Emulex MD21/S2, Техническое руководство

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SCSI Bus Overview
2.3 SCSI BDS OVERVIEW
The Small Computer System Interface (SCSI)* is a standard interface
established to support mass storage, printer output, and network
communication for microcomputers and minicomputers. The interface
is an eight-port, daisy-chained bus. The SCSI command standard for
the MD21 Controller is based on the ANSI SCSI Interface
Specification listed on page vii.
The SCSI bus can support up to eight SCSI host adapters and/or
controllers. Each controller can be connected to a maximum of
eight devices (called Logical Unit Numbers, or LUNs). The MD21
Controller hardware supports any combination of host adapters,
intelligent controllers, or intelligent peripherals connected to
the SCSI bus. The MD21 Controller supports one or two LUNs (ESDI
disk drives). Three basic SCSI configurations are supported with
the MD21 Controller and SCSI bus:
• Single initiator, single target
• Single initiator, multi target
• Multi initiator, multi target
Communication on the SCSI bus occurs between a host adapter and a
controller. When a host adapter and a controller communicate, one
acts as the Initiator and the other acts as the Target. The
Initiator (usually the host adapter) originates an operation, and
the Target (usually a peripheral controller, such as the MD21
Controller) performs the operation. Sample system configurations
supported by MD21 Controller hardware are shown in Figure 2-3.
Some SCSI bus functions are assigned to the Initiator and some
functions are assigned to the Target. The Initiator can arbitrate
for control of the SCSI bus and select a specific Target. The
Target can request the transfer of command, data, status, or other
information via the SCSI bus. In some circumstances, the Target
can arbitrate for control of the SCSI bus to reselect an Initiator
and continue an operation. Sometimes, the Target becomes an
Initiator and arbitrates for control of the SCSI bus.
SCSI bus data transfer operations are asynchronous and follow a
defined request/acknowledge (REQ/ACK) handshake protocol. (This
protocol is defined in the ANSI SCSI specification.) One eight-bit
byte of information can be transferred with each handshake.
The SCSI bus consists of 18 signal lines. Nine signal lines are
for an eight-bit data bus with parity; the other nine signal lines
are for control and status signals that coordinate data transfer
operations between the host adapter and SCSI controllers. SCSI bus
signals are described in more detail in subsection 7.2.3.1.
2-6 General Description