Hardware Reference
In-Depth Information
theribbon,and
rounding
thecablecausessomeofthedatalinestorunparalleloradjacent
toeachotheratrandom,therebycausingcrosstalkandnoiseandresultinginsignalerrors.
Of course, many people use rounded cables with success, but my knowledge of electrical
engineering as well as the ATA standard has always made me somewhat uncomfortable
with their use.
PATA Signals
This section describes in more detail some of the most important PATA signals having
to do with drive configuration and installation. This information can help you understand
how the cable select feature works, for example.
Pin 20 is used as a key pin for cable orientation and is not connected to the interface. This
pin should be missing from any ATA connectors, and the cable should have the pin-20
hole in the connector plugged off to prevent the cable from being plugged in backward.
Pin39carriesthedriveactive/slavepresent(DASP)signal,whichisadual-purpose,time-
multiplexed signal. During power-on initialization, this signal indicates whether a slave
drive is 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 jump-
er settings to work with other drives. Standardizing this function to allow for compatible
dual-driveinstallationsisoneofthefeaturesoftheATAstandard.Thisiswhysomedrives
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.TheCSELfunctionisthemostwidelyusedandisdesignedtocontrolthedesigna-
tion 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 ac-
complishedthroughacablethathaspin28missingfromthemiddleconnectorbutpresent
intheconnectors oneachend.Inthatarrangement, withoneendpluggedintothemother-
board and two drives set to cable select, the drive plugged into the end connector is auto-
maticallyconfiguredasmaster,whereasthedriveattachedtothemiddleconnectoriscon-
figured as slave. Note that although this is the most common arrangement, it is also pos-
sible to make cables where the middle connector is master (and the end is slave), or even
touseaY-cablearrangement,withthemotherboardATAbusconnectorinthemiddle,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.
