Although current IDE drives can only stream data at about 10MB/Sec it takes so long for the host computers to issue commands, that buffers fill up and can not be emptied in the 16.7mb bursts between delays waiting for commands. After allowing for the 275uSec plus command delays on typical 4kb transfers, the effective transfer rate of a fast PC is only 7.8MB/Sec. In order to support a 10mb/Sec drive, the PC would have to transfer the data in about 175uSec -- which requires a burst transfer rate about 24 MB/Sec. Failure to keep up with the disk results in adding the time for a full disk revolution to each data transfer. That can cut the actual transfer rate for contiguous data to about a third of the 7.8MB/Sec theoretical rate.
Ultra-ATA works by allowing data to be transferred on both transitions of the data clock instead of only one transition. That not only allows doubling the transfer rate, but it does not require massive design changes to support either mode. Ultra_ATA also includes some changes in bus control that reduce propagation delays for some control signals by having them originate at the drive rather than at the host when it makes sense to do so.
An improved CRC verification scheme is added in ultra-IDE.
At Ultra DMA speeds, data cable characteristics begin to become important. Although 24 inch cables are often used for slower modes, the use of 18 inch cables with extra ground wires is recommended for PIO Mode 4 operation.
Return To Index Copyright 1994-2002 by Donald Kenney.