From its inception, PXI has made an impact on the electronics industry by reducing cycle time, development time, size, and cost.
Now there is Revision 2.0 of the PXI Specification, approved by the PXI Systems Alliance only a few months ago, which provides these same benefits for larger, more sophisticated ATE.
Revision 2.0 defines the implementation of multiple PCI bus segments and the routing of synchronization and timing signals across multiple bus segments. It also addresses the needs of high-slot-count ATE and opens the door for more applications that can be accommodated with PXI.
Multiple PCI Bus Segments
The Peripheral Component Interface (PCI) bus is the de facto standard bus used in virtually every desktop computer throughout the world as well as on the PXI backplane. The PCI bus has a multitude of strengths, including processor independence, high data rates, software and operating-system support, peripheral vendor support, and built-in features for future enhancements.
However, the PCI bus is limited to eight loads. In desktop computers, the eight-load limit allows for only three or four peripheral slots because both the card-edge connector and the plug-in peripheral count as loads. Because PXI uses high-performance connectors that do not count as loads, you can get seven peripheral slots.
More peripheral slots can be added through the use of bridges. Desktop computers, industrial computers, and PXI all leverage PCI-to-PCI bridges. The PCI-to-PCI bridge replicates the PCI bus to create an additional PCI bus segment. Each PCI segment is limited to eight loads, with the bridge counting as one. Consequently, a PCI-to-PCI bridge provides desktop and industrial PCs with an additional three slots and PXI with an additional seven.
However, PCI-to-PCI bridges come with trade-offs. While they perform well, more than three bridges on one PCI bus can greatly degrade the performance of the bus. When building high-slot-count, high-performance ATE, this performance drop can greatly increase cycle time and even prevent the use of multiple high-performance instruments in one system. But, because PXI has more peripheral slots per bus segment, it is possible to build high-slot-count systems without sacrificing a high-performance bus.
Synchronization and Timing Signal Routing Across Bus Segments
PCI-to-PCI bridges have been used extensively by the computer industry for several years. PXI uses these same low-cost bridges to replicate the PCI bus across the PXI backplane as found in desktops and industrial PCs.
However, PXI also has advanced synchronization and timing signals on the backplane: a dedicated 10-MHz reference clock, a PXI trigger bus, a star trigger bus, and a slot-to-slot local bus (Figure 1). Revision 2.0 of the PXI specification clarifies how to implement these signals across PCI bus segments.
The PXI specification requires that the 10-MHz reference clock be available to every peripheral slot in every bus segment. An independent buffer that is source-impedance matched to the backplane drives the 10-MHz clock to each peripheral slot.
The PXI specification also requires that the PXI trigger lines be bused between segments rather than directly connected. The trigger lines on different bus segments must be logically connected rather than physically connected to maintain signal integrity and allow for incident wave switching of Type A trigger drivers.
Type A trigger drivers are used for clock transmission over the trigger bus and are capable of incident wave switching on rising edges, preventing jitter degradation due to transmission-line effects. Revision 2.0 of the PXI specification allows for the star trigger signals to be routed to peripheral slots beyond the first two bus segments. It also removes the rule that the local bus could not be routed between adjacent slots if adjacent slots were on different bus segments.
Revision 2.0 outlines alternate routings of ID Select (IDSEL) and Interrupt (INT) lines of peripheral slots to accommodate PCI-to-PCI bridges, making alternate local bus routings possible. All of these specifications make designing high-performance, high-slot-count PXI chassis possible.
Do More With High-Slot-Count PXI Systems
High-slot-count PXI systems allow you to do more in a single system because of the advanced synchronization and triggering signals. With high-slot-count systems, you can contain all the instrumentation needed for automated test in one system. So whether you need multiple high-speed digitizers, DMMs, arbitrary waveform generators, switches, or other instruments, you can synchronize your instruments across the PXI backplane to achieve test accuracy and efficiency.
Synchronization and triggering signals increase your system’s accuracy when making time-domain measurements and efficiency when making steady-state measurements. Time-domain measurements characterize the variation of the output of a unit under test (UUT) over time. For these measurements, the accuracy of the measured response not only depends on the accuracy of its magnitude, but also on the time you measure signals.
You can see from Figure 2 that obtaining accurate, repeatable results requires precise triggering. In the steady-state case, the measurement process depends on the time of the measurement. If you measure the signals too early, accuracy suffers because the source output may not have fully settled.
Although you can measure the signals accurately any time after the output has settled, you must minimize the delay to reduce test time. Many test developers insert an arbitrary delay in their test programs to ensure accurate results. While this is a simple fix, test time suffers.
Many test applications call for measurements of several channels simultaneously. For example, with PXI, you can synchronize an arbitrary waveform generator and 10 two-channel high-speed digitizers with a trigger signal that has less than 5 ns of uncertainty.
For steady-state measurements that do not need to be recorded simultaneously, to lower your cost, you will use switches to route the signals to a single DMM. By passing handshaking signals across the PXI backplane between the DMM and the switches, you can optimize the time spent accessing all the nodes. And now with high-slot-count PXI systems, you can perform both of these functions and many others in one test system.
Figure 3 shows an example of a PXI system for automated mobile phone testing. Many measurement functions are combined into one high-slot-count PXI system to perform all test functions required for mobile phones, including keyboard, LCD, RF antenna, speaker-quality, battery, and circuit-board testing.
GenRad Versa Automotive Tester
The GenRad Versa Automotive Tester (GR Versa AT) already leverages multiple PXI bus segments to provide a high-performance test system. It addresses the needs of automotive manufacturers, contract manufacturers, and systems integrators who require a new, compact, tightly integrated test platform.
The GR Versa AT contains three PXI bus segments to deliver a system that houses up to 19 PXI instruments. It includes multiple analog and digital I/O lines, a matrix switching solution, active and passive loads, and multiple communications protocols. With all these devices in one PXI system, the GenRad Versa AT delivers economical, high-speed testing.
Rohde & Schwarz Cellular Phone Tester
The Rohde & Schwarz TS7100 Cellular Phone Tester also leverages multiple PXI bus segments to deliver a high-performance tester for the telecommunications market. It performs parallel testing of several UUTs to obtain maximum speed in highly automated production.
The TS7100 includes digital I/O; a digital multimeter; RS-232 serial, GPIB, general-purpose, and AF switching; and power relay devices. The TS7100 offers a test solution with high throughput and low cost.
PXI already has helped reduce cycle time, development time, size, and cost in ATE. But now with Revision 2.0 of the PXI specification, you can apply all these benefits to high-slot-count ATE. Whether you need to synchronize a large number of PXI instruments, switch a large number of channels, or both, multiple PXI bus segments make it possible without sacrificing performance, test speed, or budget.
About the Author
Brad W. Smith is the PXI product manager at National Instruments and chairman and CEO of the PXI Systems Alliance. He also is an executive-level member of the PCI Industrial Computer Manufacturers Group. Mr. Smith holds a B.S. in mechanical engineering from Texas A&M University. National Instruments, 11500 N. Mopac Expressway, Austin, TX 78759, 800-258-7022, e-mail: [email protected].
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