Conventional IDE (or parallel ATA) controllers support two devices per controller; these are designated Master and Slave, and need to be configured as such with jumpers on the devices.

In later years, another option was available: the use of Cable Select. In this arrangement the drive was not jumpered as Master or Slave, but a third option that would allow the drive to select either option depending on its position on the IDE cable. I believe this was done by only connecting pin 28 to one of the two device connectors on a compatible IDE connector, the presence or absence of which would be detected by the device.

I can picture situations where the ability to have the drive configure itself based on the cable location would be useful, such as a machine with a number of removable drive bays. I never had occasion to use it myself (or at least never got it working), so continued to use the Master / Slave options until the advent of SATA devices.

My question is as follows:

When did IDE devices (and controllers/cables) supporting Cable Select become available?


Cable Select (CS) appeared as an optional component of ANSI standard X3.221-1994 in 1994, the first published standard describing “IDE” (and later known as ATA-1). This also seems to correspond to the availability of drives implementing the standard; for example, Conner’s CFS-210A, introduced in October 1993, doesn’t support it, but their drives introduced in 1994, such as the CFS-350A, do. I first came across CS in Compaq systems around that time.

As you describe, CS was implemented using pin 28. That line on the bus changed purpose before the standard was published: initially, it was the drive address latch enable line, then it became the spindle sync (SPSYNC) line (used to synchronised two drives’ rotation, e.g. for mirroring setups), and finally cable select. Technically, at least in early drafts of ATA-1, line 28 is dual-purpose, and drives can implement SPSYNC and/or CS. See this page for details.

CS has also changed over time. With 40-wire cables, drive 0 is the drive closest to the controller, and the cable is left dangling in single-drive systems. This causes issues with signal reflection, so with 80-wire cables, drive 0 is at the end of the cable.

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  • 2
    +1 for the change in CS wiring between 40 and 80-wire cables! – Kaz Sep 7 '19 at 14:42
  • The ATA (80 wire) connectors come in three varieties; blue (for the controller) at one cable end, grey (for a slave) in the middle, and black (for the master) at the other end. The jumper-wire connections are made in the connector innards. – Whit3rd Sep 10 '19 at 19:57

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