Editing Parallel ATA (section)

=== Multiple devices on a cable ===
If two devices are attached to a single cable, one must be designated as ''Device 0'' (in the past, commonly designated ''master'') and the other as ''Device 1'' (in the past, commonly designated as ''slave'').<ref>{{Cite web |date=2020-05-12 |title=An Introduction to Parallel ATA (PATA) - Definition and History |url=https://www.minitool.com/lib/pata.html |access-date=2023-12-12 |website=MiniTool |language=en-US}}</ref> This distinction is necessary to allow both drives to share the cable without conflict. The ''Device 0'' drive is the drive that usually appears "first" to the computer's [[BIOS]] and/or [[operating system]]. In most personal computers the drives are often designated as "C:" for the ''Device 0'' and "D:" for the ''Device 1'' referring to one active primary partitions on each.

The mode that a device must use is often set by a [[Jumper (computing)|jumper setting]] on the device itself, which must be manually set to ''Device 0'' (''Master'') or ''Device 1'' (''Slave''). If there is a single device on a cable, it should be configured as ''Device 0''. However, some certain era drives have a special setting called ''Single'' for this configuration (Western Digital, in particular). Also, depending on the hardware and software available, a ''Single'' drive on a cable will often work reliably even though configured as the ''Device 1'' drive (most often seen where an optical drive is the only device on the secondary ATA interface).

The words ''primary'' and ''secondary'' typically refers to the two IDE cables, which can have two drives each (primary master, primary slave, secondary master, secondary slave).

There are many debates about how much a slow device can impact the performance of a faster device on the same cable. On early ATA host adapters, both devices' data transfers can be constrained to the speed of the slower device, if two devices of different speed capabilities are on the same cable. For all modern ATA host adapters, this is not true, as modern ATA host adapters support ''independent device timing''. This allows each device on the cable to transfer data at its own best speed. Even with earlier adapters without independent timing, this effect applies only to the data transfer phase of a read or write operation.<ref>{{cite web |url=http://www.pcguide.com/ref/hdd/if/ide/confTiming-c.html |title=Independent Master/Slave Device Timing |access-date=2008-08-08 |author=Charles M. Kozierok |date=2001-04-17 |work=The PC Guide}}</ref> This is caused by the omission of both overlapped and queued feature sets from most parallel ATA products. Only one device on a cable can perform a read or write operation at one time; therefore, a fast device on the same cable as a slow device under heavy use will find it has to wait for the slow device to complete its task first. However, most modern devices will report write operations as complete once the data is stored in their onboard cache memory, before the data is written to the (slow) magnetic storage. This allows commands to be sent to the other device on the cable, reducing the impact of the "one operation at a time" limit. The impact of this on a system's performance depends on the application. For example, when copying data from an optical drive to a hard drive (such as during software installation), this effect probably will not matter. Such jobs are necessarily limited by the speed of the optical drive no matter where it is. But if the hard drive in question is also expected to provide good throughput for other tasks at the same time, it probably should not be on the same cable as the optical drive.