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.

USB FACTS
Network Size:    Max 127 nodes
Network Length:    5m point to point
Data Rate :    1.5 Mbit/s to 480 Mbits/s
Bus Topology :    Tiered Star
Addressing:    Point to point
Network Features :    USB supports Control, Interrupt, Bulk and Isynchronous transfers