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October 4, 2008

Playing with the ALIX LEDs

Filed under: UNIX & Linux — Tags: , , — martin @ 7:24 am

I have started to use an ALIX machine by PC-Engines as my printserver using CUPS. Being the playful kid that I am, I wanted the machine to somehow indicate that it has queued print jobs, using the LEDs.

Controlling the LEDs is fairly simple. I started on OpenBSD, found a very simple method for controlling them through gpioctl right away and quickly rolled it into this very simple shell script that allows for commands such as “led 1 on” or “led 3 off”, you get the idea.

I found that OpenBSD didn’t work too well for me as a CUPS printserver for multiple USB printers. The helpful OpenBSD mailing lists were down on that weekend, so I just decided to install Debian Linux onto the ALIX.

The Linux Kernels currently in circulation don’t however support the ALIX LEDs. Support is available, though. I downloaded the kernel module source from here (leds-alix_0.0.1.orig.tar.gz) and after compiling them and loading the leds_alix module, my shell script was ready to be extended into a “multiplatform” LED control wrapper.

(The LED drivers on Linux allow for a lot more fun, such as a “heartbeat” feature or acting as an “IDE” LED, but that’s not my point here. 🙂 )

On top of my little shell wrapper, I have now implemented a Perl script, the “CUPS LEDs daemon”, cupsledsd, which starts cycling the LEDs when there are pending jobs and uses the wrapper for turning them on and off. Of course, this could easily be used for something more sophisticated, such as displaying the number of queued jobs.

Update, 2008-10-07: I found that the original cupsledsd, as posted above, was a bit too hard on the CPU, using 5-10% for just blinking. Here’s a Linux-only cupsledsd that uses no system() calls. It accesses the CUPS queue directly (searches for data files in /var/spool/cups, probably unsupported), writes directly to the Linux /sys filesystem and blinks more frantically while not using a significant amount of CPU.

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