Problems accessing serial ports via symlinks (on Linux)

[Ben Jones]

I have been trying to setup udev rules for my various USB devices since each time I unplug them and reconnect they get assigned a different ttyUSB* name. 

For instance I have RFXCOM and ZWave USB devices with the following symlinks defined;

   /dev/rfxcom

   /dev/zwave

However when using these syslinks in my openHAB configuration file the bindings always fail to connect. Using the direct ttyUSB* device name works just fine.

After some digging I discovered this is a known issue with th RXTX library being used by openHAB (http://rxtx.qbang.org/wiki/index.php/FAQ).

Therefore by adding the following to the connection logic before attempting to open the port;

// work around to allow device symlinks to work in Linux

System.setProperty("gnu.io.rxtx.SerialPorts", device);

// get port identifier

CommPortIdentifier portIdentifier = CommPortIdentifier.getPortIdentifier(device);

The bindings work with symlinks. I have tested this on the RFXCOM binding, but not ZWave as I don't have the latest code for that from Brian.

Can all bindings that use this RXTX library be updated to include this work-around so users like myself can use symlinks instead of having to update our config files every time we unplug a device?

Cheers,

Ben

[Ben Jones]

Ok - this didn't work as expected. Setting the system property in each binding is no good, since it is a 'system' property and each binding effectively overwrites the other! Therefore my RFXCOM binding was starting up ok, but ZWave was failing. After some more digging I have found that you either need to add a property to the java command line, or add the values to a properties file.

I have updated my startup script to include "-Dgnu.io.rxtx.SerialPorts=/dev/rfxcom:/dev/zwave" in the Java command line. Now everything is working as expected.

So ignore my first post about updating bindings - I think we leave the bindings as they are - and if you want to use symlinks with the RXTX library you need to set this gnu.io.rxtx.SerialPorts yourself - either as I have done in the command line args or in a properties file in the classpath (property file should be name gnu.io.rxtx.properties - see http://create-lab-commons.googlecode.com/svn/trunk/java/lib/rxtx/README.txt).

[Kai Kreuzer]

Thanks for this update, Ben!

Would be probably worth an entry in the tips&tricks section of the sample wiki (https://code.google.com/p/openhab-samples/wiki/Tricks), don't you think?

Regards,

Kai

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[Ben Jones]

Yep - good idea - done!