART protocol analysis. There are clients available for Linux and Windows.

There is a newer effort called Sump Pump going to port the Gadget Factory LA to a dedicated board with similar specs, but a price in the $30 range instead of $100 or so. Specs are similar, but with additional features such as trigger outputs, and external SRAM to expand sample depth.

The grand-daddy of these projects, and the inspiration for other open source LAs, is a thing called Sump, which was done circa 2007. It is based on a xilinx Spartan 3 eval board which was widely available then, but which is obsolete now.

There is a project called CheapLA, which is based on current Spartan 3E or 3A eval boards. This is an interesting project in that it incorporates a MicroBlaze processor that runs a WWW server, so that you interact with it using a web browser. There is also PC client software that includes “sophisticated post processing. I couldn’t find much in the way of technical specs but I imagine it is similar to the Gadget Factory projects since it uses similar hardware. CheapLA uses external RAM to provide much deeper sample memory, and uses internal FPGA ram to buffer the RLE encoded data, giving it the ability quickly buffer bursty data while capturing longer traces at a lower average sample rate.

I also found miniLA. This project, based on a xilinx CPLD, really puts the mini into miniLA. Nonetheless, performance is similar to the above, 32 channels at 100 MHz, 128Kb sample memory for each channel. Features are limited to a single level combinational trigger with a programmable trigger counter, external trigger input, and selectable trigger position. Like most open source projects there are a few versions, this one comes in a parallel port interface version as well as a USB version. There are also a variety of clients for this board, supporting Linux, Windows, and even DOS!.

What about used commercial equipment?

The hobbyist looking to outfit his workbench on a budget has another source of affordable test equipment: Ebay. There is an Ebay category specifically for logic analyzers, and you can filter by brand, model, and specification, so it’s easy to shop there. Logic analyzers can be had for $50 or less, but they are pretty old, certainly not offering USB or ethernet connectivity. Filtering the list of used candidates for models that offer some way of getting data from the machine into a PC, and sample/state rates of 100 MHz or better, we find plenty of HP 16500 class systems starting around $250.

Many of these are “As-is”, and vary widely in terms of what cards are in installed, and what cables/accessories are included. Tested and relatively complete units start in the $500 range. The 16500 offers a touchscreen interface but can also use a VGA monitor and PC keyboard. It has an ethernet connection and can communicate with an external X server, and it also has an internal HTTP server. For you DOS users it also has a 1.44MB floppy drive. Many options are available for this mainframe: oscilloscope, pattern generator, state/timing analysis, memory expansion, etc. Benchtop units are also plenty wide and deep, 102 channels x 4K deep is typical. For the price this is a pretty compelling alternative to an open source alternative, however the size of a benchtop unit may be a problem.

Conclusion?

Personally I prefer used commercial equipment, so I have an older HP LA and much older Tek scope. Neither connects to my PC but mostly that’s OK. However the open source projects do have some advantages: price, PC connectivity, size, gadget appeal. The eval board based projects have the additional advantage of being able to be repurposed with a new FPGA design. While I probably wouldn’t buy an eval board with the intent of using it as a LA, if I already owned one of those Spartan boards I would certainly have the LA design downloaded and would probably use it on occasion. Coming up next… open source oscilloscopes. Stay tuned…

Steve Ravet

The EDN gadgeteer

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