IDE: Master and Slave; was: Re: Pet Peeves

["David Neeley" <dbneeley -at- gmail -dot- com>]

Mike Starr wrote:

"With IDE disk drives, each connector on the motherboard can communicate
with
two IDE drives. All IDE drives (Integrated Drive Electronics) contain a
drive controller. In order to use two drives attached to a single connector
with a ribbon bus cable, only one drive controller can be used. "

That is incorrect. Each IDE has the controller integrated, true enough.
However, when one of the drives has a slave jumper, pin 29 of the IDE cable
carries a signal called "Drive Active Slave Present" (DASP). When this
signal is present, the controller on the master drive is queried by the
controller on the slave when the slave wishes to send data to the peripheral
bus. If the master is itself not sending data, it returns a clear to send
signal and the slave executes the transfer. That way, the data lines of the
IDE interface do not have contention.

Note that a drive controller fulfills several functions. It must know the
geometry of the device--where and how information is stored. Before the IDE
spec was designed, drive controllers had to know how *all* drive geometries
worked in one way or another--and not just hard drives, but other devices
such as CDs and the like. The controller must also know, of course, how to
communicate with the peripheral bus. By building specialized controllers for
each IDE device, they were made simpler and higher performing without having
to deal with future devices that might come along.

This also illustrates the aptness of the "master" and "slave" nomenclature.
While "primary" and "secondary" could easily enough have been used, the fact
remains that they were not. It is also useful to use very short names due to
the physical limitations of space for labeling on the drive labels and such,
I suppose.

If you want a very good explanation of all this, see:
http://electronics.howstuffworks.com/ide4.htm

David
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