Hi
I have just bought a used K8400 and tried to update the firmware.
When trying to load the firmware to the printer I get the error message that the “com3 is allready in use. Please unplug any other unit using Com3”.
The problem is that nothing else is connected to Com3.
I have tried to restart the computer without any luck.
Please help.
Did you use Arduino software 1.0.6 or lower?
Did you put the jumper on Jprog?
Hi Wrong Way
Yes and yes 
This is strange.
When Device manager is open and you unplug the printer does the com port go away?
Hi,
I have exactly the same issue, finished building the K8400, by compiling firmware error message: “com5 is allready in use”
Arduino 1.0.6
Jprog jumper set
com 5 disappears when I disconnect printer.
Was there a solution for this issue?
Hi, it seems like the cause is internal port configuration in my laptop, because I repeated the hole procedure on another PC with success!
This problem is solved! Up to the next…