When Is It Best To Use VXI, PXI, Or GPIB For Your System?

DESIGN VIEW is the summary of the complete DESIGN SOLUTION contributed article, which begins on Page 2.

Choosing the right platform (or combination of platforms) for your ATE system can save you money and labor—plus improve measurement results. You can assemble an instrumentation system in several different ways. One option is to "rack and stack" standalone boxes and connect them to a computer via the well known General Purpose Interface Bus (GPIB), also called the IEEE-488 bus. This makes it simple to swap out instruments and create new configurations quickly and easily.

Another route is to use instruments on VME eXtensions for Instrumentation (VXI) and PCI eXtensions for Instrumentation (PXI) cards that plug into a backplane. Such a solution seems ideal, with the computer and the instruments it controls all in the same card cage. However, not all instruments are availlable as plug-in cards.

Experience has shown that it's seldom possible to put together a complete system using all bus-based or all standalone instruments. Generally, there will be some of both.

This article reviews each of the platforms in detail, discussing their benefits and limitations. It also gets into the mixing and matching of the buses for an optimal solution.

With so many choices, it's possible to put together a system to meet just about any need. It's nice to know, too, that these technologies will stick around for a long time.

HIGHLIGHTS:
GPIB	The General Purpose Bus Interface is a byte-serial, bit-parallel bus that can connect up to 15 devices to one controller. GPIB has a huge installed base, making instrument connection a snap. It's primary drawback is limited bandwidth.
VXI	VXI, which is based on the VMEbus, works with signals ranging from low-level analog to microwave. It offers the advantages of performance and ruggedness, but it could cost two to three times more than alternatives
PXI	PXI, based on CompactPCI, falls midway in complexity between GPIB and VXI. PXI lets users standardize cards and card cages. And because it's based on the PC architecture, software is readily available. Limitations include space inside the enclosure, as well as power and density. Also, GPIB equipment is often needed to complete a test setup.
Combining Buses	Mixing and matching standalone boxes, VXI, and PXI is probably the most optimal solution. Because systems can connect to each other via GPIB and to the world via Ethernet, this circumvents VXI's and PXI's inability to supply or handle enough power.

Full article begins on Page 2

One option is to "rack and stack" stand-alone boxes and connect them to a computer via the well-known General Purpose Interface Bus (GPIB), also called the IEEE-488 bus. This makes it simple to swap out instruments and create new configurations quickly and easily. Another route is to use instruments on VME eXtensions for Instrumentation (VXI) or PCI eXtensions for Instrumentation (PXI) cards that plug into a backplane. Such a solution seems ideal, with the computer and the instruments it controls all in the same card cage. However, not all instruments are available as plug-in cards.

Experience has shown that it's seldom possible to put together a complete system using all bus-based or all stand-alone instruments. Generally, there will be some of both.

GPIB At one time, all electronic test equipment was built in individual boxes. When it came time to connect them to a computer, the natural choice was to use the computer's RS-232 port, but this proved inadequate for anything other than the simplest tasks. The Hewlett-Packard Interface Bus (HP-IB), developed in the mid-1960s, quickly became the most popular instrumentation bus of all time under the name GPIB. Keithley Instruments' sixth Survey of Measurement Trends (Sept. 2002) found that 53% of respondents currently use GPIB and 42% plan to use it in the future. It's an international standard governed by IEEE-488.1 and IEEE-488.2 in the U.S., and IEC 60625-1 and IEC 60625-2 internationally.

GPIB is a byte-serial, bit-parallel bus that uses a three-wire handshake and can connect up to 15 instruments (devices) to one computer (controller) (Fig. 1). It uses a 24-conductor cable with up to two meters between devices, and 20 meters overall length in star (Fig. 2a), linear (Fig. 2b), or mixed topologies. The maximum data rate is 1 Mbyte/s, although as cable length increases, this can decrease to 250 to 500 kbytes/s. Usually, the devices on the bus set the overall data rate.

The programming language for GPIB systems, called Standards Committee for Programmable Instruments (SCPI), enables the same commands to be used for any instrument that can execute them. For example, MEASURE:VOLTAGE:DC? means "read a dc voltage" to any instrument that can make a dc voltage reading. SCPI also includes instrument-specific commands that work only on certain devices.

SCPI's major advantage is that it allows many instruments to be controlled by the same language. Plus, it can be used with both GPIB instruments and VXI. The only drawback regarding SCPI is that, because it's very detailed, it can take a while to learn. Syntax is fairly straightforward, however, and it has great power. The programming language is controlled by the SCPI Consortium (www.scpiconsortium.org).

Key benefits of GPIB include the fact that many vendors provide a vast assortment of instruments incorporating GPIB. Thus, it has a huge installed base. This well-known and standardized interface uses a standardized programming language, and any sized instrument can be connected to it. In addition, for the same level of functionality, GPIB generally costs less that VXI.

GPIB's primary limitation is its bandwidth. With a fair amount of overhead and a maximum transfer rate of just 1 Mbyte/s (and even that's limited to the transfer speed of the slowest instrument on the bus), downloading large data files can take a while. For most applications, though, GPIB provides ample bandwidth.

GPIB Variations In recent years, there has been a push to develop a faster version of GPIB. HS488 was intended to increase the data rate of IEEE-488 to 8 Mbytes/s by making changes to the handshake protocol between sender and receiver. Proponents claim that it's backward-compatible with GPIB. Opposition has come from a number of places, though, usually in regards to compatibility issues. HS488 was first proposed in the early 1990s, and after bouncing around the IEEE for about a decade, was recently accepted as a standard. HS488 products have been available from several manufacturers since 1993.

GPIB can be mapped onto other networks. For example, LAN/GPIB gateways will allow GPIB devices to be accessed via Ethernet. Also, the IICP method from the IEEE-1394 Trade Association simulates GPIB on IEEE-1394 (FireWire). Bridge products make it possible to connect GPIB instruments to a computer's USB port, which can be useful with newer PCs. For low-cost instruments, the IEEE-1174 maps the GPIB protocols onto a serial RS-232 line. This provides connectivity for one instrument, but no networking.

What Lies Ahead For GPIB? Some people insist that IEEE-488 is passé and will be superseded by newer buses like Ethernet, USB, and perhaps FireWire. Still, there's a huge installed base of GPIB equipment, many users who have familiarity with it, and a wide variety of products available from a host of vendors. Some of the largest test and measurement companies, such as Agilent, Keithley Instruments, Rohde & Schwarz, and Tektronix, implement GPIB as their main instrument bus and supplement it where necessary with USB or Ethernet. GPIB is also used to connect VXI and PXI systems to external controllers. In addition, some specialized instruments, like Keithley's recently introduced Model 2800 RF power analyzer, only come in GPIB-connected boxes.

It's possible to get state-of-the-art performance from a GPIB-based system by increasing the level of integration within the individual instruments. A good example of that is the Integra series developed by Keithley, which puts a DMM, data-acquisition system, and complete switching setup in one enclosure with GPIB connectivity. For applications that need longer-range communications, there's a model with 10/100BaseT Ethernet. With all this going on, GPIB's place seems assured into the future.

VXI One of the first practical methods for building test instruments on cards that plugged into a backplane bus was VXI. Based on the VMEbus, VXI was developed in the late 1980s with the aim of combining the ease of GPIB integration and the speed of VMEbus. It's governed by IEEE-1155. Keithley's measurement trends survey found that VXI is currently used by 16% of test engineers, with 22% planning to use it in the future.

VXIbus works with signals ranging from low-level analog to microwave, requiring the addition of shielding to the VMEbus backplane and between the modules. For this reason, VXIbus devices are mounted on 1.2-in. centers, instead of the 0.8-in. centers of VMEbus (VMEbus modules will fit into VXIbus systems, but not the other way around). The extra space makes room for wraparound shielding on the individual modules. The VXI specification also covers power-supply and cooling-air issues for both mainframes and modules.

The VXI specification defines four module sizes (Fig. 3). Its starts with the same A and B sizes used in regular VMEbus systems, and adds two larger sizes (see the table). All VXIbus modules share the 96-pin P1 connector called out in the VMEbus specification. A second 96-pin connector (P2) is optional on B-size and larger modules, and a third (P3) is optional on D-size modules.

As many as 256 devices are allowed in a VXI system. Depending on its complexity, a single device can share a module with another device, or it can spread over several modules. A VXI mainframe holds up to 13 modules, and it's possible to connect up to eight VXI mainframes together via the MXIbus using a bus extender module plugged into each mainframe. It should be noted that this will lead to increased delays between mainframes.

The VXIbus backplane has a theoretical data-transfer limit of 80 Mbytes/s, and under the just-ratified 3.0 specification, that will increase to 160 Mbytes/s. VXI can use distributed intelligence with multiple microprocessors. It integrates well with VMEbus systems and is fairly easy to upgrade.

Synchronization is important in high-performance systems, and VXI allows for several different methods:

There's a trigger bus on the P2 connector, with eight TTL and two ECL trigger lines. D-size modules have four more ECL trigger lines on the P3 connector.
C-size and larger modules can use a 10-MHz ECL clock generated by the Slot 0 module and distributed over the backplane via P2. D-size modules can also use a 100-MHz ECL clock connecting via P3.
In cases where the 2-ns maximum delay of the ECL trigger lines is excessive, VXI also provides (in D-size modules) a star trigger bus, which carries synchronizing signals to all modules via dedicated and equal-length lines.

The VXI specification defines register- and message-based devices. Register-based devices communicate via the backplane bus, like VMEbus systems, and are programmed in a low-level binary code. Register-based devices can move a great deal of data at high speeds, but also be laborious to program. Message-based devices communicate in ASCII using a word-serial protocol. Going this route is costly in terms of board real estate, and the communication speed is similar to that of GPIB.

There are three basic arrangements for controlling a VXIbus system: external control via GPIB, internal control with an embedded computer, and external control by a high-speed link called MXIbus.

According to the VXIbus Consortium, external control using a GPIB controller connected to a GPIB-VXI interface module plugged into the VXI backplane is the most common arrangement, and also the least expensive. The external computer can be PC- or UNIX-based. Going with an embedded VXIbus controller makes for a smaller and faster system Using MXIbus is physically similar to GPIB, but it's programmed like an embedded controller.

Message-based devices can also communicate via IEEE-1394 (FireWire) or VXIbus by installing the appropriate device. It's also possible to go the other way, using an embedded VXIbus computer as a GPIB controller to control box-type instruments.The VXIbus offers the advantages of performance and ruggedness, but could cost two to three times more than alternatives. In general, VXIbus is a good answer when extremes of ruggedness are required, or when only the highest performance level will do the job. For many other applications, it's overkill.

VXI Software The VXIplug&play Alliance (www.vxipnp.org) was formed to provide better interchangeability among devices from different manufacturers. VXIplug&play uses a series of frameworks based on different operating systems. The frameworks specify instrument drivers, DLLs, help files, knowledge-base files, C-function libraries, I/O libraries, soft front panel executable programs, and more. The Alliance's specification for I/O software, called Virtual Instrument Software Architecture (VISA), defines the API for instrument communications, although there still tend to be some inter-vendor compatibility issues.

According to the SCPI Consortium, VXIplug&play drivers can use SCPI protocol for message-based VXI instruments that speak SCPI. But the protocol can be adapted for register-based instruments as well. The same SCPI code can be used on systems running different operating systems (Windows and various flavors of UNIX) with just a recompile.

PXI Just as VXI is based on VMEbus, PXI is based on CompactPCI. Intended to fall midway in complexity and cost between GPIB PC-based systems and the more-elaborate VXI systems, PXI performs 32- and 64-bit data transfers at 33 MHz for 132- or 264-Mbyte/s peak data rates. It's frequently used in production and factory environments.

PXI uses two module sizes: 3U (100 by 160 mm, with two connectors) and 6U (233.35 mm by 160 mm, with up to five connectors). PXI adds such system-level specifications to CompactPCI as timing, synchronization, active cooling, temperature ratings, location of the controller (at the left end of the rack), environmental testing, EMI testing, and software.

PXI also adds multiboard synchronization to the basic CompactPCI design via a 10-MHz reference clock distributed to all peripheral devices. And there are eight bused trigger lines that can be used as desired. On top of that, for more precise timing, a star trigger bus is available. A 13-line daisy-chained local bus can be utilized to pass analog signals, or for special high-speed side-band communications among peripherals.

Unlike VXI and VME, it's possible to use CompactPCI modules in PXI systems and vice versa, although there may be some loss of functionality. PXI allows up to seven peripherals per bus segment. By using PCI-PCI bridges, it's theoretically possible to have up to 256 slots per bus segment.

Also, unlike VXI, all PXI systems are Windows-based, and all peripherals must be supplied with a Win32 driver. For this reason, a great deal of software is available that will run on PXI systems. Users can program PXI systems with LabVIEW, LabWindows/CVI, Visual Basic, Visual C/C++, and Borland Turbo C. Furthermore, PXI, like VXI, uses the VISA architecture for peripherals.

PXI systems have plug-and-play capability, and can connect to box-type instruments via GPIB and to VXI systems via MXI. PXI specifications are controlled by the PXI Systems Alliance (www.pxisa.org).

Several significant advantages can be realized with PXI. It lets users standardize cards and card cages, so they can configure systems as needed. It's based on the PC architecture, so software is readily available. Lots of development work is currently under way on PXI cards. PXI is fast and although it's not cheap, it costs less than VXI.

Drawbacks to PXI include limitations on space inside the enclosure, power, and density for switch cards. For switching, it's only cost-competitive when it comes to midrange channel counts. Also, a user often needs GPIB equipment to complete a test setup. For example, it can be difficult in a PXI system to get the 240 channels of dc-to-40-GHz switching that's available in, say, Keithley's System 41, or to put 32 channels of dc-to-40-GHz switching in a 2U cabinet.

Combining Buses As mentioned earlier, it's possible to mix and match standalone boxes, VXI, and PXI. Systems can connect to each other via GPIB and to the world via Ethernet (Fig. 4). This gets around one of the major drawbacks to the VXI and PXI platforms: their inability to supply or handle enough power. This is evident with dc power supplies, ac-dc sources, and electronic loads that still use a GPIB interface. It also allows for specialized measurements. When the test system requires a unique measurement, there is no substitute for a specialized GPIB instrument.

Some typical applications are functional test systems, avionics test benches, telecom repair stations, and automatic power-supply test systems. Each of these applications takes advantage of the versatility of combining a specialized GPIB instrument with either a PXI or VXI mainframe, or even both. In some unique applications, such as mixed-signal ATE, the integration of all three hardware architectures for production test and verification of mixed-signal devices is a must.

Using either GPIB or Ethernet as the system I/O backbone makes it easy to integrate an instrument with a mainframe back to a standalone PC. It allows system test engineers to use the feature set of a specialized instrument while keeping the space-saving feature of a mainframe. Ultimately, a mixed system provides the best of both worlds.

For example, a telecom repair station may have a waveform digitizer, waveform generator, and some digital driver for control within a VXI chassis. But it would still need a universal telecom instrument like an RF power analyzer to make power measurements—all running on a GPIB backbone.

Another example would be a functional test system. It may have an analog-to-digital data-acquisition card, counter-timers, and some digital-to-analog converter analog outputs in a PXI chassis, while still needing a precision digital multimeter to switch and make multiple precision measurements—all running on an Ethernet backbone.

The sheer volume of GPIB-controlled instrumentation (new and used) and the free availability of Ethernet I/O on almost every PC makes these two backbone I/O buses the mainstay of mixed systems. Other proprietary I/O buses may have unusual performance capabilities, but none can compare to the GPIB instrumentation availability and the free Ethernet I/O availability on most every PC sold today.

The good news for engineers is that with so many choices, it's possible to put together a system to meet just about any need. It's nice to know too that these technologies will stick around for a good long time.