Instrumentation and The Universal Serial Bus

Contents

•USB Development
• Usage Trends in Other Data Communication Buses
•External Bus Selection Criteria
•Outlook for Instrument Designs
•Reference

The Universal Serial Bus (USB) promises to make peripheral additions to PCs plug-and-play without having to reconfigure the PC and without the need for another interface card. This data communication standard gives hardware manufacturers the tools to build keyboards, modems, storage devices and other peripherals that are ready to run the moment they are connected to a USB-equipped PC. The standard provides for data transmission rates of up to 12 Mb/s. Also, a USB peripheral device can request a certain amount of guaranteed bandwidth from the PC, thereby ensuring that important data is transmitted in a timely fashion.

However, on industrial measuring instruments, USB has not yet become a standard interface and user demand is spotty. There are several reasons for this slow adoption. This paper explores these USB trends and the outlook for this protocol on industrial instruments. Criteria for selecting an appropriate communication bus also are presented.

USB Development

Recognizing the need for a standard that would be easy to use and support high data transfer rates, several manufacturers, including Compaq, Intel, and Microsoft, developed the Universal Serial Bus (USB). This standard gives hardware manufacturers the tools to build phones, modems, keyboards and other peripherals that are ready to run the moment they are connected to a USB-equipped PC. Although USB was created initially for consumer oriented applications, it will have far-reaching effects on a wide variety of PC-based systems, including those used for test and measurement. A block diagram of typical networked instrumentation systems is shown in Figure 1

Click here to see Figure 1.

USB Characteristics—The universal connectivity of USB is primarily intended for desktop and laptop peripheral interfacing. USB is an external bus, with the connected devices having built-in intelligence. This intelligence expands the concept of plug-and-play computer devices to include those installed outside the PC, thereby making PC-based system construction simple and fast.

USB cables have four conductors and different connectors on each end, one designed to fit the USB port on the PC and the other to fit the USB port of the peripheral. Employing different port and connector designs on the PC minimizes potential connection mistakes. Almost all new PC designs from major vendors will have built-in USB connections.

USB accommodates data transmission rates up to 12 Mb/s. This is more than adequate for many test and measurement applications, including those that involve high-fidelity audio, highly compressed video, such as MPEG-1 and MPEG-2, high frequency shock and vibration signals and transient waveforms. Also, the peripheral device can request a certain amount of guaranteed bandwidth from the PC, thereby insuring that the device can transmit important data in a timely fashion.

In a USB system, once a peripheral is connected to the PC, recognition of that device by the PC is automatic. You do not need to reboot the PC or run its set-up procedure. The operating system of the host PC detects when devices are added and removed. The PC automatically allocates the resources, including driver software and bus bandwidth that each peripheral needs, and then makes those resources available without user intervention. As newer peripheral products come to market, however, they may not be supported by a PC's resident drivers and will require a diskette containing driver updates.

Benefits of USB—Since USB peripherals are automatically configured as soon as physically attached to the PC, this allows a user to switch easily between compatible peripherals. They can be connected and disconnected as easily as a telephone or lamp. Hot swapping is no problem, and once plugged in, the device is ready to run.

USB's easy connectivity allows additional devices to be added without the configuration nightmares previously associated with PC-networked systems. Peripherals can be daisy-chained together using dedicated USB hubs that have one input and four outputs to allow networking of different device types to the PC. Devices such as monitors and keyboards are capable of acting as plug-in sites for other peripheral devices. Ultimately, up to 127 USB-supported peripheral devices can run simultaneously on a computer, allowing the PC to support a variety of sensors, measurement instruments, and monitoring equipment.

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Usage Trends in Other Data Communication Buses

Will older serial buses and GPIB fade away? It's nearly a no-brainer to say "Probably not any time soon." You can look at almost any data communication standard and see a slow evolution into and out of hardware supporting the standard. In the case of RS-232, RS-422/485 and EPP, the evolution will probably require less time than the transition away from GPIB. Most of the reasons for this can be gleaned from Table 1, principally the first six rows. There are other practical reasons why GPIB is likely to survive for some time:

Bus speed isn't the principal throughput limiter in many applications, GPIB instruments can provide resolution and accuracy not possible with other test and measurement solutions There is a large body of engineers and technicians familiar with the menu and SCPI programming used in GPIB instruments GPIB instruments can also be used in a slotless environment (that is, as standalone measurement solutions)

Table 1. Salient Features of Selected Data Buses

Click here to see Table 1.

Survey Findings—This intuitive analysis is supported by recent research. According to a Keithley study completed in 1998 (Table 2), most test and measurement applications today use the serial port, 4-20mA, and IEEE-488 bus communication protocols. Though the survey respondents indicated that use of these protocols will decline, some observers feel that they will continue for applications not requiring remote access. However, users who also need control capabilities are likely to migrate to faster protocols, such as Ethernet, USB and Firewire. Engineers in the study also predict that remote measurements will become more common; 62% of respondents say that future measurements are somewhat or very likely to fall into this category.

Table 2. Communication Protocols Used To Capture Measurement Data.

While USB will support many test and measure applications, initially much of its industrial use is likely to be in data acquisition (DA). Migration to USB is expected to be most vigorous for DA applications now using PC serial and parallel ports. Besides the obvious benefits of USB, another reason is the smaller investment base of instrumentation with RS-232, -422, -485 and EPP interfaces compared to those with an IEEE-488 interface.

Given that transition of test and measurement equipment from older protocols to USB will be evolutionary, and your favorite instrumentation is not yet USB equipped, what should you do? The next section attempts to answer that question by examining external bus selection criteria. This type of analysis will be useful even when USB becomes more common.

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External Bus Selection Criteria

Instrument Constraints—Before getting concerned about the lack of a USB interface in an instrument, recognize that the bus often is not the limiting factor in data transmission throughput. Instrument constraints may prevent you from taking advantage of maximum bus speed. This is particularly true if we extend the definition of instrumentation to include switching matrixes and component handlers typical of those used in production test systems.

For example, in a production system with a cross-point switch matrix between the measuring instrument and component handler, individual switching times can range from about 100µs to 10 ms, depending on whether solid state or mechanical relays are used. Then the component handler typically takes about 100 ms to remove one device from the test station and insert another one. With appropriate programming, many GPIB-based measuring instruments can stay ahead of a component handler and switching matrix, but their reading rate may be slower than the maximum USB data transmission speed.

In applications where other criteria support the use of a bus other than IEEE-488, a number of interface controllers have been developed. These are modules supported by software drivers, located outside the PC, that allow control of IEEE-488 instruments to take place over another type of bus. These interface controllers include, IEEE-488 to Ethernet, USB, RS-232, RS-485, and the parallel port (EPP and SPP). Although this adds hardware cost, it allows the use of legacy instruments and software, which often provides a significant offset.

Legacy System and Software Considerations—Trends driving the integration of PCs and instruments in production systems may provide compelling reasons to stay with an older network protocol. Today, it is increasingly common for measurement systems to be tied into an enterprise business computing network. Whereas plant floor tests are used for product/process adjustments, quality control and parts binning, business analysis of the same data looks for ways to improve quality and the bottom line. The plant network may be using Ethernet, CAN, DeviceNet, SDS, Foundation Fieldbus, Interbus-S, LONworks or PROFIBUS, many of which incorporate some of the features found in RS-232, -422 and -485 protocols. In any case, IT personnel are now concerned with how test and measurement systems interface with broader enterprise networks.

This will probably affect test and measurement software development. Graphical development environments, which might be appropriate in a standalone or simple USB-based system, will not be familiar to many business programmers trying to tie a customized measurement systems into the enterprise network. Business IT personnel are more likely to be familiar with open software standards, such as Visual Basic, Visual C++ and ActiveX; they will want to use as much of their legacy software as possible.

A key argument for using USB is because PCs equipped with it have software support built into their operating system (OEM version Win 95 OSR 2.1); separate drivers and interface cards are not required in the PC. While it may be easy to create and expand an isolated USB test system (when such instruments are available), this could be offset in legacy systems by the need to create or modify software to integrate the USB system with a larger enterprise network. On the other hand, adding a measurement system that uses an older PC without USB requires additional controller cards and/or drivers.

A limitation of the USB standard is its cell orientation. In other words, USB devices must be used in clusters because of cabling limitations (five-meter cables and a maximum of six cable segments). This geographically constrains USB devices within an application cell having a local PC, which makes data sharing across a broader plant network much more difficult. Ethernet tends to be a better solution for broad data sharing.

Shared Bandwidth/Multiple Devices and Drops—When integration within a larger network is not a concern, USB does offer an easy method of connecting multiple sensors and measurement devices. However, other industrial buses and Ethernet also allow shared bandwidth and multiple drops. Depending on the protocol, devices may be daisy-chained or utilize a router hub. In all cases the protocol must provide interrupts to control the PC's processing activities. A disadvantage of other buses compared to USB is that a PC interface card is required, as well as a unique software driver for every instrument connected. While the bus itself may have an open configuration, all the software drivers are proprietary. See Table 1 for other bus characteristics, including cabling and connector requirements.

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Outlook for Instrument Designs

During the transition to USB, look for instrument manufacturers to provide functions that overcome some of the disadvantages of IEEE-488 and other external buses. For example, GPIB instruments are evolving in ways that prevent the IEEE-488 bus from limiting throughput. You can expect to see more instruments with hardware ports for digital I/O and trigger inputs that don't require use of the bus. Also, more instruments will have internal memory and other functions that eliminate bus usage for communicating major portions of a test program.

As the transition to USB gets underway, the interest in IEEE-1394 (Firewire) is growing. Firewire is in its infancy as far as industrial systems are concerned. Originally designed for multimedia use, it will have a slower take-off in test and measurement applications. Although it has high speed (400Mb/s; 1Gb/s planned), few PCs have it today. Nevertheless, the Instrumentation and Industrial Control Product Design Work Group of the 1394 Trade Association has been formed to promote use of Firewire in these applications. One of the group's aims is to figure out how GPIB commands can be used with 1394-based systems, thereby allowing the same test program code to be used with either data communication protocol. As these problems are solved, and more PCs are equipped with Firewire, industrial use of the IEEE-1394 bus should accelerate rapidly.

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Reference

1. "Measurement Needs Tracking Survey", Keithley Instruments, Inc., Cleveland, OH, 1998.

About the Author

David Howarth is Data Acquisition Product Manager with Keithley Instruments, Inc. located in Cleveland, Ohio. He has more than 15 years of experience in engineering and software development for industrial computers and peripheral equipment. He holds a BS degree in Computer Engineering from Case Western Reserve University.

Copyright 1999, Nepcon-West Proceedings. Reprinted with permission of Reed Exhibition Company.