Upgrading and Repairing PCs

Plug and Play SCSI

Plug and Play (PnP) SCSI originally was released in April 1994. This specification enables SCSI device manufacturers to build PnP peripherals that are automatically configured when used with a PnP operating system. This enables you to easily connect or reconfigure external peripherals, such as hard disk drives, backup tapes, and CD-ROMs.

To connect SCSI peripherals to the host PC, the specification requires a PnP SCSI host adapter, such as PnP ISA or PCI. PnP add-in cards enable a PnP operating system to automatically configure software device drivers and system resources for the host bus interface.

The PnP SCSI specification version 1.0 includes these technical highlights:

• A single cable connector configuration

• Automatic termination of the SCSI bus

• SCAM (SCSI Configured AutoMagically*) automatic ID assignment

• Full backward compatibility of PnP SCSI devices with the installed base of SCSI systems

Note "AutoMagically" is not a misspelling. The word is actually used in the official name for the specification, which the X3T9.2 committee designated X3T9.2/93-109r5.

This helps make SCSI easier to use for the average user.

Each SCSI peripheral you add to your SCSI bus (other than hard disk drives) requires an external driver to make the device work. Hard disks are the exception; driver support for them typically is provided as part of the SCSI host adapter BIOS. These external drivers are specific not only to a particular device, but also to the host adapter.

Recently, two types of standard host adapter interface drivers have become popular, greatly reducing this problem. By having a standard host adapter driver to write to, peripheral makers can more quickly create new drivers that support their devices and then talk to the universal host adapter driver. This arrangement eliminates dependence on one particular type of host adapter. These primary or universal drivers link the host adapter and operating system.

The Advanced SCSI Programming Interface (ASPI) currently is the most popular universal driver, with most peripheral makers writing their drivers to talk to ASPI. The A in ASPI used to stand for Adaptec, the company that introduced it, but other SCSI device vendors have licensed the right to use ASPI with their products. DOS does not support ASPI directly, but it does when the ASPI driver is loaded. Windows 9x/Me, Windows NT/2000/XP, and OS/2 2.1 and later versions provide automatic ASPI support for several SCSI host adapters.

Future Domain (now merged into Adaptec) and NCR created another interface driver called the Common Access Method (CAM). CAM is an ANSI-approved protocol that enables a single driver to control several host adapters. CAM is widely used by Linux and Unix operating systems for SCSI device interfacing using open-source drivers.