CHOOSING A HARD DRIVE &

CONTROLLER

CHOOSING A HARD DRIVE AND CONTROLLER

W

ith so many different drives and controllers on the market, where do you start? Begin with software requirements. Narrow your choic(~s by eliminating drive interfaces or controllers that are not compatible with your application. For example, an IDE drive might not offer sufficient performance for your netw-ork software, or an older rnachine might not be compatible with Enhanced IDE. In gen-eral PC applications, IDE drives are the most conlpatible since nearly all operating syste:ms will run an IDE drive without additional soft-ware drivers.

In terms of performance and fleXibility, SCSI is always the best choice. Unfortunately, almost all advanced operating systems like Win 95, OS/2 and Windows NT require software drivers for full perfor-mance and support of SCSI controllers. Determine the availability of software drivers for your applications before choosing SCSI.

Conside:r future expandability and upgradability. SCSI controllers offer the nlost flexibility and expandability in the long run. With a SCSI controller, you can daisy-chain 7to 15 different devices, including SCSI hard drives, CD-ROMs, erasable optical drives, DAT and other SCSI peripherals from the same controller.

Most interfaces other than SCSI and IDE are now obsolete. Use them only ilf you are upgrading an older system that already has them installed. 1ESDI, RLL, and MFM drives are still available. They may not be the fastest or most economical choice, but they may be a good choice for upgrading an older system.

If you arle building a new IBM compatible system, you also have a choice of motherboard bus/controller card configurations. The most popular choices are ISA and PCI. Each bus has it advantages and limitations.

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52 Hard Drive Bible

ISA refers to the original 16-bit bus that IBM designed into the first 80286 based AT computers. The IBM ISA specifications strictly limit-ed bus spelimit-ed to 8MHZ and set firm rules about bus timing. Newer clone motherboards violate this specification and permit operation up to 16MHz. The ISA bus design is capable of accommodating most hard drives and I/O cards without a bottleneck. Its main limitation is video.

With the advent of programs like Microsoft Windows, large amounts of data must be transferred quickly to the video card as windows are opened, closed, and scrolled. The original AT bus lacks the band width for acceptable video performance.

To solve the AT-Bus performance problem, a committee called the Video Electronics Standards Association was formed. The VESA local bus standard was established to improve video performance while maintaining conlpatibility with ISA bus peripherals. VESA bus moth-erboards have two or three local bus slots that are connected directly to the 32 bit bus of the Intel compatible CPU chips. This permits up to three VESA peripherals to operate at any speed up to the full speed of the processor. The main problem with the VESA bus design is bus loading. As VL-bus speed is increased (VESA bus speed is linked direct-ly to processor speed), the number of adapter cards that can be used decreases. For example, lll0St 50MHz VESA motherboards will support only one or (maybe) two cards. Due to these limitiations, the VESA VL-BUS standard has lost popularity and is now found only in older sys-tems.

A new standard, the Peripheral Connection Interface (or PCI) bus has now taken front stage. The PCI bus offers high performance (up to 133MB/sec in burst mode) and easy installation. PCI doesn't suffer from a limited number of supported slots as VESA does. PCI boards are also auto configuring (an advantage over VESA and ISA). As more PCI peripherals become available and prices drop, the price/perfor-mance ratio of PCI will make it the only practical bus for SCSI drive interface cards. Insist on both PCI and ISA if you are building up a new system. famous for conlpatibility problems between video and controller cards. PCI cards are generally all compatible, but inserting one low performance PCI card in your system will lower the performance of all the other cards.

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With standard IDE controllers, bus speed is normally not an issue.

With fllemory or I/O mapped SCSI controllers, you will need an avail-able interrupt and rsufficient address space in the base 640K memory to support the footprint of the controller BIOS. ISA bus mastering controllers of any type can be a nightmare. Bus on/off times and refresh rele:ase rates often need to be adjusted to get things working.

With a negligible performance difference between bus mastering and memory mapped controllers, you are best off steering clear of bus mastering controllers. ISA bus mastering controllers may also have compatibility problems or performance limitations in machines with more than 16MB of memory.

Our ove:rall recommendations: A fast PCI SCSI controller for new systems. Couple this controller with the largest SCSI drive you can afford. If you are interested in a small capacity drive and controller, an EIDE drive 'will offer the most for the money. Weigh your storage and speed requirements. For Network server applications, go with the fastest wide: SCSI drive you can afford. For workstations or light data-base applications, a larger capacity drive with a slower access time and lower cost may be preferable. In notebook and portable applica-tions insist on a drive with good shock tolerance. When selecting a drive capacity, be sure to think to the future. It's better to start with a large capacity drive now than to replace the entire drive in the near future.

In sumtuary, for most low capacity applications we recommend a small, inexpensive EIDE drive with an imbedded controller. For maxi-:mum software compatibility in sizes below 2GB standard EIDE drives are a good choice. For top performance and upward compatibility with the ability to daisy-chain additional peripherals, choose a SCSI drive and controller.

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