USB Communication with Computer PSU vs 12v Supply

I was using using an ATX PSU (500W) with my RUMBA for a while now, but I just replaced it (because it liked to shut off in the middle of prints, and the 1A load on the 5v trick didn't work) with a dedicated 12v 30amp power supply. I have everything wired up, and I can get the board to turn on with the new supply, except USB communications doesn't work. Thought I burned something out, but when I went back to the ATX PSU, everything was fine - or at least the USB communication still worked. I assume it would still have the shut-down issue, let's not worry about that for now. I want to make sure im not missing something before I accuse the 12v supply of being cheap (which it is) and broken (which im not sure about). For diagnostics i've got a cheap multimeter. Nothing like an oscilloscope, that would be too useful.

Not sure if it makes a difference, but when I move the jumper to "USB power" from "stand alone", I still need the separate power supply (the ATX PSU). Some USB ports do nothing, some make the power light blink for a second, then turn off. I'm not sure if the USB power option used to work, I thought it did.

the name for what i think you have is on the tip of my toung, gaaaaa.

i think it has to do with the grounds, if the ground lever are different things don't wanna talk.

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[mike-mack.blogspot.com]

Ground loop.

How are you wiring up the AC side of the dedicated PSU? A pic speaks a thousand words here.

A lot of people connect AC Ground to DC -VE (or DC Gnd - in this case same thing). If DC ground on the PC is a fixed voltage different from the DC ground on the PSU, you get current flowing. This can either cause the USB to shut down, or in some cases cause the USB cable to get hot, as the current flows from one end of the USB cable to the other "trying" to make the voltages the same.

Dedicated PSU has the power cable going into the terminals as they are labeled. Green into ground (symbol), white into "N", black into "L". I have heard of the problem in other situations, I suppose i should have recognized it. I remember using a 3-prong to 2-prong converter being a solution, but that didn't help (i just disconnected the ground at the supply end). The RUMBA now resets when i plug in or unplug the usb cable, so something changed.

I wasn't clear on if you thought putting the mains ground into DC -V was a cause or a solution, I can try that too. And I keep testing the ATX PSU... still working.

Usually it's the problem, as you get a loop caused between:

Mains ground on Printer Supply to Mains ground on PC.
Mains Ground on PC to DC ground on PC's USB.
USB cable from PC to USB cable on RUMBA (or any USB based electronics)
DC ground on RUMBA to Mains ground on Printer Supply.

This forms a loop. If instead of these all being the same voltage level, there is instead a voltage difference at one point (eg: Say 1V between Mains ground on PC to DC ground on PC's US, then you get current flowing around the rest of the loop "trying" to make them equal. Depending on all the bits in the way, the result could be erratic behaviour, failure of things to work, or cables getting very hot.

With the PSU's off, you could measure resistances between AC ground and DC ground. When on, you could try measuring DC (or even AC) voltage. A lot of people recommend shorting DC ground and AC ground on those dedicated Brick-style PSU's, but I'm hesitant unless it's proven that doing so is actually safe (especially when many of these dedicated PSU's are of unknown origin or compliance, as they're usually made and sold on the cheap).

Well, to close this out, the solution was connecting the DC ground to the mains ground. Thanks for the tip about measuring the voltages, I checked between the +12v DC to mains ground; the ATX supply read 12v, while the dedicated read as an open circuit. I'll update if anything goes wrong. Like a fire.

Sadly, I think I broke the USB-power mode on my RUMBA board in the process of trying to get this working, but luckily, that's not something I need most of the time. What (and if) I actually broke isn't something I'm capable of diagnosing (my guess is a diode?), but if someone wants to try... it's clearly not directly related to my original issue. But I'd appreciate it. Maybe. If I every find out I need that functionality.

Most likely it's a diode or fuse on the USB +5V line on the RUMBA (or possibly even a track). I'm not particularly familiar with the Rumba schematic, but that is what I'd be looking at.