When Plug-and-Play Really Isn’t

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When Plug-and-Play Really Isn’t

Industry standards such as Ethernet and USB help ensure the interoperability of the computers, peripherals, and networks we depend on every day. Compliance testing is essential because any level of incompatibility can be costly in time and money for vendors and end-users.

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Here’s a big surprise: if you peruse the compliance criteria, plug-and-play may be less robust than we might imagine. The underlying issue is the cascade of incompatibility percentages. When the interoperability numbers from connected devices are multiplied, the result is low enough to introduce significant risk. This is especially true for automated test systems.

Clearing two different pass/fail hurdles

Test and measurement (T&M) equipment are generally available in three form factors: handheld, benchtop, and modular. With its space efficiency and resulting “test density,” modular has become widely used in test systems that provide crucial go/no-go decisions on the production line.

Most modular test hardware is based on PCI Extensions for Instrumentation (PXI), a rubric built on the IT industry’s Peripheral Component Interconnect Express (PCIe) standard. The PCI Special Interest Group (PCI-SIG) relies on two types of testing—compliance, and interoperability—to ensure broad compatibility between IT products from different manufacturers.

The same concept applies to PXI. Here, compliance testing pits a T&M product against an array of standardized test modules: electrical, configuration, link protocol, transaction protocol, and platform BIOS. Interoperability tests let member companies check their devices with other members’ products.

Every test yields a “pass” or “fail” result. For a product to be labeled “compliant,” it must pass all of the compliance tests and also achieve a minimum score of 80 percent on the interoperability tests.

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Crunching a few numbers

That’s right: just 80 percent. This is where cascading incompatibilities start to affect interoperability.

Imagine a measurement system built with PXI hardware. Suppose the system includes one PXI mainframe (or “crate”) and three modules, each from a different vendor. All are certified as compliant.

If all four units—crate and modules—passed interoperability testing at the minimum threshold of 80 percent, the worst-case potential for overall compatibility is about 41 percent (i.e., 0.8^4). Even at higher levels, the numbers do not inspire confidence: 0.9^4 is 65.6 percent, and 0.95^4 is 81.5 percent. Reaching 90 percent requires an average score of 97.41 percent across all four pieces of hardware.

Leveraging de facto standards

Software, drivers, and connectivity are the other crucial components of an automated test system. Over time, T&M has accumulated four generally accepted guides: Standard Commands for Programmable Instruments (SCPI), the Virtual Instrument Software Architecture (VISA) communication channel, Interchangeable Virtual Instrument (IVI) drivers, and the LAN Extensions for Instrument Control (LXI) interface.

The IVI Foundation manages SCPI, VISA, and IVI. While none are formal standards, the Foundation maintains a set of specifications to assist T&M product developers. The intent is to create greater consistency in the programmatic control of various classes of benchtop and modular test equipment—oscilloscopes, function generators, signal analyzers, and so on.

It isn’t a perfect solution, but it meets the intent at a basic level: it is possible to remove analyzer A, install analyzer B, change the IVI driver, and get similar results. However, if a test depends on analyzer A’s advanced capabilities, doing the job with analyzer B will probably require the time and expense of software changes.

Relying on formal compliance testing

In a test system, the LXI interface enables communication and control via LAN. As with the underlying IEEE 802.3 standard, the LXI Consortium requires conformance testing of all devices.

To support the pursuit of compliance, member companies have access to a suite of LXI test software. Many use this as a pre-test before submitting a product for compliance testing. Once the product is ready, the manufacturer can choose to have it tested by an approved test house or during a consortium-sponsored “plug fest.” This raises the bar for interoperability by surfacing potential problems early in a product’s life.

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Accelerating system creation

In a complex test system, the cascading incompatibilities of hardware, software, drivers, connectivity, and the host computer, present a real risk. For systems based on PXI, LXI, or both, end-users should ask T&M vendors to provide information about the level of interoperability they achieved during compliance testing. At a minimum, this will make it easier to isolate incompatibilities and accelerate system integration. It also increases the chances of getting valid go/no-go results in manufacturing and thereby sending good products into the marketplace.