INITIALLY the Universal Serial Bus (USB) was designed to connect peripheral devices, such as keyboards and mice, to PCs. However, USB has proven useful for many other applications, including measurement and automation.
The USB 1.1 specification specified a maximum throughput of 12 megabits per second (Mb/s), but recent USB developments increased the speed, as outlined in the USB 2.0 Specification below.
The USB host automatically detects when a new device has been added, queries for device identification, and appropriately configures the drivers. Due to the bus topology, up to 127 devices can run concurrently on one port. Conversely, the classic serial port supports a single device on each port. By adding hubs, more ports can be added to a USB host, creating connections for more peripherals.
USB, the PC, and measurement and automation
MANUFACTURED PCs are currently equipped with USB ports; therefore it is not necessary to purchase a dedicated controller to interface to a USB-based data acquisition device or instrument.
USB-based devices are supported by Windows 2000/SP/Me/98. USB delivers an inexpensive, easy-to-use connection between data acquisition devices/instruments and PCs.
USB provides a step up from conventional serial port technology by featuring faster performance, hot-pluggable functionality, built-in operating system configuration, and multi-drop cabling, which allows you to connect multiple devices from the same port.
Because USB features are an excellent fit for measurement and automation applications, USB usage in measurement and automation is expected to become more popular.
How’s this for a good fit?
USB is easier to use than some of the traditional internal PC buses, such as PCI and ISA. The devices that use USB are hot-pluggable, eliminating the need to shut down the PC to add or remove a device.
A USB bus also has automatic device detection; once you plug in your device, the operating system software should detect, install and configure the device automatically.
High-speed data and control applications benefit from the ability of a USB to deliver data through either isochronous or asynchronous data transfers.
With isochronous transfers, USB guarantees bandwidth with time-based delivery of data packets. Isochronous transport guarantees that a transmission is completed within a given amount of time, but it does not guarantee that the transmission is received error-free.
The USB protocol guarantees devices that have requested isochronous bandwidth a predetermined number of data packets in each frame. Data-intensive applications often demand isochronous bandwidth.
Measurement and control systems commonly require the ability to respond to events. USB allows any device to generate an asynchronous event. Asynchronous transport guarantees accurate delivery, and devices with urgent messages can be given priority over all other devices.
Applications for asynchronous transport include delivering control messages and changing device parameters.
Aspects of the specification
THE USB 2.0 specification divides USB devices into three categories based on transfer rates:
• Low-speed devices, such as keyboards and mice, operate at 1.5 Mb/s.
• Full-speed devices operate at 12 Mb/s.
• The new high-speed class of devices delivers throughput of 480 Mb/s - 40 times faster than the USB 1.1 standard.
The primary differences between USB 1.1 and 2.0 specifications that impact test and measurement devices are the addition of lower-latency 480 Mb/s (Hi-Speed) transfers and improved host software specifications.
USB 1.1 compliant devices will not become obsolete because all USB 1.1 devices are compatible with the USB 2.0 standard (they are classified as full- or low-speed devices). All National Instruments USB products are USB 2.0 compatible, and many take advantage of the fast transfer rates of Hi-Speed USB.
How can I use USB in my measurement system?
NATIONAL Instruments recently released several new data acquisition (DAQ) devices to simplify hardware installation and setup:
• High performance USB DAQ: DAQ devices with up to 16-bit accuracy and sampling rates up to 800 kS/s.
• USB DAQ for sensors and high voltages: Measurement devices with built-in or modular signal conditioning for sensors such as thermocouples, RTDs and more. These devices also include modules for measuring high voltage signals up to 60 V. Some devices offer isolation as well.
• Low cost USB DAQ: DAQ devices with basic functionality and up to 14-bit accuracy and sampling rates up to 48 kS/s.
NI offers several other types of USB-based devices, including: a 15 MHz digital oscilloscope (NI 5102 for USB), data loggers (NI 4350 for USB), and digital I/O devices (DAQPad-6507/8).
Serial or GPIB to USB converters for instrument connectivity are also available. The USB-232 and USB-485 devices offer up to four serial ports per USB connection.
The GPIB to USB converter, GPIB-USB-HS, allows you to control up to 14 programmable GPIB instruments through a PC USB port. The GPIB-USB-HS uses the industry standard NI-488.2 application programming interface, which automatically converts USB to GPIB.
* Commentary by National Instruments