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Functional Description
2.3
SCSI Bus Technology
The interface used by the BI-0316 is the "Small Computer System Interface" (SCSI) bus standard. Dif-
ferent versions of this standard exist. A brief introduction to the standard is given in the following sec-
tions.
2.3.1
Overview
SCSI is a local I/O bus that can operate over a wide range of data rates. The primary objective of this
interface is to provide host computers with device independence within a class of devices. Thus, differ-
ent disk drives, tape drives, printers, optical media drives, and other devices can be added to a host
computer without requiring modifications to generic system hardware or software.
2.3.2
Data Transfer Options
The SCSI protocol is designed to provide an efficient peer-to-peer I/O bus with up to 8, 16, or 32
devices depending on the data path widths implemented, including one or more hosts. Data may be
transferred asynchronously at rates that depend primarily on device implementation and cable length.
Synchronous data transfers are supported at rates up to 10 megatransfers per second. Three data path
widths are allowed, 8-bit, 16-bit, and 32-bit. The corresponding maximum transfer rates are 10, 20, and
40 megabytes per second.
Data transfers are divided into three speed classes: asynchronous, slow, and fast. A transfer period
specifies the minimum time allowed between the leading edges of successive REQ pulses and of suc-
cessive ACK pulses while using synchronous data transfers.
•
Fast data transfers require a transfer period greater than 100 ns. and less than 200 ns. with a REQ/
ACK offset greater than zero. If a fast data transfer is agreed upon, fast timing shall be observed
even though the actual data transfer period is greater than or equal to 200 ns.
•
Slow data transfers require a transfer period greater than or equal to 200 ns. with a REQ/ACK offset
greater than zero.
•
Asynchronous information transfers require a REQ/ACK offset of zero. The transfer period does not
apply to these information transfers.
The BI-0316 supports all three classes.
2.3.3
Electrical Options
There are two electrical alternatives for the SCSI bus:
Single-Ended and
•
•
Differential.
Single-Ended and Differential devices are electrically incompatible and can not be mixed on the same
physical bus.
When the BI-0316 contains Single-Ended drives it can be used in a Single-Ended SCSI bus. When the
BI-0316 contains Differential drives it can be used in a Differential SCSI bus.
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Page 2-2
BI-0316
Technical Manual
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