Dual Channel PCI SCSI Adapters (133492)

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Multiplexed dual-channel PCI SCSI adapters are two SCSI controllers on one card. The controllers often share a single interrupt, but each has its own I/O address and interface. Although typical SCSI adapters use a single RISC processor, a dual-channel SCSI adapter has two RISC processors (with each processor controlling its own SCSI bus).

There are several reasons for using dual-channel PCI SCSI adapters, but the main benefit is a dual-channel controller saves one slot over two single controllers. By using one controller that switches between channels, dual- channel adapters can support up to 14 devices while theoretically providing the same throughput as a single-channel SCSI-2 adapter (which can support just seven devices). On a dual-channel adapter, the two SCSI buses operate independently. Data is transferred across the PCI bus by one controller at a time, but there is no throughput loss because the PCI bus operates much faster than the two SCSI buses combined. This assumes an aggregate SCSI-2 transfer rate of 20 MB per second (2 channels x 10MB per channel), compared with the 132 MB per second burst-mode speed of the 33 mHz PCI bus.

Software-based disk striping, disk duplexing, and RAID support also work better with dual-channel SCSI adapters. Disk striping offers the best performance if there are multiple disk drives without disk duplexing or RAID enabled. Dual-channel SCSI adapters can assume two basic configurations during operation. The first distributes devices equally between the two controllers. The second one takes a hint from dual-channel EIDE (Enhanced IDE) controllers. Slower devices such as scanners, printers, CD-ROM drives and tape drives are placed on one channel, whereas faster devices such as hard disks and optical disks are placed on the other channel. Separating slow and fast devices works better when not using striping, duplexing, or RAID because each channel can run at the most efficient speed for the devices attached to it. Microsoft Windows 95 and Windows NT take better advantage of the dual-channel adapters because they can issue multiple disk requests simultaneously.