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- What would happen ....
- Man-wai Chang
July 28, 2007, 12:04 pm
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... if I short the USB port? Will the whole motherboard die or just the
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Re: What would happen ....
This is from the USB2 standard, in a paragraph just before section 188.8.131.52 .
Short Circuit Withstand
A USB transceiver is required to withstand a continuous short circuit of D+
and/or D- to VBUS, GND, other data line, or the cable shield at the connector,
for a minimum of 24 hours without degradation. It is recommended that
be designed so as to withstand such short circuits indefinitely. The device must
not be damaged under this short circuit condition when transmitting 50% of the
time and receiving 50% of the time (in all supported speeds). The transmit phase
consists of a symmetrical signal that toggles between drive high and
drive low. This requirement must be met for max value of VBUS (5.25 V).
The +5V rail usually has an overcurrent capability. It can take the form of
a Polyfuse, for example. A Polyfuse is a polymer based fuse that closes when
it cools off again. Thus, the motherboard USB port can recover after the
short is removed.
In terms of extraneous voltages, the ESD rating of some USB pads, is on the
order of 6KV. The very best electrical interfaces, are found on conventional
serial RS-232 interfaces, and some of those can withstand 15KV. Many other
types of integrated circuit pads, can only take 1KV or 2KV of ESD insult. So
the USB ports are still susceptable to static discharge, if the discharge is
violent enough. Under the right conditions, a human can develop a potential
of 25KV to 50KV. So that is one example of a source of possible problems for
a USB interface. (And more so on the computer case front USB ports, due to
the poorer electromechanical design typical there.)
For the USB interfaces that do die from time to time, the root cause could
be static electricity.
Re: What would happen ....
The USB specification is actually much better policed than most
interface standards due to the USB logo certification requirements.
Looking at the mandatory tests I see that powered hubs (including
the root hub inside your computer) _must_ have current overload
protection, and that this must be resettable without mechanical
intervention. However, I note that this protection may allow up
to 5A before tripping: more than enough to do damge in many instances.
Of course, there is nothing that mandates the certification and
testing regime other than the USB logo requirements - I'm familiar
with several companies that resell USB VIDs/PIDs so that you can
develop and sell USB devices without talking to the USB-IF at all.
However, from my observation they tend to be short run specialised
products rather than mass-market stuff, no matter how cheap. After
all, even cheap tat seems to have the logo as a matter of routine.