SCSI vs. EIDE : What's the best choice?
I first became interested in SCSI because I wanted a CD recorder and that was the only connection type available; the same was true for scanners and optical drives. So I installed an Adaptec 2940 controller card and set up a SCSI system, which I felt operated pretty fast. I continued to add devices to my system with relative ease; an external Zip drive, another hard drive and a tape drive. That was five years ago, and I'm still using all the components in one system or another. Let's look at the differences between SCSI and EIDE connections.
SCSI stands for Small Computer System Interface. Here's a bit of an explanation of how drives connect. Most computers use IDE ( integrated drive electronics ) or EIDE ( enhanced … ) type hard drives and CD-ROMs because : the connection is built into the computer and adds no cost, drives are much cheaper, connecting devices is pretty straightforward. IDE and EIDE are sometimes referred to as ATA, but it's basically all the same thing. I should note that EIDE devices that aren't hard drives, like CD-ROMs and tape drives, are sometimes referred to as ATAPI ( ATA Packet Interface ). Both EIDE and SCSI have had numerous versions and specifications, adding great confusion and grief. The current spec for EIDE is ATA66, which allows for a burst speed of 66 MB per second. The current spec for SCSI is Ultra160, which allows for a burst speed of 160 MB per second. Note burst speed; this is not the sustained speed it works at all the time.
An EIDE bus has can accept two drives, a master and slave. Most computers ship with two EIDE connectors, allowing four drives. Of course you can add more EIDE connectors to your PC via an adapter card, but this takes up more resources and starts to complicate matters.
SCSI has several advantages : a separate adapter is used to control the devices which frees up the CPU. A SCSI bus can accept 7 to 15 devices, it's faster than EIDE and you can have external devices.
SCSI gets a bad rap for being difficult to set up, which I feel is bogus. For EIDE devices, you set the drive to either master or slave; for SCSI you set an ID between 0 and 15. The only point where there can be some confusion lies in termination; for SCSI the last device on the chain must have the terminator jumper set, which follows a fairly simple principle. A signal is sent along the cable, and the terminated device sends it back indicating the end of the chain.
Another advantage is resources, whereby the SCSI adapter uses one IRQ and memory address for all its devices. EIDE uses an IRQ and memory address for each connector, which can start to hog things up. A PC has fifteen IRQ's ( interrupt requests ) which are used to let devices talk to the CPU; a standard PC uses IRQ 14 and 15 for it's two EIDE connectors. This is a legacy from the original IDE spec; I've got a Promise EIDE controller card that has two connectors allowing four devices all the while using one IRQ. SCSI uses a separate adapter which requires one IRQ only for it's fifteen devices. You can get high end SCSI adapters that have multiple buses; Adaptec makes a card that can handle 45 devices and still only uses one IRQ.
EIDE devices work in sequence; the CPU handles the requests in order. SCSI allows for concurrent connections to all it's devices, and since the adapter handles the requests the CPU utilization is very low. To be fair the ATA66 spec and it's predecessors have brought EIDE a long way; almost all devices are available in this format. In fact I'm starting to see devices only available in EIDE; Plextor, king of SCSI CD-ROMs, just released their new 8x4x32 CD rewriter in EIDE format with no SCSI drive in sight.
Speed is the difference most quoted regarding these two connections, but one that has lost most of it's luster. In the beginning SCSI was much faster and the clear champ in this category, but with the recent ATA33 and ATA66 specs EIDE has narrowed the gap. SCSI has a slight advantage with rotation speed and access time; 10,000 RPM drives with a 5 MS access time are common, while the ATA66 drives are capped at 7200 RPM and around 8 MS access time. Rotation speed is the revolutions per minute that the hard disk platters spin, and access time is the number of milliseconds it takes the drive to find the platters. This is an ongoing battle that has benefited the consumer greatly.
Capacity has been another SCSI stronghold; the largest SCSI drive is just over 70 GB, while the big EIDE drive is 40 GB. Since SCSI has always been aimed at the server market, it's been important to get as much data in there and get it out as fast as possible.
Price is where SCSI really loses out; you're going to pay anywhere from 50 – 100 % more for practically the same component. Why? The EIDE market is huge and demand is high, which is a place SCSI will probably never be. I don't believe it's more than a few pennies extra to put a SCSI connector on the back of a hard drive or CD-ROM instead of an EIDE connector. SCSI has always been used for the server and high end workstation market where manufacturers make a large profit, which is a two edged sword. Drive innovation is always done to the SCSI market, a high profit is made, then it settles down to the EIDE market where profits are razor thin. Western Digital has stated that the prices are far too low for EIDE drives, and if the big manufacturers keep going this way they'll be out of business. For now it's a buyers market as they compete to produce the largest and fastest drives and still only charge around $400 for the best.
The big question is which to choose: SCSI or EIDE. For regular computer users and gamers who are using one or two applications at a time, EIDE and SCSI will perform roughly the same. In fact for the average user EIDE fits the bill perfectly, allowing for a wide range of devices. Throw in USB ( universal serial bus ) a new plug and play connector for external devices, and you'll never look at SCSI.
Then why bother? If you're a power user, involved in data intensive work like engineering or modeling, then you need the extra performance of a SCSI system. If you want to share external high speed devices like hard drives then SCSI is your best bet. If you need more than four internal drives than SCSI is the way to go. Finally, if you like to tinker and get the best of what's available, like myself, then there really is only one choice. SCSI isn't going away any time soon, with a new Ultra320 spec around the corner and the growing importance of servers and networking in this wired world.
by Scott VanderPloeg
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