KB103436 - Small Computer System Interface (SCSI) Bus Configuration

The following table describes the configuration attributes of the standard SCSI interface.

           Bit   Cable  Pin    Max Transfer  Max SCSI

Standard Width Name Cnt. Rate MB/sec Devices Description

SCSI-1      8      A     50       5            8      Asynchronous
SCSI-2      8      A     50       10           8      fast
SCSI-2      16     A+B   50+68    20           8      fast+wide **
SCSI-2      32     A+B   50+68    40           8      fast+wide **
SCSI-3      8      A     50       10           8      fast
SCSI-3      16     P     68       20           16     fast+wide *
SCSI-3      32     P+Q   68+68    40           32     fast+wide **

*  = with 1 cable
** = with 2 cables

NOTE: Windows NT currently supports only eight SCSI IDs.

Standard: The name of the SCSI standard as defined by ANSI.

Bit width: The number of bits that gets transferred by the SCSI bus during the data transfer phases.

Cable Names: A is most common, P is getting more popular, A+B is currently not popular due to cost and space issues.

Pin Count: The number of pins in the cable. Refer to the above table for specific numbers.

Max Transfer Rate (MB/sec): Number of bits transferred over the SCSI bus in one second.

Max SCSI Devices: The Maximum number of devices that can be connected to the SCSI bus with one host adapter installed.

Description:

   Asynchronous: A handshaking protocol that requires a
   handshake for every byte transferred (Synchronous  transfers a
   series of bytes before handshaking occurs, which speeds data
   transfer rate)

   Fast: Fast SCSI is an option that doubles the synchronous data
   transfer speed. The speed is achieved by removing excess margins
   from certain times and delays. To use the fast SCSI option, high
   quality cables are required. This option is compatible with normal
   synchronous SCSI and has:

- Up to 10 (megabytes) MB/second over an 8 bit bus.

- Synchronous Data transfer negotiation required.

- Single-ended implementation recommendations: max cable length of 3 meters and active terminators.

   Wide: Wide SCSI is an option that adds a second SCSI cable of 68
   conductors. This cable provides a data path for 16- or 32-bit data.
   This path has separate handshake signals and is for data transfer
   only. The transfer rate is two or four times the present transfer
   rate of SCSI-1. With the second cable, SCSI-2 remains compatible
   with the 8-bit SCSI.