Caution: Writing to this register while not connected may cause the loss of a selection/reselection by clearing the Connected bit.

Register: 0x02

SCSI Control Two (SCNTL2)

Read/Write

7

6

5

4

3

 

2

1

 

0

SDU

CHM

SLPMD

SLPHBEN

WSS

 

VUE0

VUE1

 

WSR

 

 

 

 

 

 

 

 

 

 

0

0

0

0

0

 

0

0

 

0

 

 

 

 

 

 

 

 

 

 

SDU

SCSI Disconnect Unexpected

 

 

 

7

 

This bit is valid in the initiator mode only. When this bit is

 

set, the SCSI core is not expecting the SCSI bus to enter

 

the Bus Free phase. If it does, an unexpected disconnect

 

error is generated (see the Unexpected Disconnect bit in

 

the SCSI Interrupt Status Zero (SIST0) register, bit 2).

 

During normal SCRIPTS mode operation, this bit is set

 

automatically whenever the SCSI core is reselected, or

 

successfully selects another SCSI device. The SDU bit

 

should be cleared with a register write (move 0x00 to

 

SCNTL2) before the SCSI core expects a disconnect to

 

occur, normally prior to sending an Abort, Abort Tag, Bus

 

Device Reset, Clear Queue or Release Recovery

 

 

message, or before deasserting SACK/ after receiving a

 

Disconnect command or Command Complete message.

CHM

Chained Mode

 

 

 

 

 

6

This bit determines whether or not the SCSI core is programmed for chained SCSI mode. This bit is automatically set by the Chained Block Move (CHMOV) SCRIPTS instruction and is automatically cleared by the Block Move SCRIPTS instruction (MOVE).

Chained mode is primarily used to transfer consecutive wide data blocks. Using chained mode facilitates partial receive transfers and allows correct partial send behavior. When this bit is set and a data transfer ends on an odd byte boundary, the LSI53C875A stores the last byte in the SCSI Wide Residue (SWIDE) register during a receive operation, or in the SCSI Output Data Latch (SODL) register during a send operation. This byte is

4-26Registers

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LSI 53C875A technical manual Scsi Control Two SCNTL2 Read/Write, Sdu, Scsi Disconnect Unexpected, Chm, Chained Mode