Ultra ATA/66 versus Ultra ATA/33 Comparison

A Brief Background

To see why the addition of a slower device prevents us from running at Ultra ATA/66 speeds, let’s take a closer look at the technology behind Ultra ATA/66.

The goal of the Ultra ATA/66 specification was to double the burst transfer rate of the interface, from 33 MB/sec to 66 MB/sec, while at the same time keeping other significant changes to the Ultra ATA specification to a minimum. This goal was met, as the only change required, other than the obvious need for an Ultra ATA/66 drive and controller, was the addition of an 80-wire IDE cable. This cable contains the exact same pin terminations as the previous 40-conductor IDE cables, however, an additional 40 “ground” wires are included. These ground wires serve to eliminate “noisy” signals and to allow the data lines to stabilize more quickly after each pulse of transmitted data. Since the lines can stabilize more quickly with the additional ground wires, the amount of delay, or “setup time”, can be halved, effectively doubling the maximum transfer rate.

Since the Ultra ATA specification makes a point of ensuring backward compatibility, and IDE channels are not very good at switching modes on the fly, all of the devices on the cable must be Ultra ATA/66 compatible (including the controller) or else the controller will revert to Ultra ATA/33 speeds.