Linux on Track
Linux proved to be an absolutely stable platform for software development and autonomous data acquisition. The three finger salute (ctrl-alt-del), well known on certain widespread desktop program launchers, is never necessary on Linux.
Using A/D conversion boards with on-board memory precludes all real-time constraints. Boards with too little memory are not easily supported. The fact that scheduling is sometimes suppressed for more than 100ms is considered a bug and first resulted in some hectic and active kernel debugging in cooperation with Ingo Molnar (Wien). It turned out that there seemed to be more than one reason for the problems, and they were reported to the kernel developers by Mr. Molnar. However, since we could not wait for the problem to be corrected (a simple patch seemed not to be enough), the solution described above was chosen.
Programming feature-rich A/D conversion boards proved to be more complicated than expected. Even the driver for the well-documented RTI-834 was not easy because of the many dependencies in time and logic between subcomponents of the board. It seems as if a general problem with A/D conversion boards is that designers put too many features on one board introducing dependencies and side effects only they are able to deal with correctly. This might be the reason why it is usually not possible to get good documentation—it simply does not exist, because nobody is able to write it.
A new and very interesting trend in measurement devices was recently initiated by Intelligent Instrumentation (a Burr Brown company). Their EDAS (Ethernet Data Acquistion System) is a 16 channel, 12 bit, 100KHz A/D conversion device which can be hooked to the Ethernet. For UNIX they deliver a library in source code to talk to the device, i.e., program it and read the data. No new device driver must be written. The device can either be connected to a local network or, if continous high speed transfer is necessary, it can be connected to its own “network”--a direct line between the device and a dedicated Ethernet board in the computer. However, while this idea is very nice and is similar to those fashionable WebCams, the EDAS is a bit broken for two reaons: A minor annoyance is that it does not understand RARP (reverse address resolution protocol). To set its IP address, it has to be connected to a computer via a serial line. A more major problem is the device's inability to continuously pump the 100KHz it samples onto the Net. After the first enthusiasm we were very disappointed when the German distributor told us that the EDAS' microcontroller can fill the internal 32 kilometer samples of memory at 100KHz, but that it is too slow to stream the data to the Ethernet at the same speed.
Considering the price of 2500 DM (about $1400 US), it would be cheaper to combine a single-board PC (1000 DM) with an A/D conversion board (1000 DM) and, say, some flash RAM as replacement for a disk into a small case. Install a minimal Linux and a suitable daemon as an interface between IP and the device driver of the A/D board, and you have an iDAB (Internet Data Acquisition Box). Depending on the application, you can even install software to preprocess the data before it is passed to the network.
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With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
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