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Printer disconnecting or USB port disappearing in Windows Answered

First I thought to make an Instructable out of it but realised there are too many different printer models out there, so this time no images.

What is this about you might wonder?
Well, let me tell you my story first and you might see similarities to your problem.
It all started with me getting a new PC as the old one got memory problems (RAM modules failed).
After a few successful prints I noticed errors coming up in the log window.
Mainly things like communication problems and that some data is sent again.
Realised that on my new PC I did not check the speed settings for the COM ports, so I adjusted them to match the printer board and moved on without even bothering to check the logs.
Then, half way through a bigger print, it all stopped and I could not even connect to the printer anymore.
After a power cycle on the printer all was fine again but the error kept coming back every now and then.
At this point I started to read up on the problem and the most common recommendation is to print over SD - too bad if your printer does not support it and too bad it does not address the issue at all!
A few more technical answers pointed to the Logitech drivers, especially mouse, keyboard and 3D vision.
As I was using the same outer hardware as before and also the same drivers (and same version numbers) I simply ruled this one out too, although it might be a vital clue for others.
When starting to get frustrated and after opening a cold blonde I remembered that I had a similar issue a long time ago and that it was related to loose wires on the screw terminals for the power connection.
Measured it all but according to my trusty multimeter all was fine.
Now comes the fun part:
I friend of mine with a HiFi fetisch was here when I testing the connection and he started laughing at my attempts.
He explained that ALL his connections, no matter if power or audio signal are oversized!
Here I started to wonder if he is up to something and looked up similar circuit board mounted power plugs.
To my surprise they are all rated for anything between 1 and 5Amps.
Even without a heated bed powered by the printer board I think everyone will agree that a max 5A connections is not enough.
My frined then offered to check the plug and connector at his place - what a great thing to have friends :)
He used a signal generator and small speaker with the plug as a connector between them.
With an oscilloscope connected to both input and speaker you could see, while moving the plug, that the audio signal become somehow unclean - there where spikes and missing bits everywhere depending which way you wobble the plug.
After taking the whole thing apart the destructive way we saw the cause: overheated contact areas with discolored surfaces.
I replaced the plug now with a 250V/20A one from an old laser printer and had no USB or connection issues ever since.

Ok, what's the thing with power and USB problems on a 3D printer?
Almost all printer boards have the ground connections bridged to avoid interference on the USB signals.
This mean, in case of a faulty power connection or one that is "dirty", the USB port on the computer can receive back EMF signal or even a voltage spike.
In return a smart bios either disconnects the port or disables it until the problem is solved, in our case by restarting / reconnecting the printer.
But even with no obvious signs of power problems you can get  a so called "dirty" connection.
Dirty covers all from corrosion, worn springs for battery compartments, overheating or in the old days burnt relay contacts.
For our printer it usually means that either the soldering connection on the board or a screw came loose, in rare cases like mine an undersized connector can burn out due to being unable to handle the currents.
The bad thing is that you can not always spot these problems the easy way...

Is there an easy way to tell that my USB problem is caused by a faulty power connection?
Yes and no.
Some boards offer indicating LED's, you connect power and the LED stays on even if the printer is disconnected from the PC.
If it is more than just a Power on" indicator" this LED will be off after the printer stops working and in the device manager your serial port for the printer is gone.
You might also hear the warning sound from Windows in regards to a device being disconnected.
If all the above is true than most likely your power connection has a problem somewhere - if in doubt replace all screw and plug connectors for the power.
Sometimes the problem is less obvious.
Your print software might show communication problems in the log window or re-send a lot of commands.
If the speed selection for the com port is the same as for the firmware of the printer board and as set in the printer software, it could also be a power connection problem.
But to be on the save side try a shielded cable with ferrite cores on both ends first for the connection between computer and printer.

Can I take any measures to prevent the problem?
Sure, you could solder everything and make sure the wires are fixed in place so they won't move.
But a simple indicator might be enough:
Solder a piece of LED strip to the power connection on the circuit board - if the connection has trouble the LED's will show it in most cases by changing light levels or flickering.
On top they act as a nice light to see what's happening during a print.