21 September 2010

CANBoat Design part III - Choosing the server hardware

The server that CANBoat runs on was always designed to run 24 by 7. This means that it must be reliable and use little power. Power usage adds up over longer periods. Something that uses 3W will use 6 Ah @ 12 V per day. So every Watt counts!

For a long while I thought that I would end up with some form of embedded ARM board running Linux. I bought a number of ARM based systems to run field tests.

However as I started to develop software it soon turned out that the criteria list contained three items, not two:
  • Power usage - as low as possible IN REAL LIFE.
  • Connectivity - USB 2.0 Host, Ethernet, Wifi.
  • Up-to-date Linux kernel with distribution that provides lots of packages.

I was surprised to find that in practice a small x86 server based on the AMD Geode ran with almost as little power as an ARM board. Unlike the ARM boards it had all the I/O that I wanted. Furthermore it was cheap and reliable to procure. You can get cheap ARM boards based on commercial items such as routers but these usually have a very short lifetime as manufacturers continuously drive down prices by bringing out new hardware. Embedded systems are usually a lot more expensive.

The clincher was that most ARM board manufacturers have a hard time keeping up with the Linux distributions, so they usually run out-of-date kernels and weak distributions. Support for FTDI serial converters was not a given, and these are used in many devices such as the Actisense NGT-1. I had to backport many fixes for the FTDI serial converters to the older Linux kernels to get reliable serial USB connections. Not Good.

So in the end I decided on low power x86. The PC Engines ALIX 2d.13 has exactly the interfaces I needed, but is available in a range of different systems. They run about € 100 / $ 140 which is still very reasonable. It runs a very nice mini Debian release named Voyage Linux with a kernel that is new enough to run stuff like the FTDI interfaces reliably. As it is just a pruned Debian release getting an additional package is just an apt-get away. The processor is a 500 MHz AMD LX800 which is plenty fast for Linux server needs.

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