Emerson MVME147 manual

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CUSTOM SCSI PACKETS

NOTES:	1.	Refer to Chapter 3.
	2.	Command Table = 384 bytes
		RAM work area.

This first class is used for command execution and/or message passing through the MVME147 SCSI firmware while the module is playing the initiator role. According to SCSI definition, an initiator is a SCSI device (usually a host system) which requests an operation to be performed by another SCSI device; a TARGET is a SCSI device which performs an operation requested by an initiator. Initiator role custom packets request operations to be performed by other SCSI devices. For the initiator role, certain data structures are needed by the SCSI firmware. These are:

1.SCRIPT: A "script" is a sequence of SCSI bus phases that the initiator expects the TARGET to perform when executing a requested command. For example, a disk read (under SCSI rules) would typically require the following SCSI bus phases:

MESSAGE-OUT: The IDENTIFY message is sent from the initiator to the TARGET. This message contains the identification of the desired logical unit of the selected disk controller that the initiator wishes to read. The message also indicates whether the initiator is capable of reselection.

COMMAND: The Command Descriptor Block (CDB) is sent during the command phase to specify the block number to read, the logical unit to read from, the number of blocks to read, and whether the command is linked.

DATA-IN: The actual data is transferred from the TARGET to the initiator during the data-in phase.

STATUS: The disk controller sends the status of the command that was executed during this phase.

MESSAGE-IN: During this phase, the disk controller sends a message describing the execution of the command it just executed. The linked command information would appear in the message sent during this phase, for example.

BUS DISCONNECT: After a disk controller sends a command complete message, it disconnects from the SCSI bus by releasing the BSY* signal.

The justification for a script is as follows. On the SCSI bus, the TARGET is always the SCSI device that dictates the sequence of bus phases that occurs during a communication with the initiator (this communication is

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