Since the transfer modes associated with the IDE/ATA interface
are constantly being improved, new devices support faster transfer modes than older ones
do. In addition, hard disks often support faster transfer modes than ATAPI devices such as
optical drives do. Yet, these devices can be combined on the same IDE/ATA channel, raising
the question of compatibility when the devices are together.
The ability of an IDE/ATA channel to operate a master
and slave device using different transfer modes is called independent device timing.
The hard disk controllers integrated on modern chipsets
all pretty much support independent timing, as do modern add-in controllers, but this was
not always the case. Independent timing can be an issue if, for example, you upgrade an
older PC and get a new, high-speed drive, but want to continue to be able to use the older
one on the same channel with the new one.
If your system does not support independent device timing, and you use a newer hard
disk that supports PIO mode 4 on the same channel as an older hard disk that operates only
at PIO mode 0, the system will knock down the PIO mode to 0 for both drives. This
will hamper the performance of the newer hard disk. The lack of independent device timing
on many older systems is one reason why placing ATAPI devices like CD-ROMs on the same
channel as a fast hard disk is usually not recommended. (It should be pointed out that we
are talking here about the interface or external transfer speed of these devices.
Reducing the speed of the interface only causes big issues if the effective interface
speed becomes less than the sustained
transfer rate of the disk. For a full discussion of these issues, please see this page.)
Again, today's chipsets all pretty much support independent timing, so this is less of
an issue than it once was. However, there are still other good reasons to be careful about
how you assign drives to the IDE/ATA channels in your system; see this page for a full discussion of these issues.
Note: It is not possible
to use PIO modes to control one device on a channel and DMA (or Ultra DMA) modes to control
the other one. Do not mix devices that don't support DMA with ones that do on the same
channel, if you want to use DMA.
Next: Windows Drivers and DMA Support
