- As the name suggests, USB is a serial transmission of data, but at a much higher speed than, for example, RS-232 interfaces. This means that the bus is also suitable for demanding applications, e.g. in data acquisition.
The USB transmission takes place via a single data line with a clock frequency of 12 MHz. This results in a maximum gross data rate of 1.5 Mbytes / s. No separate line is used for the clock; rather, NRZI coding ensures that the clock can be regenerated from the data. For this purpose, after no more than six bits with '1', a bit with a '0' must follow or be inserted. In extreme cases, this can reduce the available bandwidth by 16.67%.
- The data transmission is divided into so-called frames with a duration of 1 ms. A frame can therefore contain up to 1500 bytes of data. However, some of these are required for the protocol and control functions, so that in the worst-case scenario, around 1100 bytes of user data remain available per frame. When implementing the USB, it was possible to reserve fixed bandwidths for certain participants on the bus. This is particularly important for applications such as telephony, audio or measurement technology, in which certain transmission rates must be available. The remaining gaps within the frames can be used to transport less time-critical data, such as controlling printers or scanners.
- While the usual transfer rate is 12 Mbit / s, a special "low-speed" mode with 1.5 Mbit / s is provided. In this operating mode, only 8 bytes of data can be transmitted per frame; the time required for this is already taken into account in the frames. The low-speed operation is particularly suitable for peripheral devices such as mouse or joystick, which only generate small amounts of data.
Up to 126 devices can be connected to a USB. For this purpose, distributors (hubs) are used, which are integrated in monitors and keyboards, for example. The cable length between the hub and the peripheral device can be up to 5 m. A four-pole cable is used, one pair of wires is provided for data transmission, another is used for power supply. Depending on the power consumption, peripheral devices can be supplied directly via the USB, so that the plug-in power supply units that are often used today are in many cases superfluous.
- An important property of USB is its "hot-plug" capability. This allows devices to be added or removed without having to reboot the computer. The hubs are queried for new devices at regular intervals. If new peripherals are added, this is signaled to the computer. Information about the device itself is also communicated to the computer. This enables real plug-and-play operation.
- Due to the relatively high transfer rate, USB is also very suitable for applications in measurement technology. A practical example should clarify the bandwidth: the simultaneous transmission of music in stereo with CD quality (2 channels with 16 bits at 44.1 kHz sampling rate) as well as ISDN data transmission on both B channels (128 kBit / s) requires only 50% the available bandwidth. A data acquisition system with a 100 kHz sampling rate for the analog inputs, at the same time a 100 kHz update rate for the analog outputs, also only requires around 50% of the bandwidth.
- With the PCI-20xxx, Intelligent Instrumentation is bringing a USB data acquisition system onto the market that offers the same performance features as a PC plug-in card. In addition to 16 analog inputs with a total sampling rate of 100 kHz, the system has 2 analog outputs with the same speed. In addition, the system offers 16 digital channels as well as clock generators and counters. A FIFO (First In, First Out) memory in the system enables continuous data acquisition or output even at maximum speed.
- In the past, the reason for the limited widespread use of USB was primarily due to the small number of peripheral systems available. These were not offered because there was a lack of software support for USB. This changes with the launch of Windows 98, as USB support is built directly into the operating system. Owners of older computers can then use the advantages of USB without any problems. For computers without integrated USB hardware, expansion with appropriate plug-in cards is possible. Even owners of notebooks can finally use a larger number of high-performance peripheral devices.
- There will also be a solution for even higher bandwidth requirements in the foreseeable future. With Firewire, a serial standard is under development that offers the same performance as today's PCI bus for devices connected externally to the PC. Then it is quite conceivable that computers without internal slots will become the standard, since all peripherals can also be connected externally.
Further information about USB for both users and developers can be found on the Internet, e.g. at
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