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Introduction
Serial ATA (SATA) is the next generation bus used for computer internal storage interconnections, designed to replace
the ageing parallel ATA (PATA) technology (commonly referred to as 'IDE'). SATA is the evolution of the common ATA interface from a parallel to a serial
bus architecture. This improved architecture overcomes the serious electrical constraints that are increasing the difficulty
of continued speed improvements for the classic PATA bus. SATA, put simply, allows for faster transfers between the hard disk and the rest of the system.
I often get asked about different aspects of SATA, so the purpose of today's article is to explore this technology and how it works, understand the differences between SATA and its older cousin PATA, as well as why we actually need SATA in the first place. We'll also discuss the requirements for SATA, and the installation of SATA disks.
SATA - Definition
SATA stands for Serial Advanced Technology Attachment. SATA is a newer computer bus to connect IDE hard drives as well
as some DVD Burners to the motherboard of a machine. Currently there are two specifications for SATA - SATA I (150MB/s) and SATA II (300MB/s).
The previous mainstream specification for connecting IDE devices was called PATA (Parallel Advanced Technology Attachment).
PATA could only run up to speeds of 133 MB/s reliably, this was because at higher frequencies, problems such as 'clock skew'
and 'crosstalk' would occur.
The Differences between SATA and PATA
- As two PATA disks could be connected to one data cable, jumpers had to be set on each disk to designate
one drive as the master, and one as the slave to allow simultaneous communications from both disks to work
correctly. SATA does not share connections with other devices like PATA, so there is only one cable per drive.
This means there are no jumpers to set anymore!
- SATA cables are much thinner compared to PATA cables - this results in better system airflow, and it also makes the
cables easier to hide compared to PATA cables (making the system more tidy as a result).
PATA/SATA cable comparison: PATA on the left, SATA on the right.
- While PATA cables are limited to around 18 inches, SATA doubles that to a whole meter (39.37 inches) before
losing signal strength. It is possible to have even longer cables but, but due to attenuation, these longer cables
are generally more trouble than they are worth.
- SATA data cables and power connectors are different from PATA connections. The SATA data cable has only 7 pins (compared
to 80 pins of PATA) and can be inserted only one way into an SATA slot.
Three of these pins are ground pins, as shown below:
The SATA ground pins allow 'Hot-Swapping', not to be confused with 'Wife-Swapping'.
The ground pins are longer than the data lanes to allow 'Hot-Swapping' of drives (the ground pins make
contact first when being plugged in).
The SATA power connector has 17 pins
(compared to the 4 pins of a standard Molex connector), although you can buy Molex>SATA power adapters from most good retailers.
PATA/SATA power plug comparison: SATA on the left, PATA ('Molex') on the right.
- Hot-Swapping, or removing/adding a drive while the PC is still on, is an additional feature to SATA because of the
design of the connectors, as mentioned above.
- Reduced operational voltage. Serial ATA uses LVDS (Low Voltage Differential Signalling) with voltages of 250mV; while
PATA is based on an older 3.3V TTL (Transistor-Transistor Logic) signalling system (which must be designed to tolerate
up to 5V).
- Serial data transfer instead of parallel data transfer. I know that one sounds quite obvious, but it's rather an important difference!
Serial ATA uses a single signal path to transmit data serially, or bit by bit, and a second serial path to return
receipt acknowledgements to the sender (compared to PATA which uses a 16-bit wide bus). To compensate for the loss of
parallelism, the single-bit SATA bus can be run at a much higher speeds.
IDE = Integrated Drive Electronics. All Parallel ATA and Serial ATA drives are IDE!
PATA = Parallel Advanced Technology Attachment. Older drives which use a flat, wide ribbon data cable and molex power
connector are PATA.
SATA = Serial Advanced Technology Attachment. Newer drives which use a smaller round data cable and a special
SATA power connector are SATA.
- Reduced timing. The 1.5Gbits/sec transfer rate of SATA requires bit transitions and reception to both occur within 0.667ns.
The maximum allowed switching time is 0.273ns (much faster than the 10ns transition time allowable for PATA). This is
achieved via more tightly controlled electrical design parameters.
- Enhanced Reliability. Serial ATA adds 32-bit CRC error correction for all bits transmitted, as opposed to only data
packets in the PATA spec.
- The last, and biggest, difference is the maximum bandwidth between the two technologies. The true maximum transfer
rate of PATA is 100 MB/sec (with bursts up to 133 MB/sec). With the first introduction of SATA, the maximum transfer
rate was 150 MB/sec. This is set to increase every 3 years with a maximum transfer of 300 MB/sec already achieved in 2005
(SATA II) and 600 MB/sec set to be released in 2008 (SATA III).
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