Universal
Serial Bus
USB version 1.1 supported
two speeds, a full speed mode of 12Mbits/s and
a low speed mode of 1.5Mbits/s. USB 2.0 supports
up to 480Mbits/s. The 480Mbits/s is known as
High Speed mode and was a tack on to compete
with the Firewire Serial Bus.
USB Speeds
High
Speed - 480Mbits/s
Full
Speed - 12Mbits/s
Low
Speed - 1.5Mbits/s
The Universal Serial Bus is host controlled. There
can only be one host per bus. The specification in itself, does
not support any form of multimaster arrangement.
However the On-The-Go specification which is a tack on standard to
USB 2.0 has introduced a Host Negotiation Protocol which allows two
devices negotiate for the role of host. This is aimed at and limited
to single point to point connections such as a mobile phone and personal
organiser and not multiple hub, multiple device desktop configurations.
The USB host is responsible for undertaking all transactions and scheduling
bandwidth. Data can be sent by various transaction methods using a
token-based protocol.
USB uses a tiered star topology, simular to that of 10BaseT
Ethernet. This tiered star topology, rather than
simply daisy chaining devices together has some
benefits. Firstly power to each device can be monitored
and even switched off if an overcurrent condition
occurs without disrupting other USB devices. Both high, full and low
speed devices can be supported, with the hub filtering out high speed
and full speed transactions so lower speed devices do not receive them.
Up to 127 devices can be connected to any one USB bus
at any one given time. Need more devices? - simply
add another port/host. While most earlier USB hosts had two ports, most
manufacturers have seen this as limiting and are starting to introduce
4 and 5 port host cards with an internal port for hard disks etc. The
early hosts had one USB controller and thus both ports shared the same
available USB bandwidth. As bandwidth requirements grow, we are starting
to see multi-port cards with two or more controllers allowing individual
channels.
A notable feature of USB, is its transfer modes. USB supports
Control, Interrupt, Bulk and Isochronous transfers.
While we will look at the other transfer modes
later, Isochronous allows a device to reserve a defined about of bandwidth
with guaranteed latency. Each transfer mode provides the designer
trade-offs in areas such as error detection and
recovery, guaranteed latency and bandwidth.
Where and how is USB
used?
USB is relatively a new technology
for Industrial Automation but is already becoming
intergrated into many industrial products on
a global basis. USB is a relatively inexpensive
and a simple network to implement which is
why it is gaining rapidly in popularity.
See
Anybus products supporting the USB network
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