IDE Master/Slave -- IDE (Integrated Drive Electronics) devices are used on the IDE bus on AT computers to control devices such as hard drives and CDROMs. Almost all "IBM PCs" except the original PC and PC XT are AT devices. A minority have the IDE controllers turned off and use SCSI to control storage devices.

Two IDE devices are allowed per IDE controller. One is conventionally designated as the "Master" and the other as the "Slave". When only one device is present, it should be (often "must" be) set up as the Master. Unlike many "Master/Slave" setups in other contexts, the Master does not control the "Slave". It does relay signals from the slave during initialization. The rationale is to make the two components look like two MFM drives when two drives are present. "Primary/Secondary" might have been a better choice of words.

Master/Slave status is conventionally set by three or more jumpers on the disk drive. These are no standards for the number and arrangement of jumpers. The configuration must be worked out from the drive documentation. Since 1995, some vendors have provided jumper setting information on the drive label. Check the printing on the drive circuit board and lettering stamped into the case next to the jumpers as well. There are often additional jumpers present thereby making setting master/slave by experimentation difficult. It is also possible to select the Master/Slave status of some drives via "Cable Select". Jumpering the drives for Cable Select then cutting the wire to pin 28 of the IDE connector between the two drive connectors will make the drive connected to the inner connector "Master" and the one on the outer connector "Slave" if the Cable Select signal is provided by the PC. (The "Master" Select status is line grounded and the "Slave" Select is line open)

Not all drives will work as Master or Slave with other drives. Some drives will only work as Master. Others will only work as Slave. This is especially true of older drives. One Seagate drive will only function as Master or Slave with another drive of the same model. Trying to put jumper selected drives into a machine set up for cable select (most aren't) is likely to further confuse things. Some performance determining parameters such as I/O mode are likely to be controller rather than drive specific. If so, the controller will have to be configured to support the slowest device attached to it.

By design, IDE is not fundamentally incompatible with the older MFM/RLL drives, but the controller cards often try to use the same resources in different ways. It is possible, but often not easy, to mix IDE and MFR/RLL drives in the same machine.

Updated 10/28/02

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