Storage Architectures and Devices 103
market. With a 16-bit path, the data rate is double that of
an 8-bit device for the same clock speed.
Fast/Wide Fast/Wide refers to a 16-bit data path running at 10 MHz
producing a maximum data transfer rate (or burst rate) of
20 MB/s.
Fast-20 Fast-20 is a bus running at double the clock speed of Fast,
or 20 MHz. Fast-20 typically refers to an 8-bit bus and can
also be called Fast-20 Narrow. Fast-20 Wide is the 16-bit
version, also known as Ultra SCSI.
Ultra SCSI Ultra SCSI, as described above, is a subset of the SCSI-III
specification. It is effectively a Fast SCSI bus running at
20 MHz. Ultra SCSI can produce a maximum transfer of
20 MB/s over an 8-bit data path. Wide Ultra SCSI, the
16-bit version of Ultra SCSI, can transmit a maximum of
40 MB/s.
Ultra2 SCSI Ultra-2 is the latest extension to the SCSI-III specification
that allows transfers to take place at 80 MB/s on a wide
bus, or 40 MB/s for narrow. At twice the speed of the
current Ultra SCSI, and comparable with SSA, it has the
advantage of complete backwards compatibility with
existing peripherals. The cable length maximum, as
defined by the standard, is 12 meters. This standard
requires Low Voltage Differential (LVD).
4.2.4.2 Asynchronous versus Synchronous
An asynchronous device must acknowledge each byte as it comes from the
controller. Synchronous devices may transfer data in bursts, and the
acknowledgments happen after the fact. Synchronous is much faster than
asynchronous, and most newer devices support the synchronous mode of
operation. The adapters negotiate with devices on the SCSI bus to ensure
that the mode and data transfer rates are acceptable to both the host adapter
and to the devices. This process prevents data from being lost and ensures
that data transmission is error free.
Wide refers to the width of the bus between the SCSI adapter and its
attached devices. Do not confuse this with the width of the host bus
interface (for example, a 32-bit PCI or Micro Channel bus).
Note