Cydrome Leader wrote:

Michael A. Terrell wrote:

Cydrome Leader wrote:

Rich Webb wrote:
On Wed, 12 Sep 2012 18:34:33 +0100, "Arfa Daily"
wrote:

Anyone know if any USB pins other than 1 and 4, are used as part of the USB
charger protocol ? I needed a small 5 volt PSU for a little project, so I
bought one of the latest generation 'Apple' chargers from eBay, where it's
all built into a standard sized UK 13A mains wall plug. It came with a USB
to Apple lead, so I cut the Apple connector off, fully expecting there to be
only two wires - Gnd and +5v - in the cable, but there was actually 4 thin
wires. So I then assumed that they had maybe just doubled up on the wires,
as this thing is rated at 1 amp output. But again, no. All four wires were
separate, so I checked what was coming out of it. Just over 5v between black
and red, so that's ok, but measuring between black and green, and black and
white, there is also voltage. Two point something on one pair, and three
point something on the other, I don't recall the exact figures. Could they
be just 'pullup' values to keep the "data +" and "data -" lines quiet when
it has a genuine USB device connected to it ?

AFAIK, in order to pull more than 100 mA from a USB port, a device is
supposed to do a digital dance with the host to request the higher
capacity, up to 500 mA. There may be (read: probably are) dedicated
charging ports that will cheerfully source 500 mA (or more?) to a "dumb"
device without requiring the dialog.

Isn't it sad all this garbage is needed to send 5 volts down a cable?


Needing three resistors is sad?

yeah, it is.

USB is not like ethernet where adding power happened later on and there is
a chance to do weird stuff like plug two PoE switches into each other
with a patch cable. USB doesn't support "illegal" scenarios like this due
to the connectors, so any extra signalling is just bull**** to lockout
devices or make you get stuff you should not need.


A couple cents worth of resistors to turn on the port is a simple and
sensible way to protect the port. It was designed that way, not added
at a later date. Look at the data sheets for some of the hub ICs.