A data transfer rate of more than 500 Kbytes/s is possible on the GPIB bus, provided that bus cable length is limited to 1 m (times the total number of devices), up to a maximum length of 15 m. Longer cable lengths reduce the maximum data transfer rate to less than 500 Kbytes/s.

Nonetheless, GPIB flexibility makes the configuration of an instrument I/O bus within a system a relatively easy task. However, it's necessary to ensure that each instrument on the bus has a unique address. This often requires that an instrument's address be changed manually during system configuration.

There's another drawback. Along with their connectors, the cables used with GPIB are rather large and bulky. They're also relatively expensive.

Moreover, GPIB isn't a standard built-in PC interface, so connection to a PC has to be made via a PC interface card that's installed in one of the PC's expansion slots.

Communication with instruments requires the installation of an I/O software package. Plug-and-Play drivers, IVI-COM drivers, VISA/SICL (Virtual Instrument Software Architecture library and Standard Instrument Control library), and Agilent's Intuilink are examples. These packages support popular languages such as C and C++, VisualBASIC 6.0, VisualBASIC.net, National Instruments LabVIEW, and others.

USB Makes the Scene
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USB is now emerging as one of the contenders for instrument I/O supremacy. USB was originally intended as an alternative to RS-232 serial and Centronics parallel interfaces on PCs. In those applications, it's suitable for a range of computer peripherals from slow devices, such as mice and keyboards, to high-performance devices such as scanners, printers, and cameras. Nowadays, USB is finding its way into test-and-measurement instrumentation, too.

Unlike GPIB, USB is a serial interface bus that's capable of data transfer rates of about 12 Mbits/s for v1.1, and up to 480 Mbits/s for v2.0. In addition, v2.0 is fully backward-compatible with v1.1. The only real difference is the data transfer rate.

USB is also capable of supporting up to 126 devices on a given interface. GPIB-based systems require a user to ensure that instrument addresses are unique, but USB provides this function automatically.

At the time of manufacture, USB devices are given unique identifiers based on parameters such as the manufacturer, the instrument serial number, and the product number. When the device is powered up and connected to a controller, the controller detects its presence automatically, and if the host-side software drivers are loaded, the instrument will be ready to communicate on the bus.

Note that USB cables and connectors are considerably smaller than their GPIB counterparts. However, device-interconnect configurations for USB are somewhat different from those usually seen in GPIB-based systems.

Many USB instruments aren't equipped with multiple USB connectors. This means that devices can' be daisy-chained together, but must be connected to the computer via a hub, as shown here.

Hubs provide expansion capability for USB, permitting multiple devices to be connected to a single USB port. These hubs are transparent to a controller, and can be cascaded up to five deep.

The use of hubs in a multiple-instrument environment or a systems environment offers several advantages. For example, many USB hubs include LED status lights that indicate what specified port is connected.

Also, a hardware failure at the interface to one instrument, such as a shorted line, is unlikely to cause an entire bus to fail. This makes troubleshooting an I/O interface fault in a large system with many instruments a much easier task than having to disconnect each device in turn, as you have to do in a GPIB based system.

Simple Connections
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Connecting USB instruments to a PC controller is also a simpler task. Most of the PCs produced within the last few years have several USB ports already built in (but these will probably be USB v1.1-compliant).

Even though PCs that support USB 2.0 are only just beginning to appear in the marketplace, there are USB 2.0 PCI cards available from several manufacturers. Some even have ports on both the rear and the front of the PC as a convenience.

For those PCs that don't have built-in USB ports, PC interface cards for both USB v1.1 and USB v2.0 are available. If USB v2.0 performance is desired, then Windows 2000 or Windows XP operating systems are preferred, since both provide support for USB v2.0.

Similar to GPIB, communications with instruments via USB requires the installation of an I/O software package. As mentioned above, Plug-and-Play, IVI-COM drivers, VISA/SICL, and Intuilink supporting C/C++, VisualBASIC 6.0 and VisualBASIC.net are available with USB support.

USB Has Company
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Now that I've convinced you to use USB, I can say that USB isn't the only new interface alternative! Test-and-measurement instruments can also be connected by a LAN interface.

Ethernet-based LAN is likely to be even more common than USB since Ethernet LANs are almost universally available in most industrial and commercial locations. Moreover, most PCs found in these facilities are already connected to a LAN. Ethernet-based LANs commonly support data rates of 10 Mbits/s to 100 Mbits/s, and some even operate at up to 1000 Mbits/s.