Hi,
subject:
Power supply interrupted on USB port (normal and Always-on) when performing
reboot from any Windows version but not when rebooting from Linux or BIOS
(Lenovo ThinkPad T4xx). Why the difference?
According to my research there is no standard describing the power state
(power on/power off) of USB port on computer reboot.
Is that really so?
The problem we are having is the following. We have external hardware VPN
device which is powered through USB port. In order to establish VPN, one
must access web page on the device and enter password. To be able to log-in
to domain and apply Computer Configuration part of Group Policy assigned
through AD, one must first log-in (offline, cached mode), establish VPN
tunnel, perform OS reboot and log-in to domain. Since the device is USB
powered it is mandatory that there is no interruption of power supply during
this procedure. On Dell laptops (several models) and Apple Mac Books Pros
this works perfectly. However, on Lenovo ThinkPad (tested T410 and T420),
the USB power supply is interrupted during Windows OS reboot. Strangely it
is not interrupted when performing reboot in Linux or when still in
POST/BIOS mode. Another strange thing is that power supply is interrupted
for different periods of time.
T410 WinXP 225ms (525ms), T410 Win7x86 25ms(350ms), T410 Win7x64 75ms
(350ms), T410 Win8 1050ms (1350ms), T420 Win7x86 <5ms. The values in
parenthesis denote measurements on Allways-On USB port.
Is it possible to influence USB power on OS reboot? Linux and BIOS don't
interrupt the power of USB port on reboot, only Windows (all versions we
tested do). This along with different times of power lack leads me to
conclusion that there is no standard regarding this issue. Also we are
dealing with SW+HW, since we only found this issue with Lenovo ThinkPads.
Blaz Malnersic
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