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The Industry Standard Architecture (ISA) bus is the oldest motherboard bus in common use on x86 PCs. In its earliest incarnation, the ISA bus was an 8-bit bus, but the one or two ISA slots that are still common on modern motherboards are invariably 16-bit slots that can accept either

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8- or 16-bit cards. Although a few early SCSI-1 adapters worked in 8-bit ISA slots, these devices were very slow and undesirable even a decade ago. Today's ISA SCSI cards almost invariably use 16-bit ISA connectors.

Caution

Don't confuse the 8- or 16-bit nature of the ISA bus with the 8- or 16-bit (a k a Narrow or Wide, respectively) versions of SCSI. Most ISA SCSI cards use 16-bit ISA interfaces, but implement an 8-bit (Narrow) SCSI bus.

ISA SCSI cards are best classified as belonging to one of two categories:

• PIO boards The least expensive ISA SCSI boards use programmed input/output (PIO). Like early IDE controllers, these boards require the computer's CPU to supervise every data transfer. PIO circuitry is inexpensive to produce, but the resulting CPU load causes these boards to impose a substantial performance penalty in a multitasking OS such as Linux.

• DMA boards More expensive ISA SCSI cards implement direct memory access (DMA), in which the SCSI host adapter can transfer data directly from the SCSI bus to the computer's RAM. DMA boards reduce the CPU load when performing SCSI operations, and so are preferable to PIO boards. Unfortunately, the ISA bus limits DMA transfers to the lower 16MB of RAM, so on computers with more RAM, Linux might need to move data around in memory after a transfer. DMA boards usually implement a feature known as bus mastering, in which the expansion card takes over the motherboard's bus during its DMA transfer. These boards are therefore sometimes referred to as DMA bus mastering, or bus mastering, boards. 9

There's very little reason to purchase a new DMA ISA SCSI card today, because such cards are not much less expensive than PCI SCSI cards that often perform much better. Most of the ISA SCSI cards in today's market are PIO models that are bundled with low-end SCSI devices, such as scanners and CD-R drives. If you acquire such a package and have no other SCSI host adapter, you might want to try using it under Linux. You can then evaluate the device's performance and, if you notice an unacceptable slowdown of your system when using the SCSI device, replace the ISA board with a better PCI model.

A few of the very low-end ISA boards include only an internal or only an external SCSI connector. If you have only an internal connector but want to use an external device, you can attach an internal/external connector (see Figure 9.6) to the end of an internal SCSI cable, thus extending the chain outside the case.

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Figure 9.6

An internal/external SCSI connector lets you use external devices with a SCSI host adapter that has only an internal connector.

Figure 9.6

An internal/external SCSI connector lets you use external devices with a SCSI host adapter that has only an internal connector.

Few or no ISA boards implement any SCSI standard more advanced than Narrow Fast SCSI-2. Indeed, the ISA bus's theoretical maximum transfer speed is only 8MB/s, which is slower than Fast SCSI-2's maximum transfer speed of 10MB/s. There really is no point in supporting higher speeds than this in an ISA card.

EISA Bus and VL-Bus

Many 80486 computers used the EISA bus or the VL-Bus as a higher-speed alternative to the ISA bus. SCSI adapters were common on such busses, and, if you have such a computer and a matching SCSI host adapter, there's a good chance that you can use the host adapter in Linux.

DMA boards were the most popular type of EISA and VL-Bus SCSI host adapters, but a few PIO models were produced. Like ISA cards, few EISA or VL-Bus boards implement anything more sophisticated than Fast SCSI-2. These motherboard busses are capable of higher speeds, and a few models did include support for Wide devices.

Both EISA Bus and VL-Bus SCSI host adapters are very difficult to locate in the marketplace today, because of the demise of the bus type. If you must purchase such a board, your best bet is to look in used computer shops or on auction Web sites such as eBay (http://www.ebay.com). You might want to consider upgrading the entire computer, or at least the motherboard and components, to a more modern design.

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