Machine Design

Sorting Out PC Interface Technology

Connections of fast peripherals generally are handled with SCSI (Small Computer Systems Interface) format interfaces, which promote fast datatransfer rates and burst-transfer rates. The latest versions of SCSI are SCSI-2 and double-speed UltraSCSI, both available for the PCI bus found in multimedia PCs and Macs.

SCSI interfaces handle up to 15 peripherals, depending on the type of SCSI, using cables that can be up to 20 ft long. The earliest SCSI standard, now termed SCSI-1, provides a 5- Mbps maximum data-transfer rate and is an eight-bit interface. SCSI-2, also called Fast SCSI, is also eight bits and works at 10-Mbps rates. It additionally specifies a means of transferring 16-bit data, dubbed Fast and Wide SCSI, providing a 20-Mbps rate.

One factor to note is that different SCSI formats use different cables and connectors. Fast and Wide 16-bit SCSI carries a 68-pin cable, but most eightbit drives sport a 50-pin Centronics connector. Thus connecting an eight-bit drive to a 16-bit adapter requires a cable with a 50-pin connector on one end, and a 68-pin connection on the other.

UltraSCSI Fast 20 uses a 20-Mbps transfer rate at eight bits, UltraSCSI Fast 40 uses 40 Mbps with 16-bit transfers. High-capacity hard drives and disk arrays can make use of the fast transfer rates to communicate efficiently over the PCI bus. This format is also backward compatible with existing bus standards. It employs a 68-pin cable like that of Fast and Wide SCSI and can accommodate cables up to 10 ft in length.

SCSI adapter cards can incorporate features such as on-board processors to optimize data flow. Built-in RAM may allow caching of data and instructions on the adapter itself, minimizing the load on the host PCI bus.

The other widely used PC interface format is EIDE (Enhanced Integrated Drive Electronics). EIDE interfaces can support data transfer rates of up to 16.7 Mbps when connected to a single drive. EIDE handles four peripherals at most and EIDE drives cannot be daisy chained. EIDE cables for external drives can extend about 1.5 ft.

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