Emerson MVME147 manual Servicing the TARGET Requests

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TARGET ROLE ROUTINES

the user command table. The service of this command is provided by you. This service is provided by a custom SCSI sequence -- TARGET sequence, which is described in the next section.

IMPLEMENTATION NOTE: The TARGET LUN is deciphered from the identify message sent immediately after the selection phase. If the LUN identified during this phase is not enabled by you, the firmware rejects the message (by sending the MESSAGE REJECT message) and then disconnect from the bus. If an identify message is rejected, it is a signal that some other LUN on this MVME147 TARGET address must be enabled, otherwise the SCSI firmware would not have responded to the selection in the first place. If a selection interrupt is ignored because the TARGET role was not enabled in the firmware, the firmware ignores the interrupt after the SEL signal is removed from the bus. In any case, if this happens, the SCSI bus is tied up unless the initiator times out and gives up by removing its signals from the bus.

Servicing the TARGET Requests

After a particular LUN is enabled for TARGET role, a selection interrupt begins the interrupt-driven service for TARGET role and the TARGET role stays enabled until the system is reinitialized. (The selection interrupt enable through the WD33C93 is turned on and only reinitialization turns the interrupt enable off. Disabling the selection interrupt is accomplished by clearing the select enable register in the WD33C93 and/or turning the PCC SCSI interrupt enable off through the module control register.) Issuing a board reset also disables TARGET role on the MVME147.

If TARGET role is enabled, a selection of the MVME147 causes the firmware to respond to the selection by asserting the BSY signal. The following sequence is typical of SCSI rules.

If the ATN line is asserted with SEL, the firmware takes the SCSI bus to the message-out phase and reads the message. The firmware then takes the SCSI bus to the command phase, reads the command (6, 10, or 12 bytes), and stores the command count and the command in the command table. If the received message is "identify" and if the identified LUN has been enabled, the firmware continues and save the messages and command in command table area. No command interpretation is done in the firmware; this job is left for the user application. If disconnect is allowed then next phase directed by the firmware is the message-in phase: the DISCONNECT message is sent. Then a bus disconnect is performed by releasing the BSY signal. The first TARGET role

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