SCSI Chapter 13

SCSI Small Computer System Interface introduced in 1979 as a means of mass storage Common SCSI devices Hard drives- Scanners Tape backup units- CD-ROM drives Removable hard drives- Printers

SCSI SCSI manifests itself through a SCSI chain, which is a series of SCSI devices working together through a host adapter Host adapter attaches to the PC picture on page 866

SCSI Chains All SCSI devices can de divided into 2 groups: external and internal External devices stand alone and hooked to the external connector of the host adapter Internal devices attach to the host adapter internally and the devices sit in the PC SCSI has 50-pin, 68-pin, or 80-pin cables

SCSI Chains If the SCI cables are plugged in incorrectly, they can damage the devices External devices all have 2 connections in the back, which enable you to daisy-chain SCSI can have a maximum of 8 (the host plus 7 others) or 16 (host + 15) depending on the SCSI chain

SCSI IDs The values of the ID numbers range from 0 to 7, or 0 to 15 No 2 devices can share the same ID number Host adapter is usually set to 7 Doesn’t matter what ID # other devices are set to Try to use ID #0 for bootable hard drive You can set SCSI IDs by setting jumpers

SCSI chains Each end of the chain must be terminated examples of termination on page 877 some devices self-terminate, but others you have to do yourself usually set with jumpers; sometimes switches can damage devices if terminating improperly

SCSI Flavors Types of SCSI: SCSI - 1: 8-bit SCSI - 2: 8-bit SCSI - 2: 16-bit (wide) SCSI - 2: 32-bit (wide) SCSI - 3: numerous Refer to chart on page 884 for types

Bus mastering Why is bus mastering an important part of SCSI? devices that are on a SCSI chain can communicate with each other with very little use of the CPU, which frees up the system resources

SCSI Cable and connectors SCSI-1 & SCSI-2 (8-bit) used Type A: 50 pins SCSI-2 (16-bit) used Type B: 68 pins Some of the higher end SCSI-3 cables use 80 pins

ASPI Advanced SCSI programmer interface mandates a standard way to write BIOS device drivers for SCSI devices you can install a standardized set of device drivers for all your SCSI devices

Compatibility with IDE IDE and SCSI can both exist in one machine IDE will get priority for the boot drive; you should use CMOS setup which allows SCSI to take priority Not a good idea to mix them

SCSI SCSI adapters need an IRQ and usually take 3, so be careful with conflicts with COM ports. Usually only happens with legacy PCs Read the cost and benefits section on 893

SCSI vs. IDE SCSI used to be much bigger, now the advantage is disappearing SCSI is much faster, uses bus mastering, and can support up to 15 devices now but also more expensive