In the case of floppy drives, the drive ID is fully decoded and the cable provides for one of four drive ID's being selected. Cable select is implemented by flipping the ID 0 and ID 1 drive select wires as well as associated ground wires between the two drive connectors. Drives are shipped selected to drive ID 0. The drive on the Inner connector will be 0 (A:), that on the outer connector will be 1(B:) One affect of this is that attempting to hook up a 3.5 inch drive floppy cable backwards with the twist adjacent to the controller will cause both drives to be selected as B: and will require that manual selection be used on the drives. That won't be an issue with 5.25 inch drives as the connector used on 5.25 inch drives has a different form factor than that at the controller.
Cable select is not commonly used on IDE hard drives. The original definition apparently put the master on the middle connector of a two drive system. That was undesirable because it required a single drive to be moved from the end connector to the middle connector when adding a new drive in order to keep it from changing from C: to D: if a second drive was added. UDMA cable select allows (requires sort of) that a single drive be on the end connector.
Most disk drives allow CS to be selected as an option along with Master and Slave. Modern high speed drives generally won't be connected as a single drive on the middle connector, because they won't work at full speed in that configuration. CS may become more popular in the future.
IN ATA-2, if CS is set, a cable select line (line 28) in the drive cable is checked to determine if the drive will be Master or Slave. Slave is selected if the CS line is Low, Master is selected if the line is high. The usual implementation is simply to cut the cable select line at or between the drive connectors. The middle connector is held low by the CS line which is tied to ground. The pin connected to the unconnected line segment beyond the cut will be pulled high by the disk electronics.
Drive select cables can not be hooked up backwards as both connectors will be beyond the break and both drives will try to be Masters. Some older controllers may not tie the CS line low. It is not always obvious whether cables are or are not cable select cables. "Universal" cables that ignore the controller line 28 setting and tie the line to ground in the cable are available.
Master/Slave settings via jumpers will always override cable select.
Return To Index Copyright 1994-2008 by Donald Kenney.