This chapter introduces SCSI and ISA hardware. Objectives important to this chapter are:
Small Computer System Interface (SCSI) drives are a special case. SCSI is actually a kind of bus, and devices that attach to a SCSI bus can be daisy-chained, providing you have the proper cables, and providing your devices have SCSI input and output ports. A device that only has one port on it can only go on the end of a chain. Since SCSI is a bus, devices that are attached to it must be addressed (numbered). If your SCSI bus will accommodate 8 devices, they are addressed from 0 to 7. If your SCSI bus will accommodate 16 devices, they are addressed from 0 to 15.
The actual SCSI card that plugs into your PC gets one of these addresses, and so does each device in the chain. Because the controller card must have one of the addresses, this means that you can only attach 7 or 15 other devices to the chain, instead of 8 or 16. The controller card is called a host adapter, or it can be part of the motherboard of your computer. The host adapter is usually assigned the highest address on the SCSI bus, 7 or 15.
There is an exception to this rule: some SCSI devices contain several logical devices. The text give the example of a CD-ROM changer with several trays. In this case, the device (the jukebox) is assigned a SCSI address, and each sub-device is assigned a logical unit number (LUN). Each unit in the device can be referred to with the form SCSI Address:LUN.
An advantage to SCSI devices is that, since they are on their own bus, they can pass data to each other with passing through the processor. Of course, this is only an advantage if you need these devices to talk directly to each other. You might consider the information flow shown on page 579 a disadvantage. When a CPU communicates with an IDE drive, the communication flows from the CPU to the system bus, to the hard drive controller, to the hard drive. When this communication flows to a SCSI drive, it flows from the CPU to the system bus, to the SCSI host adapter, across the SCSI bus, to the SCSI interface controller on the device, to the hard drive controller on the device, and finally to the hard drive.
Installing a SCSI host adapter involves assigning several system resources. It has its own BIOS, but needs a DMA channel, an IRQ, and I/O addresses. Host adapters come in several bus styles: ISA, 16-bit PCI, 32-bit PCI, and 64-bit PCI-X.
Some host adapters support bus mastering, which allows the SCSI devices to access memory and other devices on the system bus without accessing the CPU. If your host adapter uses bus mastering, you do not have to assign a DMA channel to it.
Most SCSI devices must have their SCSI bus address set manually. Some have dials to choose a setting, some use jumpers. Some others use the SCAM (SCSI Configuration Automatically) system to comply with Plug and Play standards. SCAM level 1 assigns an address to SCSI devices (but not the host adapter) on system startup. SCAM level 2 assigns an address to the host adapter as well.
It is also common to need a device driver to enable a SCSI system on a Windows computer.
SCSI has evolved several varieties:
The text discusses when you might choose SCSI or IDE. In general, a workstation may be better off with IDE devices. A server may need many drives, and may need in increase in performance across those drives, which a SCSI system may provide.
A general set of installation steps is offered. The technician performing the installation should note any specific instructions for a host adapter or SCSI device that deviate from these steps, and should follow the instructions provided with their specific hardware: