Each IDE/ATA channel can support either one or two
devices. IDE/ATA devices of course each contain their own integrated controllers, and so
in order to maintain order on the channel, it is necessary to have some way of
differentiating between the two devices. This is done by giving each device a designation
as either master or slave, and then having the controller address commands
and data to either one or the other. The drive that is the target of the command responds
to it, and the other one ignores the command, remaining silent.
Note that despite the hierarchical-sounding names of "master" and
"slave", the master drive does not have any special status compared to the slave
one; they are really equals in most respects. The slave drive doesn't rely on the master
drive for its operation or anything like that, despite the names (which are
poorly-chosen--in the standards the master is usually just
"drive 0" and the slave "drive 1"). The only practical difference
between master and slave is that the PC considers the master "first" and the
slave "second" in general terms. For example, DOS/Windows will assign drive letters to the master drive before the slave
drive. If you have a master and slave on the primary IDE channel and each has only one
regular, primary partition, the master will be "C:" and the slave
"D:". This means that the master drive (on the primary channel) is the one that
is booted, and not the slave.
Devices are designated as master or slave using jumpers, small connectors that fit over
pairs of pins to program the drive through hardware. Each manufacturer uses a different
combination of jumpers for specifying whether its drive is master or slave on the channel,
though they are all similar. Some manufacturers put this information right on the top
label of the drive itself, while many do not; it sometimes takes some hunting around to
find where the jumper pins are on the drive even once you know how the jumpers are
supposed to go. The manufacturers are better about this now than they have been in the
past, and jumpering information is always available in the manual of the hard disk, or by
checking the manufacturer's web site and searching for the model number. I describe (and illustrate) the jumpers on IDE/ATA disks in
detail in the section on hard disk construction.
ATAPI devices such as optical, Zip and tape drives are
jumpered in pretty much the same way as hard disks. They have the advantage of often
having their jumpers much more clearly labeled than their hard disk counterparts. Most
optical drives, for example, have three jumper blocks at the back, labeled "MA"
(master), "SL" (slave) or "CS" (cable select).
If you are using two drives on a channel, it is important to ensure that they are
jumpered correctly. Making both drives the master, or both the slave, will likely result
in a very confused system. Note that in terms of configuration, it makes no difference
which connector on the standard IDE cable is
used in a standard IDE setup, because it is the jumpers that control master and slave, not
the cable. This does not apply when cable select is being used,
however. Also, there can be electrical signaling issues if one connects a single drive to
only the middle connector on a cable, leaving the end connector unattached. In
particular, the use of Ultra DMA is not supported in such a
configuration; see the discussion of the 80-conductor Ultra DMA
cable for more information.
As long as one drive is jumpered as master and the other as slave, any two
IDE/ATA/ATAPI devices should work together on a single channel. Unfortunately, some
older hard disks will fail to work properly when they are placed on a channel with another
manufacturer's disk. One of the reasons why drives don't always "play nicely
together" has to do with the Drive Active / Signal Present (/DASP) signal. This is an IDE/ATA interface
signal carried on pin #39, which is used for two functions: indicating that a drive is
active (during operation), and also indicating that a slave drive is present on the
channel (at startup). Some early drives don't handle this signal properly, a residue of
poor adherence to ATA standards many years ago. If an older slave drive won't work with a
newer master, see if your master drive has an "SP" (slave present) jumper, and
if so, enable it. This may allow the slave drive to be detected.
Drive compatibility problems can be extremely frustrating, and beyond the suggestion
above, there usually is no solution, other than separating the drives onto different
channels. Sometimes brand X won't work as a slave when brand Y is the master, but X will
work as a master when Y is the slave! Modern drives adhere to the formal ATA standards and
so as time goes on and more of these older "problem" drives fall out of the
market, making all of this less and less of a concern. Any hard disk bought in the last
five years should work just fine with any other of the same vintage or newer.
When using only a single drive on a channel, there are some considerations to be aware
of. Some hard disks have only a jumper for master or slave; when the drive is being used
solo on a channel it should be set to master. Other manufacturers, notably Western
Digital, actually have three settings for their drives: master, slave, and single.
The last setting is intended for use when the drive is alone on the channel. This type of
disk should be set to single, and not master, when being used alone.
Also, a single device on an IDE channel "officially" should not be jumpered
as a slave. In practice, this will often work despite being formally "illegal".
Many ATAPI drives come jumpered by default as slave--because they are often made slaves to
a hard disk's master on the primary IDE channel, this saves setup time. However, for
performance reasons they are sometimes put on the secondary channel, and often the system
assemblers don't bother to change the jumpers. It will work, but I don't recommend it; if
nothing more, it's confusing to find a slave with no master when you or someone else goes
back into the box a year or two later to upgrade.
For performance reasons, it is better to avoid mixing slower and faster devices on the
same channel. If you are going to share a channel between a hard disk and an ATAPI device,
it is generally a good idea to make the hard disk the master. In some situations there can
be problems slaving a hard disk to an optical drive; it will usually work but it is
non-standard, and since there is no advantage to making the ATAPI device the master, the
configuration is best avoided.
There are many more performance considerations to take into account when deciding how
to jumper your IDE devices, if you are using several different ones on more than one
channel. Since only one of the master and slave can use any channel at a time, there are
sometimes advantages to using more than one IDE/ATA channel even if not strictly necessary
based on the number of devices you are trying to support. There can also be issues with
using a drive that has support for a fast transfer mode like Ultra
DMA with older devices that don't support these faster modes. See this section for a discussion of performance issues and
IDE/ATA configuration.
Next: Configuration Using Cable Select
