Details Of Hard Disk (John B. Emmerson III)
Mar 4, 2010
Hard disk drives are accessed over one of a few bus types, including similar ATA, Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fiber Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses that they cannot communicate with natively, such as IEEE 1394, USB and SCSI.
Back in the days of the ST-506 interface, the data encoding scheme was also vital . The first ST-506 disks applied Modified Frequency Modulation (MFM) encoding, and transferred data at a charge of 5 megabits per second. Later on, controllers using 2,7 RLL encoding amplified the transfer scale by 50%, to 7.5 megabits per second; this also amplified disk capacity by fifty percent.
Enhanced Small Disk Interface also supported multiple data rates, but this was usually negotiated automatically by the disk drive and controller; most of the time however, 15 or 20 megabit ESDI disk drives weren't downward compatible.ESDI disk drives typically also had jumpers to set the number of sectors per track and sector size.
Modern hard drives present a constant surface to the rest of the computer, no concern what data encoding scheme is applied internally. Typically a DSP in the electronics in the hard drive takes over the raw analog voltages from the read head and apply PRML and Reed-Solomon error correction to decode the sector boundaries and sector data, then sends that data out the standard structure.
SCSI originally had just one signaling frequency of 5 MHz for a maximum data charge of 5 megabytes/second over 8 parallel conductors, but later this was increased radically. The SCSI bus velocity had no bearing on the disk's internal speed because of buffering between the SCSI bus and the disk drives internal data bus although, many early disk drives had very small buffers, and thus had to be reformatted to a different interleave when used on slow computers, such as initial Commodore Amiga, IBM PC compatibles and Apple Mackintoshes.
About the AuthorThere are many sort of hard drives in the market such as SAS,HDD,SATA 30 and it is upgrading very fast. Only the technology itself is not upgrading, but the availablity is also increasing,like it is more easier to buy Ultra320-SCSI then the past times..