Manufacturers of test-and-measurement equipment have long recognized the need to allow for automated testing. So, they usually build equipment with some type of I/O interface. The General-Purpose Interface Bus (GPIB) described in the IEEE-488 standard and RS-232 are two of the more common interfaces. But because today's computers work with the high-speed Universal Serial Bus (USB) 2.0, manufacturers and designers should consider USB as an alternative or additional I/O to the aging GPIB and RS-232 interfaces.
In manufacturing-test applications, USB's 480-Mbit/s data rate will cut test times. In an R&D environment, USB's ease of use will let engineers quickly construct a test system and characterize a design over frequency or temperature. This article presents an overview of USB benefits in automated test, possible ways to use USB to communicate with test-and-measurement equipment, and some USB design guidelines.
To understand the benefits of USB in automated test, consider how an automated test application is created. Two main tasks are involved: making the physical connection between the computer and the test equipment, and developing the test application software that executes on the computer.
The Physical Connection: To set up an automated test using GPIB, a GPIB host bus-adapter card and GPIB software drivers must be located and installed in the computer because GPIB isn't built into any standard PC. Fortunately, many manufacturers offer GPIB cards. However, they're expensive (costing $300 or more) and unavailable at the nearest consumer electronic store. This is also true for GPIB cables (about $100).
After purchasing and installing a GPIB host bus-adapter card and the necessary GPIB cables are in hand, the cables must be connected from the computer to the test equipment. GPIB cables are thick, containing 24 wires. Sometimes it's difficult to fit and maneuver them into tight spaces. After the cables are connected, GPIB addresses for the equipment must be manually configured. The GPIB addresses chosen must be unique and not conflict with the address of the GPIB card in the computer. A maximum of 15 devices can be installed in any GPIB topology, and typically achieved data rates are approximately 750 kbytes/s.
To set up an automated test using RS-232, the proper RS-232 cables must be located, which is usually easier than finding GPIB cables, although many types exist. If an RS-232 test system doesn't work, many first try a different cable. In cases where that doesn't work, many try to change RS-232 parameters, like baud rate, parity, start/stop bits, flow control, and so on. There are too many variables, which leads to frustration.
The good news is that RS-232 is built into computers, eliminating the need for host bus-adapter installation. Yet, it limits the number of available RS-232 ports to two. Because RS-232 is strictly point-to-point (with no ability to daisy-chain from device-to-device), two RS-232 ports means that only two devices can be connected. If more than two RS-232 ports are necessary, multiport RS-232 host bus adapters must be used. Due to baud-rate limitations, typical data rates with RS-232 are much slower than GPIB.
Compared to GPIB and RS-232, making the physical connection at the computer end with USB is a snap. Hundreds of millions of computers have been shipped with USB hardware over the last several years. "Universal" in the USB name is certainly appropriate. Most USB-equipped computers offer at least two USB ports. Soon, this will grow due to Intel's 845G chip set, which handles six high-speed 480-Mbit/s ports.
Each USB port is a "root port" and supports a topology (if external hubs are added) of up to 127 devices. So USB will enable new kinds of test systems, with many more instruments connected to one computer. USB hubs are readily available (about $25) and don't introduce significant delays.
With USB, making the physical connection to the test equipment also is simple, whether or not it has a USB interface. When test equipment lacks a USB interface, a converter is required. For example, if test equipment has a GPIB interface, several companies now provide USB-to-GPIB converters. These converters typically contain a general-purpose USB chip, firmware, and GPIB hardware. They convert a protocol on the USB side into the necessary signaling on the GPIB side (Fig. 1).
The benefit of employing a USB-to-GPIB converter is that an expensive GPIB host bus-adapter card no longer needs to be installed to communicate with the GPIB test equipment. Moreover, applications written for GPIB don't even have to change, as the converter can be made to appear like a GPIB host bus-adapter card. USB-to-GPIB converters also eliminate a GPIB cable because they can plug directly into the GPIB interface on the test equipment. Using such a converter may be the only solution when no expansion slots are available on the computer.
Also, if several engineers occasionally need to run an automated test, it's much easier to share a USB-to-GPIB converter with others than to share a GPIB card. When transferring large amounts of data, the performance of even 12-Mbit/s full-speed USB-to-GPIB converters matches the 25-year-old GPIB's performance. But if the test application has a critical need to perform many short transfers, such as writing to an electronic switch, then the performance of USB-to-GPIB converters may affect the overall test time. To optimize short-transfer performance, connect the USB-to-GPIB converter to a high-speed USB port.
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