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

From the perspective of the PC as a whole, SCSI host adapters are expansion devices, since they plug into a system bus and represent a peripheral device on the system bus. (Some motherboards have integrated SCSI host adapter chips, but these are logically similar to separate host adapters even if no distinct physical card is used.) Host adapters typically require several different system resources, depending on the system bus that the host adapter is designed for, and the method it is using for transferring data over the system bus.

The following resource types are typically used on various adapters:

• Interrupt Request Line (IRQ): All SCSI host adapters use an interrupt request line or IRQ. The most commonly used ones are 9, 10, 11 or 12. IRQs 14 or 15 can usually be used as well if one or the other of the IDE/ATA channels in the system are not being used (see here for details on IDE/ATA resource usage). It should be noted that PCI-based host adapters will not require an explicit IRQ assignment. Rather, they will use one the system IRQ mapped to whatever PCI slot they are placed into. PCI also supports IRQ sharing. See here for more.
• DMA Channel: Many older host adapters based on the ISA or VLB buses use DMA channels to permit the transfer of data directly from SCSI devices to system memory. Usually DMA channels 1, 3 or 5 are used. PCI-based host adapters typically make use of PCI bus mastering to improve performance, which is a separate type of DMA that does not use regular ISA system DMA channels.
• I/O Address: The I/O address is used as the place through which data is transferred to the system. There are several ranges that are used by some SCSI cards.
• BIOS ROM Memory: The SCSI BIOS that contains the commands for controlling the host adapter typically takes up a 4000h address location in the upper memory area. This is usually one of the five 4000h spaces in the address range of CC000h to DDFFFh.

Most newer SCSI host adapters, especially those using the PC bus, support the Plug and Play initiative. Plug and Play allows the system to configure resources for the host adapter automatically in many cases, reducing configuration difficulties. (Note that I am speaking here of Plug and Play at the system level, which deals with system resources. This is different than so-called Plug and Play SCSI, which is similar in concept but is applied to dynamically allocating SCSI device IDs on the SCSI bus, not PC system resources.)

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