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CR1 chassis (E)
17
7.
US BOARD
7-1. Operational Description of USB Hub
7-1-1. General Description of USB
The USB is a serial transmission bus, and it enables connection of up to 127 units of devices, if divided through Hub. Of four pins of
connector, outside two pins are for 5V and GND and center two pins are for D+ and D– data lines. The USB cable is equipped with a
rectangle type series Aconnector and a square type series B connector; the series A always comes to the side closer to the PC. The
power 5V is supplied from the series A.
Up to 12MHz signals are sent to the data lines, and D+ and D– take inverted differential signals, but when communication is not made,
their signals are DC values indicating the status and they are not inverted.
To identify data, high (1) or low (0) is not used, but the NRZI system (see Fig. 2) is used where the status changes when data is 0.
High power
bus-powred hub
Tablet
CRT
Host/self-powred hub
ASCII
self-powered
Printer
Self-powered hub
ISDN TA
self-powered
High power
bus-powered
bus-powered
IR
unit
Modem
Self-powered
Low power
bus-powered
Joystick
Mouse
Low power
bus-powered
Keyboard
Bus-powered
hub
hub
PC
Fig. 1 Kinds of devices and their connection in USB
0
0
1
1
1
0
0
0
1
1
Original signal
After NRZI
conversion
conversion
HIGH
LOW
HIGH
LOW
Fig. 2 NRZI algorithm, 1:Status unchanged, 0:Status change
In the USB, all operations are executed under the control of host, and each device notifies of its status to the host.
For the devices connected to the USB, the keyboard, mouse, hub, etc. can operate immediately after connection because the PC has
standard drivers, but for the other devices, exclusive drivers must be installed in the PC.
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