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Though ESDI and ST506/412 drives share similar looking cables, even to the point of having a twist, the actual data and control signals are very different. One should never mix components from these two drive types. While the ST506/412 interface utilizes a standard pulse code to transmit data between the drive and controller, ESDI uses a pulse code that does not require the level to return to zero between pulses.

fig. tt7

This format is referred to as NRZ, or Non Return to Zero. By utilizing NRZ, the clock that data is transferred by can be increased, thereby increasing the throughput to and from the ESDI disk. Figure TT7 illustrates an ESDI controller cable connector & pinouts (at the drive), while figure TT8 illustrates the ESDI data cable connector & pinouts (at the drive).

fig. tt8


Times were, you had a simple choice for type of disk drive... Any kind, as long as it was ST506/412. Those were the heydays of MFM drives. But many manufacturers weren't content with the 17 sectors/track that MFM provided. They devised a newer encoding scheme to pack data tighter, and called it RLL, or Run Length Limited, as opposed to MFM, or Modified Frequency Modulation. It involves using groups of 16 bits rather than each individual bit, thus achieving a sort of "compression" of the information as it is encoded. Since the same information takes up less space as RLL encoded data, more info can be written to the disk. The most common RLL technique, known as 2,7 RLL, can pack roughly 50% more data on a disk than MFM. Of course, there is always a trade-off... in the timing and media required for RLL. RLL requires a higher grade of media because of it's dense bit-packing, and timing is more critical, since the data is flowing at 50% higher rate than an MFM drive. Also, the mechanics of the drive must have tighter tolerances because head positioning becomes more critical. These requirements kept RLL drives at a premium. It has only been since 1991 that RLL drives have outsold MFM, and have all but driven them from the marketplace. This turnabout has come from the need to increase disk capacity more and more. Both ESDI, and SCSI type drives utilize RLL encoding to achieve high capacity and transfer rates (from the disk). And the newest interface, IDE, or Integrated Drive Electronics, is also based on this technology.


IDE, or Integrated Drive Electronics is the most recent drive interface to gain popularity. Often, the control circuitry is built into the motherboard, eliminating the requirement for a separate Host Adapter. There are 2 types of IDE interfaces...those for the 8-bit XT bus, and those for the 16-bit AT bus (detailed here). The cable for IDE contains 40 conductors and has no twists. Figure TT9 illustrates a typical IDE cable. IDE cables use a Dual-row Pin connector for both ends. Figure TT10 illustrates an IDE connector & pinouts (at the drive). A single cable may be used to connect two drives, or two cables may be Daisy-Chained. Most IDE Host Adapters will support two hard drives. The first drive should be jumpered as the Master drive, and the second as the Slave drive. Plug-in IDE Host Adapters are often called Paddle-Boards, and may contain a floppy controller, and serial and parallel ports.

fig. tt9

fig. tt10

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