Two types of controllers differ on ease, capacity


Many of the columns I write come from my readers, in the form of questions that arrive in the mail.

Some queries are a bit broad for the 25 column inches or so that my editor so graciously allows ("I need a computer for my business. Got any ideas?"). Others are a little narrow for a general audience ("How about a review of high-resolution X-ray management software for orthodontists?").

But this week brought a letter that will keep me busy for a while, because it contains a half-dozen questions about one of my pet peeves, the "alphabet soup" of computer terms that buyers and users have to wrestle with.

This particular reader wants to know, among other things, if I can explain the difference between SCSI and IDE, and possibly offer some enlightenment about the great ISA vs. EISA vs. PCI vs. VESA battle. This isn't as silly as it sounds; you'll actually run into people who talk about this stuff if you dip your toe in the market today. So here goes.

SCSI vs. IDE. These are acronyms for Small Computer Systems Interface and Integrated Drive Electronics, respectively. They refer to two types of controllers used to handle hard disks and other devices, and sometimes to the devices themselves.

SCSI (pronounced "scuzzy") controllers are circuit cards that plug into your computer's main circuit board to handle hard disks, CD-ROM drives and scanners. Many sound boards, for example, have a built-inSCSI controller for CD drives.

Theoretically, the industrywide SCSI standard allows a single controller to handle up to seven different devices, connected one to the other in daisy chain fashion. This eliminates clutter inside your computer and makes it a snap to add new SCSI gadgets. It also theoretically allows the same SCSI disk drive, CD-ROM or scanner to be used with IBM-compatible or Apple Macintosh computers.

Because SCSI is truly the Macintosh standard and all Macs are built by Apple, reality usually matches theory in the Mac world. But in the crazy world of IBM clones, it often takes a Ph.D. in electrical engineering or six hours on the phone with factory technicians to get two different SCSI devices working together, let alone seven. And the SCSI "standard" is actually a moving target, because you can now choose from plain vanilla SCSI, SCSI-2 and something called SCSI-Fast.

This state of affairs is improving as the industry matures, but if you want to hook up a scanner and a hard drive to the same SCSI controller, make sure you have help close by the phone.

There are, however, good reasons to buy an IBM-compatible with a SCSI hard drive. SCSI drives are fast and reliable, and for the time being, they're your only choice if you need a drive with a capacity greater than 540 megabytes.

IDE drives, on the other hand,were designed to be simple, cheap and effective, if somewhat limited in scope. An IDE drive has its controller built into its own electronics, providing an ideal match that improves performance. The IDE adapter that plugs into your computer's main circuit board is little more than a pass-through device. Unlike SCSI drives, IDE disks don't require complicated setups or special software drivers.

The downside of the IDE standard is capacity. Today, the maximum size of an IDE drive is 540 megabytes, and you're limited to two of those. This isn't much of an imposition on the average user, which is why most IBM-compatible machines come with IDE drives as standard equipment. But even these limitations are expected to evaporate over the next year as the industry develops a new IDE standard that will handle larger drives and more of them.

Now for the rest of the gobbledygook. I SA and EISA stand for Industry Standard Architecture and Extended Industry Standard

Architecture. They refer to the type of "bus," or wiring harness, that connects your computer's microprocessor with its memory and outside devices, such as disk controllers and video display boards.

Think of the bus as the highway that the little electronic ones and zeros use to march around your computer. This ISA bus, designed by IBM in 1984 for its AT-class machines, is 16 bits wide, which means 16 little ones and zeros can march abreast. The newer EISA bus is 32 bits wide, which means it can handle twice as much information at once.

The nice thing is that EISA-bus computers are backward-compatible, which means you can plug older ISA circuit boards into them and they'll still work. In fact, most of the expansion cards that handle disk drives, monitors and other gadgets today are still strictly ISA-compatible, which means they only use 16 lanes of the highway, no matter which type of bus the computer has.

The power of an EISA computer emerges when you plug in controller cards that take advantage of its 32-bit data path and other capabilities. Thus equipped, EISA machines make great high-speed file servers for large networks, but most desktop computer buyers won't benefit by spending the extra money for one.

Now we're coming down the home stretch. VESA and PCI also refer to data buses, known in the trade as "local buses." They've become popular add-ons to computer architecture in the last few yearsbecause the standard ISA and EISA buses can't move video data fast enough to support high resolution graphic environments such as Microsoft Windows.

The Video Electronics Standards Association (VESA) developed the first widely accepted local bus design. A VESA local bus is used to connect a high-speed video controller directly to the computer's microprocessor, although it also can handle a high-speed disk controller.

PCI (Peripheral Component Interconnect) is a newer, competing local bus standard developed by the Intel Corp., which claims PCI is faster and will support more devices. While the jury is still out, it appears that PCI works best with Intel's newest, high-speed Pentium microprocessors, but offers no advantage over VESA with older 486 machines. Either type of local bus will make your video display snap to attention, and computers with a local bus represent good value for the money.

Class dismissed.

(Michael J. Himowitz is a columnist for The Baltimore Sun.)

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