Lets see the differences between the two standards used in hard drives: IDE and SCSI.

IDE is based on parallel transmission. It consists in conductive threads sending simultaneous data from a device in the motherboard to the device present in the hard drive or vice verse. This interface has 40 pins and only 16 of them send information, the rest of them are basically for power supply and control signals.

So making numbers we can say that 5.24 millions clock cycles are necessary to get a 10 MB transference at 5.24MHz frequency.

As the technology was improving everyday, that wasn?t enough in a time. The industries needed to increase the frequencies, improve the transmissions and study the logical mechanisms to improve the responses.

That?s when the PIO mode (Programmed Input/Output) appeared. It was already used in some CD/DVD drives for compatibility, obtaining transferences of 16.7 MB/s.

Later on, the DMA (Direct Access Memory) was released. Why asking the processor to transfer the data between the devices if the driver of the memory can make the job. As a result we get 16.7 MB/s, the same as PIO but without needing the processor, now available for other tasks. An interesting improvement in DMA technology was the Ultra DMA, improving the transmission in two flanks of a cycle (similar to DDR), reaching in theory 33 MB/s.

ATA reached its limit when the bandwidths of the different UDMA modes got close to the bandwidth of the bus PCI that contains it: 133 MB/s.

Facing this new challenge and considering the improvements on speeds the microprocessors and memories had in the system, it was necessary to check the IDE norm to solve this ?bottleneck?. Here is when SATA (Serial Advanced Technology Attachment ?2000) was released by the Serial ATA Working Group. The new interface makes serial transmissions unlike the parallel of the original ATA. It has only 7 pins, and only 4 are in charge of data transmission. The serial bus can oscillate at very high frequencies, what makes possible transferences between devices at 150 GB/s (gigabits per second). Its second version known as SATA II can double the bandwidth to an interesting 300 GB/s.

Looking for other differences between the two technologies, that Serial ATA uses a point to point connection, each hard drive /CD/DVD is solo in the bus. Only one device at a time can be plugged with a SATA controller, unlike the ATA (parallel) that uses the MASTER | SLAVE architecture, sharing the bandwidth between the two possible drives plugged to the controller.

Resuming, SATA drives can reach 150 MB/s (SATA) or 300 MB/s (SATA II) and the 133 MB/s reached with ATA are shared by all the devices connected to the bus PCI.