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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) | SCSI Host Adapters ]

Adapter Types and PC Bus Connections

In selecting a SCSI host adapter, one of the first decisions to be made is what type of host adapter you want. There are many different kinds of SCSI host adapters on the market, and they vary in cost and capabilities dramatically. Many lower-end adapters are designed specifically are designed to keep costs down to allow easy, inexpensive access to SCSI devices like scanners or CD-RW drives. Higher-end devices provide more capabilities and performance for users who require a full-featured implementation for hard disks and other performance drives.

A key distinguishing characteristic between various host adapter models is the type of system bus the card is designed for. SCSI host adapters have been made for all of the common PC I/O buses, including ISA, EISA, VLB, MCA and PCI. You obviously need to choose a host adapter that matches the system bus(es) in your machine. Until recently, motherboards that featured both PCI and ISA slots were common, giving you a choice. As is the case with most interfaces that have a significant impact on performance, an ISA-based card is generally a bad idea since this will greatly limit the performance of the bus. You cannot efficiently use even something like Fast SCSI through an ISA-bus-based host adapter, because ISA cannot handle more than about 8 MB/s of data throughput. (Or rather, you can use it, but you won't get all the performance possible.) Of course, by today's standards Fast SCSI isn't all that fast, and higher-performance transfer modes are even less suitable for ISA. However, ISA cards can be perfectly acceptable for slow SCSI devices such as scanners, Zip drives, optical drives and so on.

After 20 years, the ISA bus is finally going the way of the dodo; Intel and Microsoft are hard at work trying to kill off this old system bus interface, and many systems come only with PCI slots, making a PCI host adapter the only real option. Of course, not all PCI cards are alike; you certainly still have plenty of decisions to make even within PCI. Here too, low-end, inexpensive cards are more suitable for simple applications, and more expensive cards are appropriate for performance-demanding applications.

As I mentioned above, a very important performance issue concerning SCSI host adapters and the system bus they use is the throughput of the bus. If the throughput of the system bus is less than the maximum throughput of the SCSI channel, SCSI performance will be limited to whatever the bus's maximum rate is. Until recently, this wasn't much of a concern as long as a PCI host adapter was used, because PCI had more than enough "overhead" to handle any SCSI bus. In the last few years, however, with SCSI channels continuing to increase in speed, even the performance of regular PCI is now reaching a limiting point. The maximum practical bandwidth of regular (32-bit, 33 MHz) PCI is a little over 100 MB/s, and the newest SCSI devices use Ultra160 SCSI, capable of well over 100 MB/s of throughput. To get maximum performance from such an interface, regular PCI is not sufficient.

To this end, some higher-end Ultra160 and faster SCSI host adapters are designed to use new enhancements to traditional PCI. These include 64-bit PCI, capable of throughput of over 200 MB/s, and also the new PCI-X bus, which promises performance of up to 1 GB/s. Cards using 64-bit PCI are readily available and are backwards-compatible with regular 32-bit PCI, so they can be used on both newer systems with 64-bit PCI slots, or systems that have only 32-bit slots.

Note: As I discuss at the end of the page on SCSI bus speeds, one should always remember that SCSI throughput specifications are for the SCSI chain as a whole, not individual devices. 160 MB/s is the maximum for all the devices on an Ultra160 bus. The limits of regular PCI only become an issue if one is using enough devices simultaneously to push the limits of the bus--such as if RAID is being used. If you are just using one or two Ultra160 devices, regular PCI is probably sufficient for your needs.

Another reason to use PCI is that most newer host adapters that run on the PCI local bus support the use of bus mastering. This can be a very important feature, as it allows for more efficient transfer of data from the host adapter to the system memory. Full performance from high-end SCSI chains requires bus mastering support.

Next: Protocol Support


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