Interfaces for Computer-Controlled Test Systems

A short introduction to some of the more popular interfaces used to connect test instruments to computers.

Almost as soon as computers became affordable for general use, engineers began using them for controlling test instrumentation. For example, Hewlett Packard began as a test instrumentation company. While HP is now known solely as a computer company, their first product, released in 1938, was an audio oscillator.

HP developed their first computer, the HP 2116A, in 1966, which was intended to be used primarily to control test instrumentation. The computers were so well-designed, however, that customers began to use them for all kinds of computing applications, not just instrument control. Computers quickly became the largest part of HP's business, and they spun off the test and measurement business in 1990 to focus solely on computers and printers.

Over the years, computers have become more and more important for test engineers and test-system development. A number of factors have led to this development and the cost of testing is certainly one of them. By automating tests, companies are able to perform tests more quickly and with fewer errors than if they are performed manually. Product complexity is another factor that demanded test innovations. As products became more complex, the tests followed suit, and computers help companies manage this test complexity.

One of the first hurdles that test engineers had to leap when using computers to control test equipment was determining how to connect that test equipment to the computers. Over the years, many different test busses have been developed to make this task easier. Let's look at a few of them:

Hewlett Packard Interface Bus (aka HPIB, GPIB, and IEEE 488)

Hewlett Packard developed the first widely-accepted instrument interface, called the Hewlett Packard Interface Bus, or HPIB, in the late 1960s. Other test instrument manufacturers adopted the bus when HP made it available for a nominal fee. At that point, it became known as the General Purpose Interface Bus (GPIB). In 1975, the IEEE drafted standard IEEE-488, Standard Digital Interface for Programmable Instrumentation, based on the GPIB specifications.

The IEEE 488 bus is a 16-bit wide parallel bus. Eight lines are data lines, while the other eight are control lines. The connector is a 24-pin connector, which also contains eight ground lines. Up to sixteen different instruments can be connected in a daisy-chain to the computer or bus controller.

HP computers were, obviously the first computers to have an IEEE 488 interface, but others soon followed. National Instruments developed an IEEE 488 interface for DEC PDP-11 computers, and there were IEEE 488 interfaces for many personal computers. Even the Commodore PET had an optional IEEE 488 interface.

While this interface has been superseded by other test-instrumentation interfaces that are faster and easier to use, many older systems still use the IEEE 488 bus. That's a testament to its design and utility.

PC-bus interfaces

When the IBM Personal Computer was introduced in the 1980s, many manufacturers developed instrumentation cards that users could plug into the PC's Industry Standard Architecture (ISA) bus to make measurements and switch signals. When more performance was needed, PC manufacturers migrated to the Peripheral Component Interconnect, or PCI bus. Introduced by Intel, it was used from the early 1990s to the early 2000s. It has since been replaced by PCI Express.

PXI, or PCI eXtensions for Instrumentation was developed by National Instruments and introduced in 1997. The standard is now supported by the PXI Systems Alliance, a trade group whose members include many of the major test and measurement equipment vendors.

PXI is based on the CompactPCI standard, but adds features, including integrated timing and synchronization and triggers that make it more usable for test and measurement applications. PXI modules are available that can meet a wide variety of test needs from many different manufacturers.

VXIbus

Similar to the PXI bus, the VXIbus was based on a standard computer bus architecture, in this case, the VME bus. VXI (VME eXtensions for Instrumentation) adds features to the basic VME standard that are useful for test and measurement applications. Just as the VME bus was, and still is, used for high-performance applications, the VXIbus is often used instead of the PXI bus for test applications that required higher performance that the PXI bus could provide. Perhaps the biggest markets for VXIbus systems are the avionics and military automatic test equipment markets.

VXIbus systems consist of one or more chassis, into which you plug controllers and instrumentation modules. Each chassis contains up to 13 slots, and VXI modules are typically 6U or C-size modules. In addtion to the bus connecting the modules, the chassis usually provides system power and cooling.

The VXIbus Consortium support the VXIbus standard, and test application software, such as MATLAB and LabView continue to support VXIbus instrumentation. 

LXI

The LAN eXtentions for Instrumentation, or LXI, extends the functionality of Ethernet-based local area network to the test and measurement world. Rather than try to change the physical connection, the LXI specification describes how test instruments should behave when connected via LAN. In this way, it leverages the advantages of Ethernet LANs and allows test engineers to easily connect their instruments to a computer and to one another.

For example, to be certified as LXI-compliant, an instrument needs to provide a Web interface that works with standard Web browsers. Via this interface, users are able to both configure an instrument, generate instrument outputs, and access measurement data.

The LXI Consortium supports the LXI specification and certifies instruments as being LXI compatible. Several times a year, they sponsor events they call “plug fests,” to which instrument manufacturers bring their latest designs for certification testing.

USB

Another standard interface that test engineers use to connect instruments to computers is the Universal Serial Bus (USB). While USB doesn't offer the timing features or the high data rates of PXI or LXI, its ease of use make it a natural choice for many applications. The USB interface is also well-supported by the more popular test application software packages.

Helpful Tips

When choosing an interface for a test system, you need to first analyze your requirements, including the number of input and output channels and the data rates at which you need to generate outputs and take measurements. That way you can see which interface makes the most sense for the application. Remember to consider future requirements as well as present requirements. If you can expand the system later, when you need to, you can avoid having to redesign the whole system.