You can get there from here

New England folklore says that if you ask someone in Maine for driving directions, the answer you'll get is "You can't get there from here." You may have the same sentiment about trying to connect numerous test instruments to a complex DUT. Sometimes, the mass of cables becomes so twisted that it looks like a Boston road map.

Mass interconnects are one of the subjects being covered at the 2004 Automated Test Summit. In this third and final installment covering topics from the summit (Ref. 1), I'll focus on the mass interconnects, switches, and software that let you weave your way from test instruments to UUTs.

Figure 1. A mass-interconnect receiver lets you attach a test fixture to a test station. Cables and switches complete the signal connections. Courtesy of Virginia Panel.

Test fixtures let you attach your UUT to a test station. The type of fixture you need greatly depends on the UUT's form factor. In Figure 1, the test fixture contains a panel that mates with the receiver. If the UUT is a board, then the fixture will route the test signals between the receiver and the UUT. Because it slides on tracks, the test fixture aligns with the receiver. You can automate the fixture positioning with motors or pneumatics, thereby obtaining consistent insertion from one UUT to the next.

If your UUT is a finished product with its own connectors and housing, then you should build or buy cable assemblies that connect it to the receiver. But because the receiver should use a set of identical connectors, you could mistakenly insert the cable-assembly connector into the wrong receiver connector. Therefore, you should mount the UUT's cable connectors on a mating panel for proper insertion to the receiver.

When should you use a mass interconnect? Tom Kohli, facility manager at Flextronics Test, says that mass interconnects work best in low-volume, high-mix manufacturing because they provide a common interface to all your test signals. Jerry Hopp, product manager at Virginia Panel, adds, "The mass interconnect doesn't care what's behind it, so you can change or upgrade instruments without impacting the system."

Behind the mass interconnect, your test system needs cable assemblies that connect the receiver to switches—often hundreds of them. Switch cards, housed in VXI or PXI modules, let you choose from several switch topologies. Single-pole single-throw switches often apply power to UUTs, cooling fans, motors, and pneumatics.

Figure 2. A matrix switch lets you connect signal sources, loads, and measuring equipment to a UUT. Courtesy of National Instruments.

In part 2 of this series, I showed how performing an audio test on one UUT while performing an RF test on another UUT can cut test time and maximize test-equipment usage. To accomplish this concurrent testing, you need a matrix configuration, which will let you can connect any signal source in the matrix to any UUT.

Some switch cards let you configure the switches in several different topologies. A new matrix switch card from National Instruments, for example, contains 512 relays that you can configure as a 4x2176 matrix or a 8x1088 matrix. See "Switch card gets dense," p. 66, for more information.

Figure 3. Software lets you configure switch cards into topologies such as a matrix for connecting test equipment to UUTs. Courtesy of National Instruments.

"The combination of high-density PXI switch modules, switch-executive software, system-level synchronization, and high-performance synchronized measurements brings engineers closer than ever to achieving the performance of large-scale ATE systems with the modular PXI platform," says Dr. James Truchard, CEO of National Instruments.

How much mass interconnect and switching do you need? Hopp says that most test systems expand their signals and test capacity over time. "When specifying a mass interconnect, plan for about 30% expansion," advises Hopp. Kohli adds, "We will often recommend a 14-slot PXI chassis even when an eight-slot chassis is initially enough because test engineers usually expand their system's capabilities."

Although the path from UUT to test equipment may seem long and winding, you can make the process of connecting instruments to products easy and reliable. If only you could do that with driving directions.