The advantage is a throughput of 20 Megatransfers/second - for an 8 bit system this provides 20 MB/s and a 16 bit system will obtain 40 MB/s. To achieve these transfer rates using a parallel connection the line transceivers have been upgraded.

FAST-20 uses incident-wave switching, which is the technology usually used for backplane buses. To allow operation with a cabled connection some limits have to be applied.

The changes are listed below alongside their equivalent SCSI-2 parameters:

For most integrators adding a FAST-20 component to an existing SCSI system will be simply a Plug 'n' Play operation. However where there are a large number of devices or long cable lengths involved it is important to comply with the limits identified above.

Technical bit

The reason for these constraints is that the AC and DC characteristics of the bus have been modified. The switching thresholds are reduced to allow for some inevitable impedance mismatch but this also lowers the S/N ratio.

An ideal cable would have zero resistance and zero inductance, similarly an ideal node would have infinite resistance and zero capacitance. Clearly this impossible. A good cable has low resistance and a distributed capacitance and inductance designed to provide a characteristic impedance of 90 ohms. A real node has a high resistance and a capacitance not greater than 25pF.

Connecting a node to the bus will add capacitance and therefore lower its impedance. In order to ensure that the reflected energy is sufficiently low to allow incident-wave switching, the added capacitance must be no more than twice the cable distributed capacitance.

Typical values for cable and device capacitance allow us to derive the 200 to 500mm separation specified above.