prevent usb 5V backfeeding

I am new to this and in the process of building a prusa with sanguinololu 1.3
as printer controller, using a raspberry pi as host controller.

When checking the wiki, I came across the usb 5V bug mentioned on
http://reprap.org/wiki/Sanguinololu#Bugs
The solution mentioned there and googled elsewhere (desoldering the 7805 or cutting the 5V track) seemed a bit drastic
for my brand new controller board I have received only this week.

I searched elsewhere and came across a similiar problem people seem to be having with raspberry pi - some powered usb hubs backfeed 5v into the raspberry pi. That "feature" can apparently be used to power the raspberry pi, but it can just as well cause all sorts of usb problems.

An entry in a german raspberry forum seemed promising and gave it a try.
http://raspberrycenter.de/forum/leid-usb-hubs
There was suggested a little adapter that cuts the usb 5v line - but, being an adapter, it can be used with some
flexibility (well, you can use it or not use it, but you have a choice and the choice can be changed even after you have made it)

I built two of these adapters today, basically just a type A usb plug and type A socket soldered together back-to-back, but with only pins 2-4 connected and pin 1 (5V) not. The gap I have filled with hot glue so the pins are protected. The cost was about 0,50 EUR per adapter, but the socket und plug could just as well be salvaged from an old computer and broken usb stick.

What can I say: these adapters seem to work really well. I connected both of them to my raspberry pi. The powered usb hub is connected to the first, the sanguinololu to the second. Both the sanguinololu (the first time I saw my steppers move) and the usb hub for the raspberry seem to be working ok. What is more, the usb problems I kept having on my raspberry for ages seem to have gone, I could even connect a second usb hard disk via the hub, which was a first.

Sorry, if this is an answer to an obvious f.a.q. but I haven't found the solution with the adapter in the forum so far.

This is very intriguing. Thanks for sharing the idea. I haven't quite figured out what I was going to do about this and I couldn't quite decide if it was even still a problem (sometimes the boards get fixed and the documentation doesn't get updated). I may try this if I can find the old components lying around. Thanks.

I couldn't find all the correct parts in my scrap pile but I did have extra USB cables. So I just took one and cut through the red +5V wire. I put some heat shrink on each end to make sure it wouldn't short against anything and wrapped it all back together and it works fine.

I was thinking to cut the trace, but the way it's setup on the Sanguinololu 1.3a board it has a trace leading to the USB connector pin from the VR and then another one leading away to connect it to some other logic. So to fix it by cutting the traces I would've had to cut both of them and then add a wire to bypass that pad. It sounded like more pain so I ended up just cutting the wire in the USB cable and now I'll just make sure to always use the "special" cable.

What a great suggestion! I would make one note; maintaining the ground connection can help shield the cables from noise. If you are using long runs of USB cable you should retain the ground connection, but break the 5V connection. There is no downside if your hub is also powering the pi.

But if the pi's ground connection is different from the one on the hub then you may suffer from unwanted current in that connection and could cause heating.

Stemer114 Wrote:
-------------------------------------------------------
> When checking the wiki, I came across the usb 5V
> bug mentioned on
> [reprap.org]
> .
> The powered usb hub is connected to the first, the
> sanguinololu to the second.

Hey Stemper114,
Have you tried connecting the board to the hub too? Because I want to try this solution today, but I want to connect two printers via a hub.
Could you try and report your results?

Thanks,
Guy
(also first time here, so hi evreyone)

Hi guysoft,
I have tried both, connecting the reprap directly to the raspberry pi and via the hub to the raspberry pi using the customized usb connector, and both worked.

However, right now I am using the reprap with a dedicated PC. My only raspberry pi is used as nfs server, media server etc. and when mediatomb was running I kept getting performance problems. Besides, printerface was too tedious to work with and octoprint kept loosing the connection. So what I did was I bought myself a used PC (athlon xp, 8 years old) complete with monitor etc. for less than what a naked raspberry would have cost, installed debian onto it and am now running repetier host on the PC - which works very well and is very comfortable to use. I wouldn't want to miss the ability to preview the sliced model before starting a print and to monitor the printing graphically anymore..

I am still using the usb connectors though, one in between the reprap and the PC and the other one between the raspberry pi and the usb hub - both without any problems.

Regards,
steffen

Well, maybe that wasn't mentioned in the original post, but of course the ground pins need to be connected. Only V+/5V is disconnected. See attached pictures (the red wire is only used as a pointer in the photo).

Drastic as it sounds, I would just cut the +5V trace on the board. There is absolutely no reason for the sanguino to be powering to usb line.

Cutting the trace takes 2 seconds with a pocket knife and costs nothing. Once cut, never have to worry about frying a usb port, blowing the 7805 or forgetting the adapter.

lajos, I would have cut the +5 trace to the USB port, but it definitely went somewhere else as well so I would've had to cut two traces and then run a bypass wire and that is definitely more than 2 seconds with a pocket knife. It actually looked pretty difficult to figure out where to run the wire as the board is small and the parts are surface mount. Fixing the USB cable was much simpler :-).

Well, strange as it may sound, my setup initially required both: running the sanguinololu without and with 5V USB (at different times) - hence, the adapter and not the cut trace.

Without 5V USB: sanguinololu connected to its own power supply running in the reprap connected to a headless raspberry pi - in my basement with no monitor or PC around.

With 5V USb: the bare sanguinololu taken out of the reprap and brought upstairs to my normal PC for firmware flashing using the Arduino IDE and powered by usb.

PS. Since that setup involved removing the sanguinololu and running up and down the stairs too often I eventually discarded the raspberry pi and went for a used PC..

You only need to cut one trace, the 5v supply at the usb connector. I'll take a picture and post it, if I dont forget This information should really be on the wiki.

Quote

This information should really be on the wiki.

Then, please, put it there. Everybody has write access.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

lajos, that would really help if you could post a picture of what you did. That way I could confirm we have the sme board and make the cut. Thanks.

This older thread has pictures. Pick only one option! [forums.reprap.org]

Attached is a photo of the +5v line cut on my board.

Flip your board over, find the pads for the USB connector (4 pads in a rectangle layout above USB2TTL label). You need to cut the track going to the top left pad. I used the knife from my swiss army. It's actually more like scratching with the tip than cutting.

Edited 1 time(s). Last edit at 07/10/2013 10:18PM by lajos.

Thanks lajos. That does look easy and seems like it should do the trick. I'm not sure why I didn't figure this out earlier when I was looking for a place on the board to make a cut. Maybe I was assuming that the trace after the USB connector went somewhere important, but it looks like it only goes to the holes that end up underneath the USB connector and aren't really used. I'll see if I can cut it this Saturday when I have a few minutes and it should work just as well as the hacked cable. Thanks for the picture!

Not to necro, but I posted an alternate way to fix this to the wiki. The difference here is that the USB chip will stay alive and available to the host PC even if the printer is off since it lets the FT232RL get power from the USB port while stopping the backfeed problem. This might be better if you're using a host server like OctoPrint that you want to leave running while the printer is off.