Hardware Reference
In-Depth Information
present on the interface. After that, each drive asserts the signal to indicate that it is active. Early
drives could not multiplex these functions and required special jumper settings to work with other
drives. Standardizing this function to allow for compatible dual-drive installations is one of the
features of the ATA standard. This is why some drives require a slave present (SP) jumper, whereas
others do not.
Pin 28 carries the cable select signal (CSEL). In some older drives, it could also carry a spindle
synchronization signal (SPSYNC), but that is not commonly found on newer drives. The CSEL
function is the most widely used and is designed to control the designation of a drive as master (drive
0) or slave (drive 1) without requiring jumper settings on the drives. If a drive sees the CSEL as
being grounded, the drive is a master; if CSEL is open, the drive is a slave.
You can install special cabling to ground CSEL selectively. This installation usually is accomplished
through a cable that has pin 28 missing from the middle connector but present in the connectors on
each end. In that arrangement, with one end plugged into the motherboard and two drives set to cable
select, the drive plugged into the end connector is automatically configured as master, whereas the
drive attached to the middle connector is configured as slave. Note that although this is the most
common arrangement, it is also possible to make cables where the middle connector is master (and
the end is slave), or even to use a Y-cable arrangement, with the motherboard ATA bus connector in
the middle, and each drive at opposite ends of the cable. In this arrangement, one leg of the Y would
have the CSEL line connected through (master), and the other leg would have the CSEL line open
(conductor interrupted or removed), making the drive at that end the slave.
PATA Dual-Drive Configurations
Dual-drive PATA installations can be problematic because each drive has its own controller, 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 secondary 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 system. When two
drives (and, therefore, two controllers) are installed, both controllers receive 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 different manufacturers on
a single cable.
You can configure most ATA drives with four possible settings:
• Master (single drive)
