In computing, parallel processing is said to be faster than computing things one at a time. It seems obvious. Not in disk interfaces though. Sending data serially is actually faster than sending it in parallel. A serial version of the venerable parallel ATA desktop disk interface is set to reach SCSI speeds and make faster disk access cheaper than ever. Here, we have a first look at the interface. Well look at SATA devices and controllers, and SATA performance and positioning in future articles.
The original ATA idea was to send the bits in a byte's worth of data, in parallel, along a bus connection. With the current Ultra ATA incarnation, the sixth since the original, mid-80s 3.3MB/s specification, two bytes, 16 bits, are transmitted in parallel with 16 control signals, meaning a wire per bit plus control signal wires. The result is an 80-wire flat ribbon cable with a 40-pin connector. In desktop PCs with floppy disks and a hard drive and a CD and a tape drive you get ribbon overload. One ribbon cable looks like another, space inside the cabinet is used up and cooling airflow blocked.
ATA runs at 100 MB/s and has a proposed development to 133 MB/sec. As files and disks get bigger that isn't fast enough. PC designers want to shrink the PC chassis. Getting rid of space-hogging ribbon cables would allow them to do that without causing over-heating.
It's expected that 2.5 inch form factor drives will be used in PCs and these smaller drives logically need smaller cables to make a smaller chassis feasible. The ATA design is a roadblock.
Seagate, LSI Logic, Adaptec, HP, Intel and others are members of the Serial ATA organisation. It has a working group which has developed the serial ATA interface. This uses a point-point single data path bus design with data transmitted bit by bit with an acknowledgment signal sent back. There are two pairs of wires and three ground lines in a single cable with a simple 7-pin connector. One pair of wires is for transmitting data, the other for receiving it. SATA is clocked at 1.5GHz compared to ATA's 50MHz. Its first iteration offers 150MB/s bandwidth but subsequent ones will go faster, much faster.
The SATA cable can be up to a metre long which makes it far easier to route around inside a chassis compared to ATA's maximum 18 inch length.
Other advances include error checking on both data and control signal transmissions - ATA only checks data transmissions - and a hot swap capability. SATA also uses low voltage signalling and this meets future PC design needs. Existing ATA has a 5 volt signalling requirement.
A Slave To Tradition
ATA supports two drives per channel (per ribbon cable) and the host bus adapter on the PC motherboard acts as the bus controller and routes information to the right drive. These have jumper settings to make them master or slave devices. The bus is locked if a command to either drive is outstanding. With SATA, each device is linked directly to a destination. There is no bus bandwidth sharing and hence no need for jumper setting or waits on one drive whilst another completes an operation.
SATA technology requires no change to operating systems or application software. The www.serialata.org web site confirms that Serial ATA supports legacy drivers for Parallel ATA. OEMs can deploy Serial ATA, today, using existing parallel ATA drivers.This compatibility is making it easier for suppliers to introduce SATA products. Drives, SATA controllers and PCI bus to SATA controller interface chips are already appearing, as are serial to parallel ATA bridge chips.
What Does It Mean?
There are over 160 members of the Serial ATA organisation. They include Dell, HP, Maxtor and Western Digital. With such wide vendor support SATA is a pretty definite next step in PC evolution.
John Monroe, a vice president at Gartner Dataquest, says: Serial ATA, because of its cost-effectiveness, performance and scalability, will not only become the standard drive interface for all desktop and notebook PCs, but will also enable high performance desktop class-drives to make much more significant incursions in multi-user applications.
It might even become possible to have RAID drive arrays on the desktop. SATA servers will be cheaper than SCSI servers and offer near-SCSI speed, hot-swap drives and more affordable RAID facilities great for SOHO and SMB users. We end users will spend far less time waiting for disk operations to complete and be more productive as a result.
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