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53C810

PCI-SCSI I/O Processor

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PREUMINARY

Chapter Five

Operating Registers

Bit 1 IARB (Immediate Arbitration)

Setting this bit will cause the SCSI core to

immediately begin arbitration once a BUS

FREE phase is. detected following an expected

SCSI disconnect. This bit is useful for multi-

threaded applications. The ARB 1-0 bits in

SCNTLO should be set for full arbitration and

selection before setting Immediate Arbitra-

tion.

Arbitration will be re-tried until won. At that

point, the 53C810 will hold BSY and SEL

asserted, and wait for a select or reselect

sequence to be requested. The Immediate

Arbitration bit will be reset automatically

when the selection or reselection sequence is

completed, or times out.

An unexpected disconnect condition will clear

!ARB without attempting arbitration. See the

SCSI Disconnect Unexpected bit (SCNTL2,

bit 7) for more information on expected

versus unexpected disconnects. .

An immediate arbitration sequence can be

aborted. First, the abort bit in the 1STAT

register should be set. Then one of two things

will eventually happen:

1) The Won Arbitration bit (SSTATO bit 2)

will be asserted. In this case, the Immedi-

ate Arbitration bit needs to be reset. This

will complete the abort sequence and

disconnect the 53 C81 0 from the SCSI

bus. If it is not acceptable to go to Bus

Free phase immediately following the

arbitration phase, a low level selection may

instead be performed.

2) The abort ~ complete because the

53 C81 0 loses arbitration. This can be

detected by the Immediate Arbitration bit

being deasserted. The Lost Arbitration bit

(SSTATO bit 3) should not be used to

detect this condition. No further action

needs to be taken in this case.

Bit 0 SST (Start SCSI Transfer)

This bit is automatically set during SCRIPTS

execution, and should not be used. It causes

the SCSI core to begin a SCSI transfer,.

including REQIACK handshaking. The

determination of whether the transfer is a

send or receive is made according to the value

written to the 110 bit in SOCL. This bit is

self-resetting. This bit should not be set for

low level operation.

CAUTION: Writing to thi~ register while not

connected may cause the loss of a selection!

reselection by resetting the Connected bit.

NCR 53C81 0 Data Manuar

5-7

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