ATA vs SCSI

Advanced Technology Attachment



(ATA, AT Attachment or "Integrated Drive Electronics", IDE) A disk drive interface
standard based on the IBM PC ISA 16-bit bus but also used on other personal computers. The
ATA specification deals with the power and data signal interfaces between the motherboard
and the integrated disk controller and drive. The ATA "bus" only supports two devices -
master and slave.



ATA drives may in fact use any physical interface the manufacturer desires, so long as an
embedded translator is included with the proper ATA interface. ATA "controllers" are
actually direct connections to the ISA bus.



Originally called IDE, the ATA interface was invented by Compaq around 1986, and was
developed with the help of Western Digital, Imprimis, and then-upstart Conner Peripherals.
Efforts to standardise the interface started in 1988; the first draft appeared in March 1989,
and a finished version was sent to ANSI group X3T10 (who named it "Advanced Technology
Attachment" (ATA) for ratification in November 1990.



X3T10 later extended ATA to Advanced Technology Attachment Interface with Extensions

(ATA-2), followed by ATA-3, ATA-4, ATA-5 and ATA-6.

Small Computer System Interface



(SCSI) A processor-independent standard for system-level interfacing between a computer
and intelligent devices including hard disks, floppy disks, CD-ROM, printers, scanners, and
many more. SCSI-1 can connect up to seven devices to a single SCSI adaptor (or "host
adaptor") on the computer's bus.



SCSI transfers eight bits in parallel (it is an eight-bit bus, but see Wide SCSI) and can operate
in either asynchronous or synchronous modes. The synchronous transfer rate is up to 5MB/s.
There must be at least one target and one initiator on the SCSI bus.



SCSI connections normally use "single ended" drivers as opposed to differential drivers.
Single ended SCSI can support up to six metres of cable. Differential can support up to 25
metres of cable.



A problem with SCSI is the large number of different connectors allowed. Nowadays the
trend is towards a 68-pin miniature D-type or "high density" connector (HD68) for Wide
SCSI and a 50-pin version of the same connector (HD50) for 8-bit SCSI (Type 1-4, pin pitch
1.27 mm x 2.45 mm). 50-pin ribbon cable connectors are also popular for internal wiring
(Type 5, pin pitch 2.54 mm x 2.54 mm). Apple Computer used a 25-pin connector on the
Macintosh computer but this connector causes problems with high-speed equipment.




SCSI was developed by Shugart Associates, which later became Seagate. SCSI was
originally called SASI for "Shugart Associates System Interface" before it became a standard.
Original SCSI implementations were highly incompatible with each other.



The original standard is now called "SCSI-1" to distinguish it from SCSI-2 and SCSI-3,
which include specifications of Wide SCSI (a 16-bit bus) and Fast SCSI (10 MB/s transfer).

Interface Factor	IDE/ATA	SCSI
Cost	Low	Moderate to high
Performance	High for single devices or single tasking, moderate to low for multiple devices or multitasking	High in most situations
Configuration and Ease of Use	High for small number of devices, low for large number of devices	Moderate to high for both small and large numbers of devices
Moderate to high for both small and large numbers of devices	Moderate to low	High
Device Type Support	Moderate to low	High
Device Availability and Selection	High	Moderate
Software / Operating System Compatibility	High	Moderate to high
System Resource Usage	Moderate to poor	Moderate to good
Support for non-PC Platforms	Poor, but growing	Good