Fieldbus Overview
The USB was developed in response to the need for an economical, modern solution for connecting peripheral components to a PC.
The most important features of the concept were:
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- cost reduction
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- connection and configuration were to be as easy as possible (plug-and-play)
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- addition and removal of devices while the PC is running (hot plugging)
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- many of the technical disadvantages of existing solutions such as RS232 were to be overcome
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- connection without opening the PC
The USB is a polled bus, which means that all actions are initiated by the host, which is, in most cases, a PC.
Basic principles
Velocity/speed
Version 1.1 of the USB supports two speeds:
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- 1.5 Mbit/s (Low-Speed)
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- 12 Mbit/s (Full-Speed)
The BK9500 supports 12 Mbit/s.
Architecture
The cable length from one USB device to another may not be greater than 5 meters. A maximum of 127 devices may be inserted in a tree structure. Branching is implemented through hubs. The maximum nesting depth for cascading hubs is 5. This means that the maximum distance of the last device from the host cannot be greater than 25 meters!
Some additional function is often integrated into a hub. A keyboard (KBD), for example, receives data from the PC (host) and passes it on to other connected peripheral devices such as a mouse, printer and so forth.
Transfer types
Four types of transfer are distinguished:
Control-Transfer
Special inquiries known as requests are sent to the USB device, typically during the configuration phase.
Interrupt-Transfer
This is the transfer method for devices in the PC environment, such as the keyboard, that classically are driven by means of interrupts. No interrupt is, however, actually used.
Bulk-Transfer
Typical examples of this include printers, where large quantities of data must be transmitted without particular real-time requirements.
Isochronous-Transfer
This transfer types is used for data with special requirements on latency time. This mode functions synchronously and with high continuity.