To make sure the usbtmc devices have lax permissions, I do the following.

First, I create a group called “usbtmc” and add myself to the group

groupadd usbtmc
gpasswd -a awebster usbtmc

Log out and log in again to get the permissions set correctly.

Next, you’ll have to edit some udev rules in /lib/udev/rules.d/51-usbtmc.rules. On Debian systems this should be put in /etc/udev/rules.d/51-usbtmc.rules.

and add the following line to one of these files

# this is the general rule that covers usbtmc devices
KERNEL=="usbtmc/*", MODE="0666", GROUP="usbtmc"
KERNEL=="usbtmc[0-9]", MODE="0666", GROUP="usbtmc"

This will make it such that every time there’s a /dev/usbtmc[0-9] device, it will belong to the group usbtmc and have 0666 permissions.

On Debian you have to reload the rules:

udevadm control --reload-rules
udevadm trigger

Unplug and re-plug the device.  Verify that it works

ls -la /dev/usbtmc0
crw-rw-rw- 1 root usbtmc 180, 0 May  5 13:09 /dev/usbtmc0

You should have free reign to the device now.