R

Reserved

[8:7]

ACK

Set/Clear SACK/

6

R

Reserved

[5:4]

ATN

Set/Clear SATN/

3

 

 

These two bits are used in conjunction with a Set or Clear

 

 

instruction to assert or deassert the corresponding SCSI

 

 

control signal. Bit 6 controls the SCSI SACK/ signal. Bit 3

 

 

controls the SCSI SATN/ signal.

 

 

 

The Set instruction is used to assert SACK/ and/or SATN/

 

 

on the SCSI bus. The Clear instruction is used to

 

 

 

deassert SACK/ and/or SATN/ on the SCSI bus. The

 

 

corresponding bit in the SCSI Output Control Latch

 

 

(SOCL) register is set or cleared depending on the

 

 

instruction used.

 

 

 

Since SACK/ and SATN/ are Initiator signals, they are not

 

 

asserted on the SCSI bus unless the LSI53C875A is

 

 

operating as an Initiator or the SCSI Loopback Enable bit

 

 

is set in the SCSI Test Two (STEST2) register.

 

 

 

The Set/Clear SCSI ACK/, ATN/ instruction is used after

 

 

message phase Block Move operations to give the

 

 

 

Initiator the opportunity to assert attention before

 

 

 

acknowledging the last message byte. For example, if the

 

 

Initiator wishes to reject a message, it issues an Assert

 

 

SCSI ATN instruction before a Clear SCSI ACK

 

 

 

instruction.

 

R

Reserved

[2:0]

5.4.2 Second Dword

 

 

 

 

31

 

 

 

 

0

 

 

 

 

 

 

 

 

DMA SCRIPTS Pointer Save (DSPS) Register

 

 

 

 

x x x x x x x x

x x x x x x x x

x x x x x x x x

x x x x x x x x

 

 

 

 

 

 

SA

Start Address

[31:0]

 

 

This 32-bit field contains the memory address to fetch the

 

 

next instruction if the selection or reselection fails.

 

I/O Instruction

5-21

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LSI 53C875A technical manual Set/Clear Sack Reserved, Set/Clear Satn