IBM 150 Manual De Usuario

RS/6000 43P 7043 Models 150 and 260 Handbook

These three serial SCSI interfaces have two major differences over their 
parallel sibling:

  • The cable has only six wires
  • The connectors are simplified into six pins

These new physical properties translate into lower costs for users and 
manufacturers.

You should take note that SCSI-III Parallel Interface (SPI) is backwardly 
compatible with SCSI-I and SCSI-II. It is ideal for those users that have 
sizable investments in SCSI-II equipment because SPI integrates SCSI-II and 
SCSI-III devices on the same chain. The use of SCSI-III provides much more 
function due to the improvements in the command set and data transfer rates. 
The migration from SCSI-II to SPI is relatively simple because all the 
connectors, cables and terminators essentially remain the same.

The SCSI-III Parallel Interface specification uses the terms

 and 

. Fast-20 Narrow is an 8-bit bus with a maximum data transfer rate 

of 20 MB/s. Fast-20 Wide is a 16-bit bus with a maximum data transfer rate of 
40 MB/s. The 

20 in Fast-20 refers to the clock speed of bus: 20 MHz, which is 

double the speed of SCSI-II Fast.

The term 

 is another name for Fast-20 Wide. Ultra SCSI is not 

equivalent to SCSI-III, but is a subset of the SCSI-III Parallel Interface (SPI).

The terminology of SCSI products can sometimes be confusing. Here are 
some definitions to help your understanding.

4.2.4.1  General Terminology
The following is a list of general terminology for SCSI.

Fast refers to the doubling of the data transfer rate from 
the SCSI 5 MB/s to 10 MB/s by doubling the clock rate. 
SCSI (that is, the original SCSI specification, or SCSI-I) is 
5 MB/s, which is produced by a clock speed of 5 MHz 
sending data down eight wires. SCSI-II Fast achieves 10 
MB/s by doubling the clock speed to 10 MHz.

Wide is used in reference to the width of the SCSI parallel 

bus between the adapter and the device. Wide means 
wider than the original 8-bit path defined in SCSI-I, usually 
16-bit. 32-bit transmission is possible within the 
specification but there are no "Wide-32" devices on the