With so many types of buses available today, is there a standout in the group? Perhaps the answer can be found in this comparison of both existing and future bus designs. It focuses on the best and the least appealing features of several buses and their possible uses in test and measurement and data acquisition applications.
The investigation starts with HP-IB, an interface developed by Hewlett-Packard in 1965 and then released as the IEEE 488 standard in 1975. This was the first high-bandwidth interface for test and measurement and some data acquisition devices. Data transfer speeds reached a few hundred kilobytes per second. By no means was this sufficient for large-scale data acquisition systems that traditionally used proprietary buses.
With the development of VME, the fast data acquisition market switched from proprietary solutions to this new bus. In the mid 1980s, most large companies providing IEEE 488 test and measurement equipment formed an alliance to define a test and measurement standard. This was necessary because connecting different boxes to a computer had become too cumbersome for medium-size or large-scale setups. This new standard (1987) was VXI, reunifying test and measurement and fast data acquisition and control.
With PCs becoming a mass-market product and with the widespread use of the PCI bus in every one, prices for components for this architecture dropped. Some manufacturers proposed an industrial PC bus. From this evolved CompactPCI and an offspring—PXI.
At this point, a look at the basic advantages and deficiencies of these buses is helpful. Table 1 (see below) shows a general comparison, but a deeper probe into all the different buses will help you understand the strengths and weak-nesses of each.
FireWire
PCI
Theoretical (Mbytes/s)
Achievable (Mbytes/s)
0.5
40
80
72
72
40,80
40
40
(large system)
high
to high
to high
to high
to high
VME
VME, an IEEE standard, was designed as an open multimaster bus structure to be used as a computer backplane. It consisted of one processing element without standardized interconnections. The theoretical bandwidth was 40 Mbytes/s, and the sustained data rate was about 10 Mbytes/s.
Revisions of the specification brought the transfer speed up to 80 Mbytes/s with a draft spec aiming at 320 Mbytes/s. The bus has a handshake design with very precise response times. As a result, it soon was used for high-speed, large-throughput data acquisition and control systems.
However, it lacked a ±15-V power supply for analog signal conditioning. Also, there was no definition of maximum power ratings for each module. In addition, the modules didn’t have housings, which created EMC problems, and cooling requirements were not standardized. For these reasons, test and measurement applications were not brought to this bus.
One strength of VME was the widespread availability of different real-time operating systems, enabling interrupt response times of a few microseconds for control applications. Two mechanical form factors exist in VME, 3U and 6U, and they both have widespread use today.
VXI
VXI stands for VME eXtension for Instrumentation. As the name suggests, it brings the benefits of VME to the test and measurement market. In its latest spec, it can accommodate a data transfer speed of up to 80 Mbytes/s. This is an improvement of two orders of magnitude over IEEE 488; however, most companies provide modules with a 2- to 4-Mbytes/s interface, which is only a fraction of the bandwidth capability. Other companies, such as Bustec with its ProDAQ modules, do offer the full theoretical bandwidth.
VXI is like VME, a multimaster bus with 13 slots for different modules. It has several means of connecting different mainframes together, such as FireWire or MXI. With the evolving multi-daughter-card approach, as with the Agilent Technologies’ and C&H’s M-modules, VXI provides an impressive number of different or mixed function cards in one crate or mainframe.
EMC, power, and cooling requirements are clearly defined. The existence of a ±24-V power supply provides the voltages needed to fulfill the most demanding analog design requirements for the test and measurement market as well as for very good signal conditioning for the data acquisition market.
VXI has a very broad acceptance by the users, and more than 100 vendors provide a variety of modules. For control applications, as in VME, interrupt latencies of very few microseconds are achievable as long as you don’t use Intel/Microsoft Slot-0 Controllers. In addition, even the most demanding trigger requests can be handled with some of the new state-of-the-art VXI modules.
Another very important factor for the success of VXI is the standardization of different software drivers. These so-called plug-and-play drivers enable users and developers to operate these different boards with off-the-shelf software packages like Agilent VEE or National Instruments’ LabWindows/LabVIEW.
VXI had the image of an expensive solution. But over the past few years, prices have dropped, and you can buy a mainframe and a corresponding Slot-0 module for roughly $4,000. The costs of different I/O cards also have come down drastically. Prices per channel for larger-scale applications no longer are any more than those of equivalent PC plug-in cards.
VXI has three defined mechanical form factors, but generally, only the 6U version is used. The 6U boards are deeper and wider than the corresponding 6U VME modules, providing more real estate for developers.
CompactPCI
The widespread use of PC cards in automation and the perceived low cost for operating systems as well as application programs gave rise to a standard based on the PCIbus with a ruggedized industrial form factor. This, in addition to the low cost of the different PCI components and the relatively high bandwidth you can achieve in block transfers, was very appealing to design houses. As a result, the PCI Industrial Computer Manufacturers Group was formed.
The first designs were available in 1996, and in the beginning, most boards were designed for the telecommunications market. This still is the biggest market for CompactPCI.
Next came boards for imaging applications, which were well served with a block transfer speed of up to a 264-Mbytes/s peak rate for the PCI 64-bit transfers. Today, you can get boards for data acquisition and, to a lesser extent, control.
Here was the first obstacle these designs confronted. Having the CPU board based mostly on Intel’s architecture and using mostly Microsoft operating systems, interrupt latencies no longer could be guaranteed. Consequently, control applications only were possible where an absolute, well-defined action on an interrupt was not required.
Interrupt latencies of several milliseconds are standard with Compact-PCI. With some CPU boards based on PowerPC designs, you can go down to roughly 100-µs interrupt latency, but you are nowhere close to the performance of VME or VXI.
The power supplies are similar to those available in VME, ±12 V and 5 V, plus an additional 3.3 V, which limits the analog performance for these boards. This is reflected in the number of analog boards being offered.
As in VME, power consumption and cooling are not well defined. The mechanical form factors are the same as in VME, allowing for both 3U and 6U boards. Here again, as in VME, the 3U form factor provides only limited usable space. The PCB size is 100 mm × 160 mm, roughly the space you have on a short PCI plug-in card.
PXI
PXI, developed and released by National Instruments, is an open standard governed by the PXI Consortium, not an IEEE standard. Electrically and mechanically, it is very close to CompactPCI and has some additional features. The mainframes normally have eight slots, one for the CPU, one (optional) for the star trigger, and six slots for I/O boards. Larger mainframes exist, which consist of two separate bus segments connected with PCI-PCI bridges and provide 12 expansion slots.
PXI also features a local bus between adjacent modules, a system clock, and two different trigger lines where the normal trigger only functions in each segment of the PCIbus. The software drivers are, as in VXI, well defined.
There is a major drawback. Windows is the mandatory operating system, which prevents any real-time capabilities.
As in CompactPCI and VME, PXI comes in 3U and 6U form factors. Both sizes are used, yet it is not clear which form factor will be the predominant one. Cooling and EMC requirements are well defined.
As all CompactPCI cards work in a PXI environment, you can choose from a large selection of CPU cards. The theoretical bandwidth for data transmission is 132 Mbytes/s for the 32-bit PCI transfer and double that for 64-bit transfers. The 3U I/O boards normally use the 32-bit transfer, and a sustained data rate of 40 to 50 Mbytes/s is achievable.
Since PXI is derived from Compact-PCI, the board developer is confronted with the lack of a clean 15-V power supply, which limits the analog capability. This is the biggest problem PXI faces at the moment and perhaps something that will change in a later revision of the spec.
In addition, PXI currently is not supported by a large number of vendors. Looking at the list of vendors on the web page of the PXI Consortium, you see 58 alliance members. Looking closer you find that only three companies (National Instruments, C&H, and Göpel) provide PXI I/O boards in the 3U size, two (LeCroy and Acqiris) offer high-speed digital scopes, three supply mainframes and switching devices, and two (GenRad and Geotest) sell 6U PXI cards.
Notwithstanding these facts, a revised PXI with clean ±15 V and a more open software approach allowing for Linux or real-time operating systems like VxWorks or Lynx OS could be a very attractive solution for test and measurement and data acquisition in small-scale systems. For a long-term success, the market definitely needs many more and varied vendors.
Serial Buses
Presently, two buses are in use, which will obsolete IEEE 488 in the future. The first one is IEEE 1394 with a data bandwidth of 50 Mbytes/s; 100 Mbytes/s will be available soon. The second one is USB 2.0 with 60 Mbytes/s. Both are standard interfaces, and the future will show which one becomes the standard installed on all computers.
Both buses are fully supported by operating systems such as Windows and Linux and well suited for test and measurement as well as data acquisition. Both buses support an asynchronous as well as an isochronous mode, guaranteeing a given interrupt response time. This enables these buses to operate in small control-loop applications. The first data acquisition and control boxes are available in IEEE 1394 and soon will be offered in USB 2.0.
Two high-speed serial buses are being defined now: Next-Generation I/O (NGIO) and Future I/O. Both buses will be high-speed serial crossbar switches operating at 1 Gbyte/s and above. While NGIO seems to be designed as a system area network, connections for ASIC-to-ASIC, board-to-board, and chassis-to-chassis are defined in Future I/O.
Whichever bus wins this race, one thing is sure. PCI will vanish, and with this kind of bandwidth, all your needs for high-speed data acquisition will be adequately handled. Will this also be a solution for the test and measurement applications? That remains to be seen.
Summary
We have compared all existing and future bus systems for their usefulness in test and measurement as well as data acquisition and control. At the moment, the clear winner is VXI, mainly because of the impressive number of products and vendors and the superior analog performance of VXI boards.
I foresee a reasonable market for PXI if three things happen:
- Some changes to the specification allowing for a ±15-V power supply and different operating systems.
- A common understanding about which form factor will be predominant.
- Adoption of this established form factor by more vendors, resulting in a far wider choice of I/O boards.
About the Author
Fred Bloennigen, Ph.D., established Bustec in Ireland in 1997 and opened a U.S. branch in 2000. Mr. Bloennigen has been working in the field of data acquisition since he studied physics in Germany. After earning a Ph.D. in nuclear physics in Grenoble, France, he worked at the University of Berkeley and Los Alamos National Laboratory in elementary particle physics. Bustec, 17820 Englewood Dr., # 14, Middleburg Heights, OH 44130, 440-826-4156, e-mail: [email protected].
Published by EE-Evaluation Engineering
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March 2001