SCSI-2

The next leap in SCSI was the SCSI-2 specification. It not only added enhancements that improved upon SCSI-1, but also tried to standardize the command set. The previous SCSI standard didn't really do that so manufacturers would implement their own command set, which led to much confusion. Although work on SCSI-2 began before even SCSI-1 was approved, it wasn't until 1994 that the final spec was approved by ANSI.

The improvements included Fast SCSI, Wide SCSI, active termination, differential signaling (HVD), command queuing, and other additions as well. Command queuing was introduced to allow simultaneous multiple command requests between devices on the bus. Also, more devices were supported on the bus to keep up with the current technology, such as CD-ROMs, Zip drives, etc. Fast SCSI doubles the bus speed from 5 to 10 MHz which means a transfer rate of 10 MB/s assuming the bus width is 8 bits. Wide SCSI increases the width of the bus to 16-32 bits instead of the original 8 bits, which increases the data throughput. If you combine Fast and Wide SCSI, you're looking at transfer speeds of up to 20 MB/s. Also, the number of devices that can be supported on the SCSI bus is doubled when using Wide SCSI (16 devices as opposed to 8).

Active termination improves upon the passive termination of SCSI-1 by using voltage regulators to more reliably terminate the bus. The cables and connectors have also been improved upon to allow for better data transfer. The cable lengths in the original SCSI spec were adequate at the time, but it became necessary to have longer cables support higher speeds. However, this is not as trivial as it may seem. As with any high speed bus, keeping the signal clean is always a problem. The greater the distance the signal has to travel, the more problems you have due to interference, cross-talk, and degradation. The faster the bus runs, the chances of these problems popping up increases. This is the major limitation of SE signaling. As you increase the bus speed, the maximum allowable cable length decreases.

To get around this problem differential signaling or High Voltage Differential was developed. This uses two wires to transmit each signal. To send a 0, zero voltage is sent on both wires, and to send a 1, a positive voltage is sent on the first wire only. The second wire contains the electrical opposite of the first wire. Once the signal is received, the device on the receiving end takes the difference between the two signals and converts it to a 0 or 1 depending whether it sees a zero or high voltage. Although this method solves the cable length problem, it adds another problem of high cost. Later on a new method called Low Voltage Differential was developed to replace the more expensive HVD.