Plug and Play SCSI

Plug and Play (PnP) SCSI was originally 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. 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 includes these technical highlights:

• A single cable connector configuration

• Automatic termination of the SCSI bus

• SCAM (SCSI Configured AutoMagically) automatic ID assignment (Note that this was removed in the SPI-3 revision of the SCSI standard. However, most current SCSI drives support an optional cable connection for 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 original specification, which the X3T9.2 committee designated X3T9.2/93-109r5.

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 is typically 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.

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) is currently 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.) All recent and current Windows versions, starting with Windows 9x and Windows NT, 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 UNIX operating systems for SCSI device interfacing using open-source drivers.

Major SCSI host adapter vendors feature drivers customized for various Linux distributions. You should see your host adapter vendor's website for details. Note that if your host adapter vendor does not support your host adapter with a driver, an open-source driver might be available.