|Main Page > Motley > MicroHouse PC Hardware Library Volume I: Hard Drives|
|Previous||Table of Contents||Next|
While ATAPI CD-ROMs use the hard disk interface, this does not mean that they look like an ordinary hard disk; to the contrary, from a software point of view they are a completely different kind of animal. They actually most closely resemble a SCSI device.
Intelligent caching controllers that are not ATAPI-aware will not work with these devices. This means that, at present, you cannot boot from an ATAPI CD-ROM and you still must load a driver to use it under DOS or Windows. Windows 95/NT has native ATAPI support, and the first ATAPI-aware BIOS that will even allow booting from an ATAPI CD-ROM are now available.
IDE Drive Configuration
IDE drives can be both simple and troublesome to configure. Single-drive installations usually are very simple, with few if any special jumper settings to worry about. Multiple-drive configurations, however, can be a problem. Jumpers have to be set on both drives; the names, locations, and even functions of these jumpers can vary from drive to drive.
Because the CAM ATA IDE specification was ironed out only after many companies were already making and selling drives, many older IDE drives have problems in dual-drive installations, especially when the drives are from different manufacturers. In some cases, two particular drives may not function together at all. Fortunately, most of the newer drives follow the CAM ATA specification, which clears up this problem. Drives that follow the specification have no problems in dual-drive installations.
The cable connection to IDE drives usually is very simple. There is a single 40-pin cable that normally has three pin-header style connectors on it. One of the connectors plugs into the IDE interface connector; the other two plug into the primary and secondary drives. The cable normally runs from the IDE connector to both drives in a daisy-chain arrangement. On one end, this cable plugs into the IDE interface connector, which is located on the motherboard in many systems but also can be located on an IDE interface adapter card. The cable then connects to the secondary (D) and primary (C) drives in succession, with the primary drive usually (but not always) being at the end of the cable opposite the IDE interface connector.
There are no terminating resistors to set with IDE drives; instead, a distributed termination circuit is built into all IDE drives. The last drive on the cable need not be the primary drive, so you actually may find the primary or secondary drive at either connector. Jumpers on the drives themselves normally control whether a drive responds as primary or secondary.
You may see a different arrangement of cable connections in some IDE installations. In some installations, the middle connector is plugged into the motherboard, and the primary and secondary drives are at opposite ends of the cable in a Y arrangement. If you see this arrangement, be careful; in some of these Y-cable installations, the cable, rather than jumpers on the drives, actually controls which drive is primary and which is secondary.
Controlling master/slave selection via the cable rather than jumpers on the drive is performed via a special signal on the IDE interface called CSEL, which is on pin 28 of the interface. If the CSEL line is connected through from the drive to the IDE interface connector, the drive automatically is designated as primary. If the CSEL line is open between a drive and the IDE interface connector, that drive automatically is designated as secondary.
In the Y-cable approach, the IDE interface connector is in the middle of the cable, and a separate length of cable goes to each drive. Study this type of cable closely. If one of the ends of the Y has line 28 open (usually a hole in the cable through that wire), only the secondary drive can be plugged into that connector. HP Vectra PC systems use exactly this type of IDE cable arrangement. This type of setup eliminates the need to set jumpers on the IDE drives to configure them for primary or secondary operation, but the setup can be troublesome if you do not know about it.
IDE Drive Jumper Settings
Configuring IDE drives can be simple, as is the case with most single-drive installations, or troublesome, especially where it comes to mixing two drives from different manufacturers on a single cable.
Most IDE drives come in three configurations:
Because each IDE drive has its own controller, you must specifically tell one drive to be the master and the other to be the slave. There’s no functional difference between the two, except that the drive that’s specified as the slave will assert the DASP signal after a system reset that informs the master that a slave drive is present in the system. The master drive then pays attention to the Drive Select line, which it otherwise ignores. Telling a drive that it’s the slave also usually causes it to delay its spinup for several seconds to allow the master to get going and thus to lessen the load on the system’s power supply.
Until the ATA IDE specification, no common implementation for drive configuration was in use. Some drive companies even used different master/slave methods for different models of drives. Because of these incompatibilities, some drives work together only in a specific master/slave or slave/master order. This situation affects mostly older IDE drives that were introduced before the ATA specification.
Most drives that fully follow the ATA specification now need only one jumper (Master/Slave) for configuration. A few also need a Slave Present jumper as well. Table 2.2 shows the jumper settings required by most ATA IDE drives.
|Table 2.2 Jumper Settings for Most ATA IDE-Compatible Drives.|
|Jumper Name||Single-Drive||Dual-Drive Master||Dual-Drive Slave|
|Slave Present (SP)||Off||On||Off|
The Master jumper indicates that the drive is a master or a slave. Some drives also require a Slave Present jumper, which is used only in a dual-drive setup and then installed only on the master drive, which is somewhat confusing. This jumper tells the master that a slave drive is attached. With many ATA IDE drives, the Master jumper is optional and may be left off. Installing this jumper doesn’t hurt in these cases and may eliminate confusion, so I recommend that you install the jumpers listed here.
|Previous||Table of Contents||Next|