An IDE interface cable has two plugs and can be attached to two devices. The first device acts as the master, and the second device acts as a slave. This interface is busy if either device is processing a request, so activity on one device blocks access to the other. It will generally be necessary when adding a new disk to a system to set a switch or connector on the disk to indicate if it is to function as master or slave.
When they designed the EIDE standard, they needed compatibility with all the existing IDE devices. So they didn't change the rules on the cable. An EIDE interface chip can support four devices, but it has two interface cables each connecting two devices. The EIDE chip looks and acts like two IDE chips. An old IDE disk can be connected to a new EIDE connector.
However, a new large EIDE disk cannot always be connected to an old PC. The original IBM programming interface limited the disk space to 528 megabytes (not a big problem when hard disks had 10 or 20 megs). Today there are 1 gig disks advertised for little more than $200. However, an old IDE disk interface chip may not support data beyond the first 528 megs. You can buy a new interface card for $40, but even then the BIOS on old systems will not support I/O to partitions that extend beyond 528 megs. You may need to load a new operating system (Windows 95, OS/2, or Windows NT) and the partitions containing the operating system files may have to reside completely within the first 528 megs of the disk.
Computers built in the last year should come with Extended IDE (EIDE). The extensions overcome limits in the original IDE design:
IDE supports only disks. EIDE supports a mixture of disks, tapes, and CDROM drives.
IDE supports only two devices. EIDE supports up to four devices on the same controller chip although it uses two cables.
EIDE allows disks up to 1 gigabyte. Larger disks may also work, but that is up to the vendor. IBM, for example, doesn't officially support EIDE disks larger than one gig.
Since EIDE simulated two separate IDE interface chips, there is an optimization that many customers do not fully appreciate. Newer operating systems (OS/2, Windows NT, and even Windows 95 to some extent) permit more than one I/O request to be running at a time. When a program wants to read something from a disk, the request is given to the disk interface and another program is allowed to run while the first program waits for data. However, the IDE interface allows only one of the two disks connected to the same cable to be active at a time, and any request to use the second disk will be blocked while data is being read from the first disk. An EIDE interface duplicates this IDE restriction, but since the EIDE chip looks like two IDE devices, a request can be made through the second interface while the first interface is busy.