designfeature By Dan Strassberg, Senior Technical Editor NOTHING GOOD LASTS FOREVER, BUT IEEE 488’S SLOW DECLINE IS MAKING THE VENERABLE INSTRUMENT-INTERFACE STANDARD SEEM IMMORTAL. ike the character in the famous Monty Python sketch, IEEE 488 is Lnot dead yet. The decades-old stan- dard’s longevity is in question, however, and you probably have good reason to care about its fight for life.Whether you design test-and-measurement instru- ments, develop or implement systems that incorporate test-and-measurement products that others design, or simply use such products in your lab or test fa- cility, the methods that instruments use to communicate with each other and with host computers matter a lot. The communications medium’s physical im- plementation, protocol, and instru- ment-specific command sets strongly influence instruments’ and systems’ speed and cost and the time EEs must spend programming and debugging test applications. In 1977, the IEEE adopted the bus structure and communication protocol that it named IEEE 488. Some others call it GPIB (general-purpose instrumenta- tion bus). The bus’s original name was HPIB (Hewlett-Packard instrumenta- tion bus). HP, which, in 1999, spun off its electronic test-and-measurement business as Agilent Technologies, was the largest manufacturer of such equip- ment, a market position that Agilent still IEEE 488: enjoys. Even before the early 1980s ush- ered in the widespread availability of PCs, large numbers of minicomputers (remember them?) were in use control- ling instruments and processing the data they generated. Until the advent of the HPIB, no stan- not dardized methods existed for interfacing instruments with computers. It’s only a slight exaggeration to say that no two in- struments interfaced in the same way. The RS-232 serial-communication standard and the Centronics parallel- printer interface did a decent job of dead standardizing the computer side of the interface and even motivated some stan- dardization on the instrument side. (RS- 232 ports became popular on slower in- struments.) Still, because it was rugged yet?and relatively speedy, was designed for www.edn.com June 12, 2003 | edn 49 designfeature instrument interfacing, and was backed by several of Agilent’s latest instruments ATAGLANCE HP’s clout, IEEE 488 remained for more support USB 2.0 and, because USB 2.0 is than two decades the industry’s primary ୴ Most test-and-measurement-industry backward-compatible, they support USB standard for enabling instruments and observers believe that IEEE 488’s days are 1.1 as well. Another external serial bus, computers to talk with one another. numbered, but that the number will be well FireWire, also known as IEEE 1394, no Even so, IEEE 488’s early era was no in excess of 3000 days. longer appears to figure significantly in bed of roses. Although the standard did the future of instrument interfacing, even a good job of defining the communica- ୴ The two top contenders for the instru- though it is technically superior to USB, tions hardware, it initially gave short ment-interfacing standard of the future are including USB 2.0. The reasons are pri- shrift to interfacing’s software aspects. Ethernet and USB. You can find one or both marily political. More than a decade elapsed before the in many instruments. Thanks to a more efficient protocol evolution of the necessary software stan- and in spite of a raw bit rate 20% lower dards, particularly SCPI (standard com- ୴ The industry is hard at work to ensure than USB 2.0’s, FireWire—even in its ini- mands for programmable instruments). that the switch to new communication pro- tial implementation—is faster than USB It took still longer to develop standard in- tocols will be nearly painless. Existing instru- 2.0. Subsequent versions of FireWire will strument drivers, whose existence re- ment-control routines should work without be still faster. Moreover, unlike the host- lieved test-system designers of a great deal modification with the computer-standard centric USB, FireWire, with its peer-to- of application-specific programming. protocols. peer topology, seems tailor-made for test- and-measurement systems that include SEEMINGLY TORPID PACE ୴ Don’t be fooled by the new protocols’ both a host computer and instruments The idea that the electronics industry high nominal bit rates; instrument interfac- such as modern oscilloscopes and logic includes a segment that marks its evolu- ing usually involves short messages. In such analyzers, which contain their own com- tion in decades rather than months must service, IEEE 488 can be significantly faster puters. (By enabling several computers to seem bizarre—even unthinkable—to than protocols that at first appear to be share control of the bus, the new OTG younger EEs whose experience stretches much faster than IEEE 488. (On The Go) extension of USB 2.0 back only as far as the go-go days of the should at least partially overcome late 1990s. However, this seemingly tor- FireWire’s advantages in applications pid pace is keeping IEEE 488 alive, even necessary. Scopes that offer communica- that embody multiple computers that if not in the most robust health, and will tion ports other than IEEE 488 are be- can act as hosts.) But, whereas manufac- continue to do so for years to come. coming increasingly common. The result turers need not pay royalties to use USB To many observers, though, propelled of the need to intermix old and new in- in their products, using the patented by ubiquitous computer-interconnect struments will be increasing numbers of FireWire technology requires royalty standards, IEEE 488 appears to have en- instrumentation setups that also intermix payments. The royalty situation togeth- tered the end game. It may not go quiet- communication protocols (Figure 1). er with the influence of a few large com- ly—powerful forces will keep it alive for The current and most likely future panies that back it has enabled USB to years, but many in the industry see its leader in replacing IEEE 488 is Ethernet. leap ahead of FireWire in test-and-meas- demise as inevitable.A decade from now, USB will also play a major role; indeed, urement applications. on test floors and maybe even in some REMOTE REMOTE LOCAL design labs, you’ll still see instruments UNIX-BASED CONTROLLER WINDOWS-BASED CONTROLLER WINDOWS-BASED CONTROLLER with IEEE 488 connectors and cables, but USER USER USER you’ll probably have to look APPLICATION APPLICATION APPLICATION Figure 1 around for a while to find them. VISA VISA The main reason that IEEE 488 will LIBRARY LIBRARY survive for so long is that, for years to come, many instruments with IEEE 488 VXI-11 VXI-11 ASCII GPIB CLIENT CLIENT ports will continue to perform useful SERIAL WINDOWS-BASED ETHERNET work for their owners. Instruments such OSCILLOSCOPE LAN GPIB-LAN ADAPTER as benchtop DMMs measure quantities WINDOWS-SIDE EMBEDDED-SOFTWARE SIDE ETHERNET WITH VXI-11 GPIB OF INSTRUMENT OF INSTRUMENT such as volts, amperes, and ohms, which LAN HARDWARE RS-232 USER will not change in 20 years—or ever. Sure, HARDWARE APPLICATION newer DMMs will be faster, more accu- VXI-11 SERVER rate, and less expensive and will measure VISA VXI-11 LIBRARY lower currents and higher resistances, but CLIENT if you own a DMM that meets your needs ASCII VIRTUAL SOFTWARE SERIAL GPIB and still reliably meets its specifications, GPIB GPIB CONNECTION you require a strong incentive to replace NON-WINDOWS-BASED it. On the other hand, some types of in- INSTRUMENTS struments, such as oscilloscopes, change The software architecture of a system that incorporates IEEE 488 and RS-232 interfaces can more rapidly. Signal speeds continue to appear daunting, but, with modern development tools and instrument drivers, application devel- increase, making wider bandwidth scopes opment is generally less difficult than this diagram suggests (courtesy Tektronix). 50 edn | June 12, 2003 www.edn.com designfeature IEEE 488 The most obvious reasons for turning derstanding instrument I/O: perfor- IEEE 488 connectors and cables. to computer-standard interfaces in place mance and protocols”). In fact, in typi- Industry groups and software vendors of IEEE 488 for instruments are cost, cal instrumentation applications, which are hard at work on the infrastructure size, cable length of instrument net- involve short messages, IEEE 488, rather problem. They aim to make customers’ works, and increasing difficulty of in- like the tortoise that won the race with libraries of control routines work with- stalling specialized peripheral controllers the hare, can prove faster than protocols out modification through the PC’s USB in newer PCs. Here, “PCs” refer not just that are nominally hundreds of times as or Ethernet port and the corresponding to laptops, but also to the new breed of fast. Besides speed, the most important ports of instruments that lack IEEE 488 small desktop machines in “sealed” en- reasons for staying with IEEE 488 are the interfaces. Moreover, several companies closures. Greater speed, an apparent ad- existence of such a large infrastructure now offer external adapters, which let vantage of the newer interfaces, is often supporting it, instruments’ long service PCs that provide only USB or Ethernet an illusion, however (see sidebar,“Un- lives, and the physical ruggedness of ports and lack IEEE 488 ports control in- UNDERSTANDING INSTRUMENT I/O: PERFORMANCE AND PROTOCOLS By Joe Mueller, Agilent Technologies With alternatives to IEEE 488 The USB implementers’ forum the system sends more short RPCs (remote-procedure becoming available for instru- has recently completed defining transactions than long ones. In calls): These calls enable func- mentation, understanding the the USBTMC (USB Test and these cases, the time to do a tions called in one computer to impact of selecting new I/O Measurement Committee) short transaction is a better pre- execute in a remote computer.