How much EMI can a USB port handle?

Thread Starter


Joined Oct 3, 2010
I've made this PTZ camera thingamajig:

I put two sliprings on it and I'm hoping to be able to connect to the camera through the sliprings via USB.
The stepper motor power shares the same sliprings.

I measured the EMI just now to determine if it is safe to pass USB signals through here:

But my measurements don't answer my question.

I get more noise when the axes are NOT moving (holding position): ~170mV max.
When axes ARE moving, about 90mV.
Frequency either way is 27kHz


Google did not enlighten me about this. All I found was information about FCC testing that only applies to EMI generated by a USB device.

NOTE I am not asking if it will be possible to transmit USB data through slip rings in a noisy environment. I am asking if it is safe to attempt.
I don't want to damage my camera in an experiment to determine that something I already suspected might not work, in fact won't work.


Joined Jul 10, 2017
The reliability will not depend on EMI. It will depend on the quality of the contacts on the slip rings and the stability of the USB voltage. A large value capacitor across the camera supply input will help to minimize dropouts but you are tempting fate if you use the same supply and slip rings to power the stepper motor. The drive pulses will generate a lot of electrical noise on the supply.

Thread Starter


Joined Oct 3, 2010
I put the cart before the horse on this one. I just now tried USB alone through the slip rings (steppers and controller removed from the circuit) and it doesn't work.

My USB mouse and keyboard will work through the sliprings but not the camera. I suspect there may be a loss or corruption of data that the mouse and keyboard drivers are designed to ride through but my camera driver does not have such seamless error handling.

So, this is a wrap I guess. Need to get a different camera which has WiFi or bluetooth.