V5C Technical Manual
3-10
Universal Serial Bus (USB) is a relatively new personal computer (PC) inter-
connect that can support simultaneous attachment of multiple devices. The host
serves as master of the bus. Data transfer is serial with two modes of signaling:
Full speed mode with a signaling rate of 12 megabits per second (Mbs), and a
low speed mode with a signaling rate of 1.5 Mbs. The peripheral devices serve
as slaves connected either directly to the host or through hubs which give addi-
tional connective capability.
The USB cable consists of four wires. Two wires, D+ and D-, are used for infor-
mation transfer using differential signaling. The two other wires are used for
power: Vbus and Ground. Low-cost unshielded cables are used for the low
speed (1.5Mbs) while shielded cables are required for the 12Mbs rate. Essentially
the USB protocol is similar to a Token Ring Network. The USB host keeps the
bus active by sending a Start Of Frame (SOF) packet every millisecond. The
available bus bandwidth is shared between simultaneously connected devices
within this 1 millisecond time interval.
Due to the capability of “Hot Swapping”, it is not necessary to power-down to
connect or disconnect a USB device. Immediately after a device is attached to
the USB host, the host controller is alerted by way of voltage change on the D+
and D- lines. Once the host detects that a device is connected, it determines the
exact port it is connected to and enables it. After a reset sequence the host can
provide the device with up to 100mA of current through the Vbus/Gnd lines. Of
course, self-powered devices do not use the power from the host.
The host uses an interrogation sequence through which it acquires information
from the connected device, gives it a unique address, and assigns a configura-
tion value. The system software then either recognizes the device and loads the
corresponding device driver, or prompts the user to provide a device driver for
the application. After the device driver is loaded, the device performs its intend-
ed function until it is detached from the host. Immediately after a device is
detached from the USB host port, specific voltage changes on the D+ and D-
lines alert the host of the action. The USB system software then acknowledges
the removal of the device and frees up any host resources the device was using.
For example the host reclaims the bandwidth, and if the device was bus-pow-
ered, it reclaims the power and makes it available to other devices.
The V5C has two USB Ports, designated USB0 and USB1. USB0 is a standard
USB connector located on the V5C front panel. USB1 is incorporated into the
P2 expansion. The V5C supports both the Full Speed (12Mbs) and Low Speed
(1.5Mbs) USB serial transfer. The V5C USB Protocol is facilitated by the
1543C Southbridge chip.
HUB
HUB
USB System Example
USB Ports
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
