Introduction
I had a lot of trouble getting my udev rule correct for the SIMM programmer board, so I wanted to explain more about how I figured out what to do.
Details
We need a udev rule on Linux because it defaults to giving a USB CDC modem port permissions such that only root and members of the dialout group can talk to it. The rule simply gives read and write permissions to all users, and gives it to the plugdev group rather than the dialout group.
udev rules work by matching certain attributes and performing actions in response if the attributes match. In this case, we want to match on the USB vendor and device ID.
To get a list of attributes for a specific device (assuming /dev/ttyACM0 in this example), type:
sudo udevadm info --name=/dev/ttyACM0 --attribute-walk
This will give an awesome list of all of that device's attributes, along with a quick explanation of how the rules will work.
Rule for the SIMM programmer board:
ACTION=="add", SUBSYSTEMS=="usb", ATTRS{idVendor}=="16d0", ATTRS{idProduct}=="06aa", MODE="0666", GROUP="plugdev", ENV{ID_MM_DEVICE_IGNORE}="1"
I added the ACTION=="add" portion because I really only care about when the device is first being added to the system -- that's a perfect time to change the permissions. The rest of the rule matches a USB device with the provided vendor and product IDs. In response, it will change the mode and group of the device node.
Many newer Linux distributions use ModemManager to automatically detect modems when USB serial devices are plugged in by opening the port and doing things with it briefly. This behavior interferes with the SIMM programmer software, so setting the ID_MM_DEVICE_IGNORE environment variable forces ModemManager to skip auto-detection on the port.