One bus or any bus?
07 November 2006
Ian Bell, Technical Marketing Manager at National Instruments talks about next generation test system architecture
Some commentators in the test industry claim that now is the right time to choose the single bus which will replace IEEE-488 or GPIB. Whilst this may appear to be a logical step, it is based on out-of-date concepts. When GPIB first came into widespread use for test instrument connectivity in the 70's and 80's, it was logical to focus on one bus. Test system controllers were low power DOS-based PCs, often running on 16-bit processors clocked below 20MHz. Signal processing algorithms could take minutes to run on the PC, so it was the right approach to have all the intelligence and specialised algorithms running on dedicated hardware in the instrument. Drivers were also a problem: it could take days to get a basic printer to work on a single application. The driver and communications issues were minimised by focusing on one dominant bus architecture.
Those days are over. The limitations in software and hardware have been overcome. The ever-increasing power of the PC, coupled with standardised communications architectures such as USB, GPIB, LAN, PCI and PCI Express, mean that test engineers now have greater freedom to design test systems which employ that right technology for their needs. To reduce the cost of test and improve performance, next generation test systems are software-defined, use modular instrumentation and a hybrid mix of buses.
Despite all this, many companies still promote a single technology as the ideal bus that meets all application needs; but in reality, each bus technology has its own strengths and weaknesses and thus is suited to different applications. History has shown the folly of claiming an ideal bus for every application. Confident of the abilities of IEEE-1394 (FireWire), when it was still a year away from commercial roll-out, Hewlett Packard opposed efforts to improve the widely accepted IEEE-488 standard. HP publicly urged IEEE members to oppose revisions to improve the bandwidth of the IEEE-488 in December 1997 press release, arguing that FireWire also showed great promise, offering a greater bandwidth.
Since then, the IEEE-488 standard has continued to meet many customer needs and at the same time users have recognised the limitations to 1394. While the 1394 is an effective bus technology for high-speed connectivity to digital camcorders, it has not achieved widespread acceptance in many other applications, demonstrating that an ideal bus does not exist. While each bus technology offers its strengths, the idea that a bus has no trade-offs or limitation is an overstatement. For example USB is excellent for easy desktop connectivity; LAN/Ethernet is well suited for distributed systems; and PCI and PCI Express provide high performance for ATE. National Instruments has a long history of supporting the full breadth of instrumentation buses, with drivers for more than 5,000 instruments from more than 200 vendors on ni.com/idnet - including USB, LAN/Ethernet, PXI, PCI, VXI and serial instruments. Each one of these buses has its place, but for modular instrumentation, the PXI platform, based on high-speed PCI and PCI Express offers the best approach.
Modular instrumentation is about more than just packaging. Users should expect three things of a modular instrumentation system; reduced cost and size, through a shared chassis, backplane and processor, faster throughput, through a high-speed connection to the host processor; and greater flexibility and longevity, through user-defined software. PXI best meets these demands now and in the future, with more than 70 vendors in the PXI Systems Alliance, and more than 1,200 compatible products available today from vendors including National Instruments, Rohde & Schwarz, Aeroflex and Agilent. The PXI market is expected to continue to crow 25% annually through 2011.
All instruments in a PXI system share the same power supply, chassis and controller. Alternative approaches duplicate the power supply, chassis and controller in every instrument, adding cost and size, and decreasing reliability. In choosing a PXI, you can balance instrument count, size and cost, as PXI chassis are available with four to twenty slots. For systems that need more instruments than a single chassis can support, you can daisy-chain chassis or synchronise them in a star configuration. For high accuracy in multi-instrument applications, PXI chassis can provide synchronisation with a 100MHz differential low-jitter reference clock and low-skew triggers. A PXI chassis provides all of these resources to all instruments in the system, which not only improves in the synchronisation accuracy, but also saves cost and size compared with alternatives in which the instrument casing is duplicated for every instrument.
Just as the PXI chassis is shared across the instruments, so also is the controller greatly reducing cost but also, enabling the user to manage the measurement and analysis software. With PXI, the controller can be a high-performance embedded slot-zero controller for performance and portability, a server-class machine for the highest in processing and streaming performance, a laptop for portability or a desktop PC for the lowest cost. When you require faster processing, you can easily upgrade a PXI system's controller. If you have existing instruments that you want to reuse, you can use PXI to control USB, IEEE 488, LAN/LXI, serial and VXI instruments without gateways or converters.
Modular instruments require a high-bandwidth, low-latency bus to connect the instrument modules to the shared processor for performing user-defined measurements. PXI, based on PCI and next-generation PCI Express, meets these needs with bandwidth as great as 2 Gbytes per second for each slot - more than 33 times as fast as USB 2.0, 160 times as fast as 100-Mbps Ethernet, and even 16 times as fast as emerging Gbps Ethernet (Figure 2).
Peripheral buses, such as LAN and USB, always connect to the PC processor via an internal bus such as PCI or PCI Express, and are therefore, by definition, always lower in performance. As an example of how high-speed buses can impact test and measurement, consider a modular RF acquisition system. PXI can stream two channels of 100MS/s, 16-bit IF (intermediate frequency) data directly to a processor for computation. Neither LAN nor USB can meet these requirements, so these instruments always include an embedded, vendor-defined processor - in which case they are no longer modular.
In a modular instrument, the high-speed connection to the host is what delivers flexibility and longevity, because it enables the software to reside on the host instead of on the instrument. With the software running on the host, the user and not the vendor defines how the instrument operates.
The user-defined nature of the software also means that you can add or modify measurements, and even instruments, as the device-under-test changes. You can also use the direct software access to monitor or control these modular instruments across the network.
RF and microwave test systems are a clear example where this approach is allowing designers to keep pace with rapidly changing standards, whilst simultaneously improving performance and lowering costs. National Instruments demonstrated a hybrid, modular system at last year's AutoTestCon tradeshow in the US by combining a VXI downconverter with a PXI IF digitiser to create an RF spectrum analyser. The US Department of Defense, in its NxTest initiative, has proposed the adoption of synthetic instruments, which use software and modular hardware to create low cost, high performance, reconfigurable test systems. BAE Systems, National Instruments and Phase Matrix Inc. (PMI) recently announced a joint initiative to develop a PXI Express-based synthetic instrument for military and commercial RF and microwave applications. PMI is currently developing a 100 KHz to 26.5 GHz family of downconverter modules which BAE Systems, a leader in synthetic instrument systems, plans to build a next-generation synthetic instrument using National Instruments PXI Express chassis, controllers and intermediate frequency (IF) digitiser modules, as well as National Instruments LabVIEW graphical development software for host and FPGA-based signal processing.
Every bus has its place in test and measurement, but modular instrumentation delivers a range of advantages through reduced cost and size via a shared chassis, backplane and processor; faster throughput through a high-speed connection to the host processor; and greater flexibility and longevity through user-defined software. Only high-bandwidth standards, such as PXI, can be the core of future hybrid test systems, connecting any other peripheral bus into the host processor and providing the software-defined approach that is required for modular instrumentation.
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