LSI 53C875A - Manual (Page 118)

4-26 Registers
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
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
7 6 5 4 3 2 1 0
SDU CHM SLPMD SLPHBEN WSS VUE0 VUE1 WSR
0 0 0 0 0 0 0 0