Dear all,

This is my first post in the Lenovo community and I have a query regarding T420 USB ports.

I have a T420 laptop and I am using a USB powered product/device with its USB port; a bus-powered USB device. The power consumption of the product I am attaching is 120mA on 5V; which I have verified using external power supply. However, as the product is not configured for USB operation, it is not enumerated with the laptop. I am suprised that even then, I am able to get it powered up successfully. As per the USB standard, a non enumerated device can draw only up to 100mA, but my product is consuming 120mA!

I want to know if T420 follows this standard or not. Asking other way, does T420 provide up to 500mA on 5V on all of its USB ports by default without the need for USB device enumeration?

Your reply would be highly appreciable.

Thank you,

KS

Anyone, please?

Thanks,

KS

Anyone from Lenovo support who could please answer my question?

Thanks,

KS

I am still waiting for any answer that you may have got. 

Thanks,

KS

Hi KS14,

Your first post says that you proved that your T420 supplies more than 100ma without enumeration. The (yellow) Always-On USB port provides more than 100ma with the system shut down, so I assume there is no enumeration happening there. I think we can conclude that full USB power is provided without enumeration, like most modern hardware (thank goodness!).

Hi bananaman,

Yes, I think what you say could be correct. I assume all USB ports have similar power architecture to always-on USB port in that they can deliver up to 500mA without enumeration.
Actually, I was confused because constantly referring to USB standard made it a bit difficult to digest the fact that I was able to draw 120mA without enumeration.

Thanks for your help.
KS