Serial Issues

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The Processing serial library allows for easily reading and writing data to and from external machines. It allows two computers to send and receive data and gives you the flexibility to communicate with custom microcontroller devices, using them as the input or output to Processing programs. The serial port is a nine pin I/O port that exists on many PCs and can be emulated through USB.

Using the Serial Port

All Platforms

  • If you're using a program to check to see if the serial port is working, then it won't be available to Processing. That means that if you're using HyperTerminal or whatever to see if your serial device is working, then you need to quit out of that application before using that port with Processing.
  • Serial.list() will only list the ports that are currently available. So if you have a program that's using the serial port that you want to use for your Processing sketch, it's not going to be available, or even listed.
  • If you get an error reading something like this:
 WARNING: RXTX Version mismatch
 Jar version = RXTX-2.0-4
 native lib Version = RXTX-2.1-7pre17
 java.lang.ClassCastException thrown while loading
 java.lang.UnsatisfiedLinkError: Native Library /Applications/processing-0090/libraries/serial/library/librxtxSerial.jnilib already loaded in another classloader

then you probably have another copy of RXTX installed somewhere. This is a messy thing to straighten out, because it could be anywhere in your PATH or CLASSPATH. Search your drive for RXTXcomm.jar or simply comm.jar. Files that might cause trouble are libSerial.jnilib (on Mac OS X), libSerial.dll (Windows), jcl.jar, and (additional notes from stendahl)

Mac OS X

  • We use rxtx (formerly version 2.1_6, as of Processing 0111, we're using rxtx 2.1_7) to handle serial I/O, which is included with the Processing release. If this is the first time you're using rxtx, you'll need to make your way to the "libraries" folder, find "serial", and inside, and run macosx_setup.command (double-click it and follow the instructions). This will make sure that things are properly set up (a few permissions need to be changed). As of release 0134, running macosx_setup.command is no longer necessary, thanks to help from the Arduino guys.
  • In the past, I've used a keyspan 28X dual port adapter, and Serial.list() returns things like

You'll probably have something similar. Don't mind the frightening names.

  • Tom Igoe was kind enough to note that you'll be in a world of hurt if you disconnect your serial adapter while a sketch is running—it'll prolly freeze the machine and require a forced reboot. This scenario may seem unlikely, but you might run into it if your adapter is plugged into a USB keyboard, and you have the keyboard plugged into a monitor/keyboard switcher. You know, those cheap KVMs that everyone likes to talk about in relation to the Mac Minis? The ones that actually cost ~$150 and up?


  • Linux also uses rxtx-2.1_7 (just like Mac OS X). If you're having trouble getting things to run, i.e. the port menu stays grayed out or you get error message spew to the console when starting the application saying "Permission denied" and "No permission to create lock file", this is because you probably need to add yourself to both the uucp or lock groups so that processing can write to /var/lock so it doesn't get in a fight with other applications talking on the serial port.
  • Alan Kilian contributes this description:
  1. I did an ls -l /dev/ttyS0 and saw that the group was set to uucp.
  2. Then I edited /etc/groups, I found the uucp group, and I added my login ID to the uucp line.
  3. I logged out, and logged back in again.
  4. The "groups" command now showed I was in group uucp, and when I started processing, the serial port menu item was not greyed-out.

It's important that you're in both groups, and that you completely log out and log back in again.

  • Running processing as root will often get rid of the errors, but that's obviously not a good solution for a million reasons (among them: beta code that runs as root and handles files? yeah great...)
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