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Saying goodbye to the Sigma 5

Monday, October 01, 2001

By Byron Spice, Science Editor, Post-Gazette

While officials at the Pittsburgh Supercomputing Center savor the new computer smell of their Terascale Computing System, another computer located two flights below their offices in the Mellon Institute building has come to the end of its scientific life.

Aksel Bothner-by, emeritus professor of chemistry at Carnegie Mellon University, shows the antiquated hard drive of the Sigma 5 computer. (Darrell Sapp/Post-Gazette)

The Sigma 5 once was a workhorse at Carnegie Mellon University's NMR Center for Biomedical Studies. With a bank of flashing lights on its control panel and a file cabinet full of punchcards for loading its programs, it now looks like a museum piece. And soon it will be.

The 30-year-old mainframe computer may well be the last of its kind in operable condition. That makes it a valuable artifact and the Computer Museum History Center, located at NASA's Ames Research Center in Mountain View, Calif., plans to add the Sigma to its collection.

"It's rare to find a complete, functional system," said Lee Courtney, a Sunnyvale, Calif., software engineer. "A lot of these machines were scrapped after trade-in. They were too big for individual collectors and companies didn't want to keep them."

The Sigmas were widely used in flight simulators, nuclear power plants and in libraries. They were first manufactured by Scientific Data Systems, which was acquired by Xerox in 1969. By 1975, however, Xerox got out of the mainframe business, said Courtney, a museum volunteer who spied the CMU machine by chance in the background of a photo posted to an Internet newsgroup.

CMU's Sigma 5 survived because it remained productive, harnessed to what was once the world's most powerful magnet for nuclear magnetic resonance, or NMR, studies. In the hands of Josef Dadok and Aksel Bothner-by, now emeritus professors of chemistry, the computer helped provide clues to the shape and structure of biochemical compounds -- information valuable for designing drugs and understanding the mechanisms of enzymes.

NMR is a chemical analytical technique as well as the basis for today's magnetic resonance imaging, or MRI, medical scanners: a combination of strong magnetic fields and radiofrequency pulses cause atomic nuclei to emit radio signals. For MRI scans, the signals are mapped to create images of internal body parts; for NMR spectroscopy, the signals are used to identify chemical composition and to study the structure of compounds.

The results of NMR spectroscopy appear as a series of spikes on a fever chart. "The whole art of NMR is figuring out what all these spacings mean," Bothner-by explained, pointing at one such tracing. One set of spikes suggests a benzene ring, he noted, using his hands to show the three-dimensional arrangement of molecules identified by other spikes on the tracing.

The Sigma 5 fell into disuse several years ago when Bothner-by and Dadok attained emeritus status. But it still works, if noisily.

"You can run an automobile today with this," Dadok said with a laugh as he fired up the computer's hard disk drive, which fills a five-foot-high cabinet. A rumble filled the room as the disk, about the size of a 45 rpm record, began to spin. That's nothing, he added, pointing at the computer's impact printer. "When this thing prints, it's like a World War I machine gun."

By today's standards, its performance is embarrassing. Its giant hard disk stored just 3 megabytes of data; today's PCs have more random access memory than that. The Sigma 5, which cost $300,000 when new, had just 16 kilobytes of random access memory; Dadok and Bothner-by never could afford the $50,000 it would have cost to upgrade to 32K.

But it works. Even after a busted hot water pipe drenched its circuit boards one night, it was as good as new after some cleaning by the Xerox repairmen.

"It's a very robust machine," Bothner-by said.



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