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PATA vs. SATA

by Mathew .. on 28 July 2005, 00:00

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What are PATA and SATA?

In this guide we will be discussing the differences between Parallel ATA and Serial ATA, both are types of interface for disk drives in the average desktop PC.

Firstly, a similarity. ATA stands for AT Attachment and ATAPI - a term that will be well used in this article - stands for ATA Packet Interface. AT is the designation used by IBM for their early range of personal computers. ATA/ATAPI is the single most popular interface for disk drives, the alternative method being SCSI, which is found where speed and reliability are essential. SCSI is not used by the vast majority of users due to cost.

Most USB pendrives, flash memory cards, CDROM drives and PCMCIA devices are also based on the ATA/ATAPI standard. These standards are developed and maintained by a governing body named INCITS – InterNational Committee for Information Technology Standards.

The ATA standard was developed in 1986 when a trio of companies (Western Digital, CDC subdivision Imprimis and Compaq) were manufacturing disk drives. At that time WD was a major manufacturer of silicon chips, having developed and produced the hard drive controller chips for the IBM PC/AT machines during 1984.

Compaq were the first to start using 3.5” drives fitted with the new interface manufactured by Connor Peripherals. The interface only started to take off when other distributors started using the drives and they started to become available in retail outlets. Since then the original specification has been improved upon and refined leading to several types of ATA standard, from ATA-1 offering 16.6MiB/s, up to and including ATA-7 used by some manufacturers providing 133MiB/s in burst mode which is the most recent version.

Serial ATA is the next evolutionary stage in the development of magnetic storage media, it differs from PATA not only in the speeds attainable (150MiB/s to 600MiB/s as opposed to a maximum of 133MiB/s burst speed with PATA), but in the physical layout of the connector and other hardware.

The most glaring difference should the two connectors be placed next to each other is the size, PATA cables are 20 pins wide in two layers giving 40 pins, whereas the SATA connector is a mere 7 pins wide in a single layer. One advantage of this is the ease with which SATA cables can be run through your case. PATA ribbon cables take up a lot of space and can block airflow through the case. PATA cables are also length limited by signal attenuation making the maximum useable length of the cable around 45cm. SATA cables can be up to a meter long before the signal degrades. One potential disadvantage with these new connectors is the increased risk of damage to the less robust pins. The power connector also differs from the standard molex, though adaptors are readily available for those who own power supplies without the correct connector for SATA devices.

The major advantage that SATA has over PATA is the way in which data is transferred. PATA cables use bidirectional channels for data, the channels able to transmit data both to and from the controller, SATA uses a different method with a pair of separate channels for data lines with this method reducing crosstalk and other types of impedance. SATA drives use a low voltage swing method of transferring data with a voltage differential of 0.125V, whereas PATA devices use a 3.3V signalling method. This low voltage swing greatly improves the energy efficiency of the device decreasing drain on the PSU and decreasing the heat output of the system as a whole; important for those large RAID arrays and allows greater speeds to be attained as signal integrity is maintained.