Each SCSI device is required to have a unique device ID in
order to be addressed over the shared SCSI bus. This has usually involved setting jumpers
or switches on devices like hard disks, and either physical or software setup for host
adapters. To eliminate this need for manual configuration, a specification was developed
to automate the assignment of device IDs. This protocol allows the SCSI bus to dynamically
shift and reallocate IDs when a new device is added to the bus, and is called Plug and
Play SCSI or alternatively, SCSI Configured AutoMatically (SCAM). (Yuck,
what a contrived acronym.) Plug and Play SCSI also supports automatic termination of the
SCSI bus.
Note: This idea is
similar in concept to "regular" Plug and
Play in the overall PC system, but remember that Plug and Play SCSI is used to assign
SCSI device IDs, not system resources. In fact, many modern SCSI host adapters also
support system plug and play over the PCI bus to allow dynamic setting of system
resources. This is a different matter altogether, though these may be sold as "Plug
and Play SCSI controllers"...
While a great idea in theory, SCAM never lived up to its potential. The main issue with
the feature is that it doesn't always work reliably--it's not a simple matter to
automatically assign device IDs or figure out which devices should enable termination.
Many people would find that they needed to disable the feature and manually configure
drives to eliminate problems. There were also difficulties if devices supporting SCAM were
mixed with other devices that did not support the feature. Eventually, hardware makers
started recommending that people avoid the feature altogether and just configure devices
manually, reasoning that this would reduce the likelihood of problems. And let's be
honest--it doesn't take that long to set the IDs of a few devices, and you typically only
have to do it once anyway.
SCAM was eventually removed from the SCSI parallel interface standard in the SPI-3 revision.
Next: Physical Installation Issues
