SCSI Control Zero (SCNTL0) register are set for full arbitration and selection before setting this bit.

Arbitration is retried until won. At that point, the LSI53C875A holds SBSY and SSEL asserted, and waits for a select or reselect sequence. The Immediate Arbitration bit is cleared automatically when the selection or reselection sequence is completed, or times out.

An unexpected disconnect condition clears IARB with it attempting arbitration. See the SCSI Disconnect Unexpected bit (SCSI Control Two (SCNTL2), bit 7) for more information on expected versus unexpected disconnects.

It is possible to abort an immediate arbitration sequence. First, set the Abort bit in the Interrupt Status Zero (ISTAT0) register. Then one of two things eventually happens:

The Won Arbitration bit (SCSI Status Zero (SSTAT0), bit 2) will be set. In this case, the Immediate Arbitration bit needs to be cleared. This completes the abort sequence and disconnects the chip from the SCSI bus. If it is not acceptable to go to Bus Free phase immediately following the arbitration phase, it is possible to perform a low level selection instead.

The abort completes because the LSI53C875A loses arbitration. This is detected by the clearing of the Immediate Arbitration bit. Do not use the Lost Arbitration bit (SCSI Status Zero (SSTAT0), bit 3) to detect this condition. In this case take no further action.

SSTStart SCSI Transfer0

This bit is automatically set during SCRIPTS execution and should not be used. It causes the SCSI core to begin a SCSI transfer, including SREQ/ and SACK/ handshaking. The determination of whether the transfer is a send or receive is made according to the value written to the I/O bit in SCSI Output Control Latch (SOCL). This bit is self-clearing. Do not set it for low level operation.

SCSI Registers

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LSI 53C875A technical manual SSTStart Scsi Transfer0