Editing Parallel ATA (section)

=== Cable select ===
A drive mode called ''cable select'' was described as optional in ATA-1 and has come into fairly widespread use with ATA-5 and later. A drive set to "cable select" automatically configures itself as ''Device 0'' or ''Device 1'', according to its position on the cable. Cable select is controlled by pin 28. The host adapter grounds this pin; if a device sees that the pin is grounded, it becomes the ''Device 0'' (master) device; if it sees that pin 28 is open, the device becomes the ''Device 1'' (slave) device.

This setting is usually chosen by a [[Jumper (computing)|jumper setting]] on the drive called "cable select", usually marked ''CS'', which is separate from the ''Device 0/1'' setting.

If two drives are configured as ''Device 0'' and ''Device 1'' manually, this configuration does not need to correspond to their position on the cable. Pin 28 is only used to let the drives know their position on the cable; it is not used by the host when communicating with the drives. In other words, the manual master/slave setting using jumpers on the drives takes precedence and allows them to be freely placed on either connector of the ribbon cable.

With the 40-conductor cable, it was very common to implement cable select by simply cutting the pin 28 wire between the two device connectors; putting the slave ''Device 1'' device at the end of the cable, and the master ''Device 0'' on the middle connector. This arrangement eventually was standardized in later versions. However, it had one drawback: if there is just one master device on a 2-drive cable, using the middle connector, this results in an unused stub of cable, which is undesirable for physical convenience and electrical reasons. The stub causes [[signal reflections]], particularly at higher transfer rates.

Starting with the 80-conductor cable defined for use in ATAPI5/UDMA4, the master ''Device 0'' device goes at the far-from-the-host end of the {{convert|18|in|mm|adj=on}} cable on the black connector, the slave ''Device 1'' goes on the grey middle connector, and the blue connector goes to the host (e.g. motherboard IDE connector, or IDE card). So, if there is only one (''Device 0'') device on a two-drive cable, using the black connector, there is no cable stub to cause reflections (the unused connector is now in the middle of the ribbon). Also, cable select is now implemented in the grey middle device connector, usually simply by omitting the pin 28 contact from the connector body.