"Stringfelowhaulkie" said:
On my K8V SE DELUXE I have two USB headers one says USB78 and the other says
USB56. On the pin diagram the 78 has a P7 and P7+ pins, the 56 has a P5 and
P5+ can anyone tell me what the difference is?
There are four wires on a USB interface. A 10 pin USB header contains
two interfaces. USB56 contains USB interface #5 and USB interface #6.
USB78 contains USB interface #7 and USB interface #8. That is a
total of four USB interfaces for the two headers.
On an individual USB interface, there are two wires for power,
and two wires for data. Power is +5V and Ground. The data wires
consist of a "plus" and a "minus" wire, implying they are
differential signals. Differential means the signals are the
opposite polarity of each other, and that scheme is used to
improve the ability to pass high speed signals. In other words,
even though there are two wires, the signals are "subtracted"
from one another, to make a single digital signal inside the chip.
At least, that is what the +/- differential convention implies.
To hook up a USB interface, all you need is to wire up four pins
on the header. You don't have to use all of them for it to work.
A natural question to ask would be:
Why do they insist on pairing two interfaces on a header like that ?
The answer is historical. At some point, when USB 1 was designed,
someone decided that a single instance of logic circuitry for USB
would be used to control two physical ports. That means that the
two ports in question, share a total of 12Mbits/sec between them.
With that pairing of two ports with one logic circuit, it means
for USB 1.0, ports were always added in pairs. So, it seems a
natural thing to do, to pair the associated interfaces on the
motherboard header. In other words, the two interfaces on the USB56
header, share the same logic block in the Southbridge. A different
logic block controls USB78. That holds true when the headers are
used at USB 1.0 rates. When the ports are used at USB 2.0 rates,
there is a separate logic block used to control all ports - one
60MB/sec logic block controls all the ports running at USB 2.0.
If the USB standard had started at USB 2.0 rates, and there was
no USB 1.0, then the interfaces on the motherboard would likely
have been a 1x4 header, rather than being paired.
If you are curious, the specs are here (or at least this is
where I got them a while ago):
http://www.usb.org/developers/docs/usbspec.zip (USB 1)
http://www.usb.org/developers/docs/usb_20.zip (USB 2) 9MB!
HTH,
Paul
