I've got arduino focuser on top of the AstroEq and the linux assigned different port every time so had to update the udev rules with a new one - make a new file for exapmple 52-arduino.rules and put the following line in it : SUBSYSTEMS=="usb", KERNEL=="ttyACM[0-9]*", ATTRS{idVendor}=="2341", ATTRS{idProduct}=="0042", ATTRS{serial}=="853343330323516091C1", SYMLINK+="ttyS80"
You can omit some of the attributes but in case you connecting 2 Megas can be sure that the correct one will be always ttyS80.