Which Bus Architecture Is Best for You?

The PC is ubiquitous in an engineering facility. You can find them everywhere, from an engineer’s desk to the production floor.

Advancements in PC technology are changing the way products are developed, tested, produced, and serviced. By replacing vendor-defined instruments with PCs equipped with instrumentation-grade hardware, you can create a personalized measurement solution for your application at a fraction of the cost.

A key component of a PC-based test solution is the bus interface between the PC processor and the measurement hardware. Many interfaces are available today; and as PC technology advances, so will the bus interface technology.

Several new bus interfaces are gaining acceptance for PC-based test systems, including CompactPCI, the Universal Serial Bus (USB), IEEE 1394, CardBus, and Fibre Channel. There also is a new way to take advantage of the Ethernet for PC-based systems.

CompactPCI

The CompactPCI bus is based on Peripheral Component Interconnect (PCI), delivering 132-MB/s transfer rates via bus-mastered direct memory access (DMA). It also offers eight slots for installing peripherals, a significant increase over desktop PCI.

But CompactPCI is more than just a bus interface. It is computer architecture that adds the features necessary for industrial computing to the desktop PC. Specifically, CompactPCI adds a high-performance connector system and Eurocard mechanical packaging to the PCI bus found on desktop computers. The result is a rugged alternative for PC-based test that runs the same operating systems and application software as your desktop PC.

The connector system is an advanced pin-in-socket connector designed to deliver the best possible electrical performance under all conditions. It also is better suited for industrial computing than the less expensive card-edge connectors inside your desktop PC. The mechanical aspects of CompactPCI are governed by Eurocard specifications, which see wide use in industrial environments.

Eurocard packaging was popularized by the VMEbus architecture and makes CompactPCI a compact, rugged system that can withstand harsh industrial environments in rack-mount installations. CompactPCI implements the PCI electrical specification, leveraging off the de facto high-performance standard found on most desktop PCs.

Universal Serial Bus

The USB is the first of several new external computer buses designed to replace the serial and parallel ports we use today. It offers several advantages over the legacy ports it is designed to replace.

First, USB is a hot-pluggable plug-and-play bus, meaning that you just plug the device into a port on your PC and it is configured automatically. No more opening the computer to add new hardware. You don’t even have to turn off the computer.

Multiple devices can be connected to your PC via USB, and the bus supplies power to these devices. USB is ideal for consumer peripherals and will be used primarily for devices such as keyboards, mice, and joysticks.

The availability and ease of use of this new bus make it desirable as a measurement platform as well. In fact, several companies already have introduced data acquisition and instrumentation products for USB.

IEEE 1394

IEEE 1394 is a high-speed serial bus that sends information via packets. It was developed as Firewire by Apple Computer and became an IEEE specification in 1995. Much like USB, the motivation behind IEEE 1394 is to consolidate and replace today’s proliferation of serial and parallel computer interfaces.

Real-time multimedia will be a typical application for IEEE 1394. In fact, Compaq recently introduced a PC with IEEE 1394 connections for this purpose.

Several features of IEEE 1394 are helping this specification gain popularity for a broader range of applications. First, IEEE 1394 devices are plug-and-play as well as hot pluggable. IEEE 1394 also can supply power from your PC to the devices on the bus. In these ways, IEEE 1394 is similar to USB.

Unlike USB, IEEE 1394 is a multiple master bus and handles DMA in the same manner as PCI. This makes IEEE 1394 more suitable for high-performance measurement systems because data transfers are off-loaded from the computer processor.

IEEE 1394 also is much faster than USB. Currently, theoretical speeds of 400 Mb/s are available, but there are plans for 3.2 Gb/s transfer rates in the future.

CardBus

CardBus is the much-anticipated extension of the Portable Computer Memory Card International Association (PCMCIA), promising faster throughput rates by leveraging off the PCI bus architecture. CardBus adds DMA capability while maintaining compatibility with existing PCMCIA cards. CardBus slots already are appearing on many new laptop computers, and Windows 98 provides operating-system support for this new architecture.

CardBus is backwards compatible with PCMCIA. Your existing PCMCIA cards will work in these new slots, but they will not benefit from the additional performance CardBus offers. CardBus is based on PCI, so theoretical transfer rates on the order of 132 MB/s will be a reality on portable computers.

Fast Ethernet adapters such as 100Base-T are among the first applications for CardBus, but the performance gains over PCMCIA make it a promising platform for high-performance measurements as well. With the recent proliferation of palmtop computers and personal data assistants, the potential exists to extend the benefits of high-performance PC-based test into the hand-held field-service arena.

Fibre Channel

Fibre Channel is a serial interface that boasts 100-MB/s transfer rates. It has been fighting for market share for years and now is beginning to emerge as a replacement for small computer system interface (SCSI) implementations for I/O subsystems and mass storage.

The early uses of Fibre Channel handle storage applications that SCSI does not address. For instance, Fibre Channel is capable of much greater distances than SCSI. Ultimately, Fibre Channel will be used for storage-area networking where multiple servers and data-storage devices are linked on a private network.

It is unclear if Fibre Channel will become a platform for measurement devices. But its 100-MB/s throughput and flexibility to accommodate other network protocols, such as Transmission Control Protocol/Internet Protocol (TCP/IP), are attractive.

Since Fibre Channel has begun as a high-speed link to disk subsystems and arrays, it may have a use in PC-based test to increase disk-streaming speeds for high-speed logging applications, particularly when multiple computers are streaming data simultaneously to a disk array. The beauty of PC-based test systems is the capability to leverage the benefits of a particular PC technology into automated test applications. This transition might be farther down the road for Fibre Channel, but it is a technology that should be watched.

Ethernet

You might be asking why Ethernet is included in a list of new buses for PC-based measurements—and rightly so. Ethernet has been in use for years, but there is a new way to take advantage of Ethernet for PC-based measurement systems.

Ethernet solutions are ideal for embedded and distributed applications. A little black box with an embedded processor sits out in the ether, logging data, performing process control, and reporting back any pertinent information via the network.

But what if you need to set up a distributed solution and are not versed in the intricacies of network protocols? You need a way to leverage off an existing network infrastructure and take remote data without learning anything new.

The answer lies within the Internet and software. For instance, driver software can be used to make a data acquisition board located in a PC visible to any other PC that is connected by a network. This means that any PC on your network or any PC that you can connect to via the Internet can be turned into a remote measurement device.

You use the same data acquisition hardware you have always used. You don’t even have to modify your application. Everything works just as if the board was in your local PC. Now, you can run a test while on the road in the same way that you check your e-mail.

You can set up a research system in one location and share the hardware among multiple facilities, all logging into the system and running unique tests via the Internet. This type of application is ideal when you have an existing computer and network infrastructure and opens the door to leverage off the most significant PC advancement of our generation—the Internet.

How Do I Choose?

There is a plethora of bus interfaces to choose from when building a PC-based system. In many cases, it is not clear which one will emerge as the de facto standard.

For example, there is a large following behind IEEE 1394. But if you do your homework, you will find that Intel is developing its own high-speed serial link that will compete with IEEE 1394 for viability.

Which buses will become universal on PCs? The answer will come with time, but you can’t wait. You have applications and systems that need to be built today. Your decision will be based, in part, on the bus that makes the most sense for your application and is available today.

There is another aspect to this decision. Considering the vast proliferation of computer buses and rapid advancement in PC technology, the key to success in PC-based test is software scalability. Before you purchase equipment, evaluate a product and vendor’s capability to scale your application to new platforms as they emerge without requiring software rewrites.

A consistent application-programming interface across both operating system and computer architecture will be invaluable down the line. You can develop your application on the platform that makes the most sense today, knowing that you can port to another platform in the future without sacrificing your software investment.

You might choose PCI as your bus interface today because of the high transfer rates, but you might prefer an external solution like USB. Since USB does not match the transfer rates of PCI, you will stay with PCI today, knowing that you can transition your application to IEEE 1394 down the line. This peace of mind only comes when vendors produce scalable software solutions compatible across their hardware product lines.

Summary

The rapid advancements in PC technology are producing a variety of high- performance bus interfaces, each ideal for PC-based test. These developments bring exciting new possibilities for PC-based test, but software will be the glue that holds applications together over the long term as bus technologies come and go. To be successful, choose the bus architecture that best meets your needs today, and look for software that makes the bus transparent to you, preserving your application investment for years to come.

About the Author

Chad Chesney is the data acquisition marketing manager at National Instruments. He joined the company in 1994. Mr. Chesney earned a B.E. in electrical engineering from Vanderbilt University. National Instruments, 11500 N. Mopac Expressway, Austin, TX, 78759, (512) 683-5590.

Copyright 1998 Nelson Publishing Inc.

September 1998