Hardware Reference
In-Depth Information
PATA Dual-Drive Configurations
Dual-drive PATA installations can be problematic because each drive has its own control-
ler, and both controllers must function while being connected to the same bus. There has
to be a way to ensure that only one of the two controllers responds to a command at a
time.
The ATA standard provides the option of operating on the AT bus with two drives in a
daisy-chained configuration. The primary drive (drive 0)is called the
master
,and the sec-
ondary drive (drive 1) is called the
slave
. You designate a drive as being master or slave
by setting a jumper or switch on the drive or by using a special line in the interface called
the
cable select (CS) pin
and setting the CS jumper on the drive.
When only one drive is installed, the controller responds to all commands from the sys-
tem. When two drives (and, therefore, two controllers) are installed, both controllers re-
ceive all commands from the system. Each controller then must be set up to respond only
to commands for itself. In this situation, one controller must be designated as the master
and the other as the slave. When the system sends a command for a specific drive, the
controller on the other drive must remain silent while the selected controller and drive are
functioning. Setting the jumper to master or slave enables discrimination between the two
controllers by setting a special bit (the DRV bit) in the drive/head register of a command
block.
Configuring ATA drives can be simple, as is the case with most single-drive installations.
Or it can be troublesome, especially when it comes to mixing two older drives from dif-
ferent manufacturers on a single cable.
You can configure most ATA drives with four possible settings:
• Master (single drive)
• Master (dual drive)
• Slave (dual drive)
• Cable select
Most drives simplify this to three settings: master, slave, and cable select. Because each
ATA drive has its own controller, you must specifically tell one drive to be the master and
the other to be the slave. No functional difference exists between the two, except that the
drivethat'sspecifiedastheslaveassertsasignalcalledDASPafterasystemresetinforms
the master that a slave drive is present in the system. The master drive then pays attention
to the drive select line, which it otherwise ignores. Telling a drive that it's the slave also
usually causes it to delay its spin-up for several seconds to allow the master to get going
and thus to lessen the load on the system's power supply.
