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

["Mike Starr" <mikestarr-techwr-l -at- writestarr -dot- com>]

Thanks for correcting my "off-the-top-of-my-head" explanation.

Mike
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----- Original Message -----
From: "David Neeley" <dbneeley -at- gmail -dot- com>
To: <techwr-l -at- lists -dot- techwr-l -dot- com>
Sent: Saturday, March 17, 2007 10:00 AM
Subject: IDE: Master and Slave; was: Re: Pet Peeves

> 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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