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09 Nov 1995

IDE or SCSI Disk

Summary: Modern computers come with EIDE (enhanced IDE) built into the mainboard. This is perfectly adequate for personal workstations. A high performance SCSI controller can be added to a new system for an extra $220. IDE and SCSI disks operate at the same speed, but SCSI has advantages for a multitasking server because it allows many devices to be performing operations at the same time.

The hard disk has one or more metal platters coated top and bottom with a magnetic material similar to the coating on a VCR magnetic tape. In the VCR the tape moves by a fixed recording and sensing device (the "head"). With a disk, the head is connected to an "arm" which is moved in and out along a radius of the disk circle. To read or write information, the computer or disk controller must figure out where the data is on the disk. The arm is then moved the correct distance, then it waits until the location on the disk where the data is located rotates around to the point where it passes under the head and can be transferred. The surface of the disk is preformatted into units that hold 512 byte of data (the "sectors").

In the first generation of PC's, the electronics to move the arm, position, and to control the recording or sensing was placed in a separate controller card. Advances in chip technology allow this function to be done by logic on the disk which can be more easily tuned at the factory to the special features of each type of device. Today there are two technologies, IDE and SCSI.

IDE (Standard on Desktop PCs)

IDE (Integrated Disk Electronics) is the least expensive current disk technology. IDE support is usually built into the mainboard, though it is also possible to get an interface card for the ISA bus for around $30. An IDE disk is connected to the mainboard or interface card through a flat "ribbon" cable. Rather than invent a new interface, the signals in the IDE cable simply duplicate the activity on the ISA bus itself.

After Nov, 1994 vendors started to ship systems with Enhanced IDE (EIDE). Classic IDE supported two hard disks of 528 megabyte or less. EIDE allows four devices, including a mixture of disks, tapes, and CD-ROM, and the hard disks can be larger.

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:

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.

If you run plain old DOS and Windows 3.x, it doesn't matter. Those systems will wait for any operation to complete before running any other program. However, if you are running a new system, and if you purchase a second IDE hard disk, then there is a performance advantage to putting the second drive on the second interface cable (managed by the second simulated IDE "device") rather than connecting it to the same flat disk interface to which the first disk is connected. On separate cables, the two disks can be active at the same time.

However, if you have two hard disks and an EIDE CD-ROM, then it is best to put the two disks on the same cable and isolate the CD-ROM on the second cable. A CD-ROM is much slower than a hard disk, and it will be busy longer. If it is on the same cable with a hard disk, it will block access to that disk when any request is made. Unless it is used very infrequently, the best performance will probably be provided by isolating the slow CD-ROM on its own cable.

SCSI (For Servers and Power Users)

SCSI provides a standard interface for all types of computers. The IDE disk and the ISA bus are peculiar to IBM-compatible Intel-compatible PC machines. SCSI, however, is used by Macintosh computers, RISC workstations, minicomputers, and even some mainframes. SCSI has always supported a mixture of disks, tapes, and CD-ROM drives. While EIDE disks may go up to one gigabyte, SCSI disks are available with 4 to 9 gigabytes of storage.

SCSI is a bus. In the SCSI architecture, the PC (or more precisely, the SCSI adapter card in the PC) is just one device on the bus. Each device is a "peer" of the other devices. In theory, a tape drive could send commands to the PC. In practice, the tape drive isn't smart enough and the PC doesn't respond to commands anyway.

In the Classic SCSI bus, there are 25 signals, each represented by a pair of wires (50 wires all together). Nine of the wires hold the eight bits plus parity of a byte of data. The other wires carry control functions. Classic SCSI can transfer data up to 5 megabytes per second. The Fast SCSI option of the SCSI-2 standard allows 10 megabytes per second on the same cable. To run faster, a Fast Wide SCSI interface is defined, but it requires more than the usual 50 wire cable.

An IDE disk must be mounted inside the computer. There is no provision for the IDE ribbon cable to run to external devices. SCSI devices can also be internal. They are connected to each other and to the adapter card using a flat ribbon cable with 50 wires (OK) or a round bundled cable with 25 twisted pairs of wires (Better).

However, SCSI devices can also be external to the computer. They can be mounted in individual boxes, or can be mounted together in larger tower enclosures. The adapter card is connected to external SCSI devices with a round cable containing 25 twisted pairs of wires. Four external SCSI plugs are in common use:


EIDE comes standard with any modern computer. The interface is built into the mainboard and requires no slots. SCSI requires an adapter card that may cost an additional $200.

IDE disks are also cheaper. A one gig EIDE disk is advertised for $220, while smaller SCSI disks start at $100 more. In most cases, the EIDE and SCSI disks are physically the same, operate at the same speed, and differ only in their electronics.

SCSI may be able to transfer data somewhat faster on the cable, but with a disk the limiting factor is the speed at which it rotates, not the electronic transfer speed.

EIDE is not a choice for devices that don't start out as PC's. Mac systems, RISC workstations, and minicomputers use SCSI. SCSI also provides for the external connection of devices ("Zip" removable high capacity drives, writable CD-ROMs units, backup tape units).

SCSI is worth the extra cost in a Server. EIDE supports two separate I/O operations to two disks (on the two different interface cables). SCSI allows all of the disk devices to be active simultaneously. Of course, only one device can be transferring data on the SCSI cable at any given time. However, a disk spends most of its time moving the arm to the right location and waiting for the data to rotate around to the point where it can be read or written. A SCSI controller can have all of its disks moving into position while one disk is actively transferring data.

It goes without saying that a good disk interface on a modern server will connect to the PCI bus. The ISA bus is unreasonably slow (by modern terms), the Microchannel is expensive, and EISA is now obsolete. However, there are both IDE and SCSI adapters that interface to the PCI bus.

An EIDE adapter will always be dumb and cheap. A SCSI adapter can be smart enough to Busmaster. As a Busmaster, the SCSI card can transfer data to or from buffers in memory directly. This frees up the CPU to do other things. To get the full benefit, the computer must be running an operating system (Windows NT, OS/2, Netware, or Unix) that can take advantage of the full capability of the card.

Currently, the most popular high performance PCI bus SCSI controller card is made by Adaptec. It goes by a variety of names (Adaptec 2940, AIC7870). This card is on the leading edge of technology, and there are updated drivers for it for Windows 95 (ftp them from Microsoft on the Internet), Windows NT (in Service Pak 2 for Version 3.51), OS/2, and Linux. Those who prefer a more sedate life should probably wait a few months for things to stabilize.


Copyright 1995 PCLT -- Introduction to PC Hardware -- H. Gilbert

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