Independent Master/Slave Device Timing

Independent Master/Slave Device Timing

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

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