Depending on your experience, the phrase PC-based instrumentation conjures up a variety of images. You may be contemplating using PC-based test for the first time, you may have used it in the past, or you may think it’s not suitable for your present application. If you’re in the last group, make sure you know how both PCs and instruments have changed before you make a final decision.
On the PC side, two new buses have addressed the time when PCs will have no internal expansion slots. And, instruments have become smarter and leaner. Recently developed low-power semiconductor technology has provided high-speed digital signal processors (DSPs) and high-precision analog-to-digital converters (ADCs) that can operate from only a few watts.
Instrument design has made better trade-offs between essential hardware functions and those things that can be done well by a Pentium-class host. The end result is smaller, lower cost instrumentation in a convenient and easy-to-use form.
When they were introduced, the universal serial bus (USB) and the IEEE 1394 bus (aka FireWire from its Apple Computer roots) were easily distinguished by their relative speeds. USB version 1.1 (USB1) peripherals include mice, keyboards, floppy disk drives, and other devices with bit rates below 12 Mb/s.
In fact, the USB1 specification established both a low-speed 1.5-Mb/s mode and a high-speed 12-Mb/s mode. However, neither was very impressive compared to FireWire’s 400-Mb/s rate.
On the other hand, whether due to better PR, FireWire license fees, or the very low cost of USB peripherals, USB has clearly eclipsed FireWire at this time. Virtually all PCs ship with built-in USB ports, but only a few computer manufacturers include FireWire.
So, one major USB advantage is ubiquity. Another is the recent version 2.0 (USB2) specification, calling for a top rate of 480 Mb/s. Of course, nothing comes for free, and comparable speed doesn’t necessarily equate to equivalent performance.
The Buses Compared
Both USB and FireWire are serial buses with hot-plug-and-play capabilities. This feature allows you to safely add or remove devices from the bus while the PC and any connected bus hubs are powered. There is one major difference between the buses: USB always requires a PC master while the 1394 bus provides peer-to-peer communications without PC intervention.
USB1 transfers a 1,500-B frame every millisecond, and the frame is shared by all connected USB devices—up to a maximum of 127. This means that actual data transfer for any one device could be as slow as one data point per two or three frames, although the useful composite rate is about 1.16 MB/s.
Passing all communications through the central PC makes possible very low-cost USB peripherals because they require minimal intelligence. The downside is increased transfer latency. It’s quite variable but as high as 8 or 9 ms for USB1. For that reason, USB1 is not a good bus choice for single-point transfers because its high latency limits you to low-speed monitoring and slowly changing temperature or pressure measurements.
USB2’s 480-Mb/s top rate improves burst transfer speed greatly. Its latency also improves because of the new 125-µs microframes rather than USB1’s 1-ms frames.
But, as described by Andy Purcell, a software design engineer at Agilent Technologies, “USB2 still is a master-slave architecture and will have an inherent fixed latency. The latency occurs because a USB slave cannot just send data when it is available. It must wait to send the data until asked for it. The latency is independent of CPU speed.”
The IEEE 1394 bus has a minimum latency of a few hundred microseconds and a worst-case delay of a few milliseconds. For large data blocks, this bus uses direct memory access (DMA) similar to PCI bus mastering that reduces the influence of software protocol overhead on the transfer rate. The 400-Mb/s top data rate supports consumer digital video equipment and data acquisition devices requiring relatively fast data transfer. Bus latencies are compared in Figure 1 and bus throughput in Figure 2.
Just as the USB specification has been upgraded, so too is there a 1394b version that will supersede the present 1394a. The proposed changes extend the top 400-Mb/s rate to 800, 1,600, and ultimately 3,200 Mb/s. However, although USB2 retains common protocol and operation with USB1, 1394b may not be entirely backward compatible with 1394a. Until the dust settles, manufacturers haven’t committed to 1394b silicon, preferring to back an unambiguous USB2.
To extend the USB realm of addressable applications further, the USB Implementers Forum (USB-IF) has proposed a USB On-The-Go subset of USB2. This specification enhancement would allow USB peripherals to exchange information directly, without the need for an intervening PC. So, you could download images from your digital camera directly to your printer without having to go through a PC between the two devices. On the other hand, a PC would be required if, for example, you wanted to crop, enhance, or otherwise edit the image prior to printing or if you needed to archive it on disk.
According to a recent article by Jeanne Graham, “USB is not necessarily a better technology than Bluetooth or 1394, but it has deployed better marketing campaigns.” Ms. Graham also quoted Bert McComas, an analyst at InQuest Market Research: “A consumer product manufacturer will say, ‘Give me one good reason to go with USB.’ Well, one good reason is that every PC in the world has a USB port.”1
The Industrial Case
The USB’s advantages for consumer applications seem to be equally valid for industrial users. Ease of use, low cost, and worldwide independence from AC supply considerations influenced Herb Figel’s decision to purchase a Dactron Photon Spectrum Analyzer.
Mr. Figel, the director of quality assurance at Hunter Fan, already had some experience with USB peripherals, having previously bought a digitizing pad and a device to synchronize his Palm organizer with his PC. He commented that the USB spectrum analyzer provided similar measurements to an older, large bench instrument, but that its user interface was much superior.
The Photon instrument has an upper frequency limit of 21 kHz and is entirely powered by the USB connection. It was a good fit to Mr. Figel’s ceiling-fan noise-measurement application with frequencies in the 100-Hz to 1-kHz range. Had he needed multimegahertz speeds, he wouldn’t have found an instrument that operated within the USB’s meager 2.5-W power limit, although fast PCI-bus cards are readily available. So, he could have retained a PC-based test system, but it wouldn’t have been as simple and convenient as that made possible by USB.
As an example, the Gage Applied CompuScope 14100 is a dual-channel, 100-MS/s, 14-bit resolution PCI card. It achieves sustained 100-MB/s data transfer rates via PCI bus mastering under single-tasking operating systems. On-board memory ranges from 1 MS to 1 GS, and the card draws from 25 to 35 W.
For Gage’s customers, a high sustained data-transfer rate is important. “High bus transfer speed, while almost irrelevant in one-shot applications like explosion testing, is essential in the acquisition of repetitive signals,” explained Andrew Dawson, the company’s product manager of board-level products and advanced measurement systems. “Examples of these applications include radar, lidar, ultrasonic imaging, and manufacturing test systems. A typical requirement is to capture 1,000 point acquisitions at a repetition rate of over 10 kHz without missing a single event.”
Also shunning USB and FireWire for the moment is Mark Cejer, the test and measurement marketing manager at Keithley Instruments. “Until an instrument comes along that offers unique features only available with USB or FireWire, there probably will be little incentive for users to buy them. Large production ATE racks consist of multiple types of instruments. What good will it do to have a USB DMM, for example, if all the other instruments are GPIB?”
Balancing this view is one that considers the need to connect new PCs to existing GPIB and RS-232 instruments. National Instruments’ solution consists of the GPIB-USB-A and the GPIB-1394 controllers that transform any computer with a USB or FireWire port into a plug-and-play GPIB controller that can handle up to 14 instruments.
For Dewetron, system simplification is a theme that runs parallel with the development of the DEWE-BOOK. Grant Smith, the company president, said, “A DEWE-BOOK is an eight- or 16-channel signal-conditioning front end with a built-in ADC that precedes our DAQ and PAD series of modules. Previously, we offered an internal ADC board that had to be connected to the PC’s printer port, but as well as tying up the printer port, it limited throughput to 20 kHz. Today, we get a very consistent 100-kHz throughput with each USB-connected DEWE-BOOK.
“In addition, it is plug-and-play, making it easier for our customers to install and get running, and USB is well supported by available software. Also,” he continued, “we had to use a separate COM port to control the settings on our DAQ and PAD modules. This meant that both a parallel port and a COM port on the customer’s PC were tied up. Now, we handle all control as well via USB. As a final benefit, by adding a hub, up to four DEWE-BOOKs can be used simultaneously with the same PC.”
The Broader Picture
Of course, USB and FireWire are just two of many instrumentation and computer buses available today. Agilent’s Mr. Purcell said, “USB1 block transfer performance is similar to GPIB, so the primary benefit for USB is the ease with which customers can connect instruments to PCs. FireWire performance is quite good, and we are seeing block transfer rates of 15 MB/s. This is 20 times better than GPIB and more than 1,000 times better than RS-232.
“USB2, with its Intel backing, may become as pervasive as USB1 is today. Its higher bit rate should enable 10 times the performance of GPIB,” he continued. “In anticipation of this, there is a growing international group of companies that has started work on a standard USB protocol for test and measurement devices.”
As a step toward industry-wide software compatibility, the VXIplug&play Systems Alliance has developed a specification for I/O software called Virtual Instrument Software Architecture (VISA). VISA provides a common foundation for the development, delivery, and interoperability of high-level multivendor system software components, such as instrument drivers, soft front panels, and application software. VISA not only allows test engineers to combine different I/O buses into one system, but also provides the necessary abstraction layer to make the transition to new buses transparent to the user.
“Although VISA solves the mixed I/O problem on the host side,” commented Vanessa Trujillo, an instrument connectivity product manager at National Instruments, “a similar architecture is needed on the device side to make the integration of new bus types seamless for the instrument manufacturer. For instrument manufacturers to embrace and adopt new buses while at the same time to support their many customers who still use one of today’s buses, they need an architecture that allows them to easily adapt the firmware they have written for one bus type to another.”
Sharing Mr. Purcell’s USB emphasis, Nick Turner, sales and marketing manager at Cytec, said “The advantage we saw with USB was the ability to daisy-chain devices to run from one PC port. RS-232 is a one-to-one bus, and GPIB is limited to 16 devices, so we thought there probably would be interest in USB. However, we’ve not done anything with FireWire because it requires licensing.”
Mr. Turner cited the USB’s 5-m length restriction as a disadvantage in his company’s automated test business. Although greater expense and complexity accrue, hubs can be stacked to a maximum of 30 m. The 5-m length limit also applies to each FireWire hop, but the 1394b version promises to span 100-m or greater distances by matching media and speed. For example, 100 m could be achieved at high speed via fiber-optic cable, where copper would be appropriate at lower speeds.
The continuing adoption of Ethernet for instrument communications goes on in the background as USB and FireWire vie for position. Mr. Turner commented, “The biggest trend we currently are seeing is more and more people wanting devices with a network interface. There is a plethora of software and hardware support for 100Base-Tx and even Gigabit Ethernet, and people are becoming increasingly accustomed to working with them.”
Agilent’s Mr. Purcell agreed, but added a cautionary note. “Ethernet connections allow instruments to communicate using http, RMI, DCOM, and RPC. Instruments can act as web servers, and users can use familiar browsers to control and view collected data. There seems to be a lot of enthusiasm for connecting instruments to Ethernet, but enhancements are necessary so that it’s easy to configure and then discover attached devices.”
Going beyond bus-tethered instruments, Gage Applied’s Dr. Dawson foresees high-speed wireless links that will transform PC-based test and measurement. “Within a few years, the primary human interface to the PC will not be the traditional mouse, keyboard, and monitor. Instead, users will interface to a portable personal digital assistant (PDA) that, in turn, will communicate through a wireless interface to a faceless connected instrument (FCI). Communications via the PDA will allow greater data sharing and free the user to control equipment remotely in a manner unavailable today,” he explained.
Reference
- Graham, J., “Approval Expected for USB On-The-Go,” Electronic Buyers’ News, www.ebnews.com, March 8, 2001.
Return to EE Home Page
Published by EE-Evaluation Engineering
All contents © 2001 Nelson Publishing Inc.
No reprint, distribution, or reuse in any medium is permitted
without the express written consent of the publisher.
August 2001