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SYM53C876/876E Data Manual
4-21
Registers
SCSI Registers
in the SCNTL1 register to verify that the
SYM53C876 is not connected to the SCSI
bus.
Bit 4
WATN (Select with SATN/ on a Start
Sequence)
When this bit is set and the SYM53C876
SCSI function is in initiator mode, the SATN/
signal is asserted during selection of a SCSI
target device. The SATN/ signal informs the
target that the SYM53C876 SCSI function
has a message to send. If a selection time-out
occurs while attempting to select a target
device, SATN/ is deasserted at the same time
SSEL/ is deasserted. When this bit is clear, the
SATN/ signal is not asserted during selection.
When executing SCSI SCRIPTS, this bit is
controlled by the SCRIPTS processor, but
manual setting is possible in low level mode.
Bit 3
EPC (Enable Parity Checking)
When this bit is set, the SCSI data bus is
checked for odd parity when data is received
from the SCSI bus in either initiator or target
mode. If a parity error is detected, bit 0 of the
SIST0 register is set and an interrupt may be
generated.
If the SYM53C876 SCSI function is operat-
ing in initiator mode and a parity error is
detected, assertion of SATN/ is optional, but
the transfer continues until the target changes
phase. When this bit is cleared, parity errors
are not reported.
Bit 2
Reserved
Bit 1
AAP (Assert SATN/ on Parity Error)
When this bit is set, the SYM53C876 SCSI
function automatically asserts the SATN/ sig-
nal upon detection of a parity error. SATN/ is
only asserted in initiator mode. The SATN/
signal is asserted before deasserting SACK/
during the byte transfer with the parity error.
Also set the Enable Parity Checking bit for the
SYM53C876 SCSI function to assert SATN/
in this manner. A parity error is detected on
data received from the SCSI bus.
If the Assert SATN/ on Parity Error bit is
cleared or the Enable Parity Checking bit is
cleared, SATN/ is not automatically asserted
on the SCSI bus when a parity error is
received.
Bit 0
TRG (Target Mode)
This bit determines the default operating
mode of the SYM53C876 SCSI function.
The user must manually set the target or initi-
ator mode. This is done using the SCRIPTS
language (SET TARGET or CLEAR TAR-
GET). When this bit is set, the chip is a target
device by default. When this bit is cleared, the
SYM53C876 SCSI function is an initiator
device by default.
CAUTION:
Writing this bit while not connected may cause
the loss of a selection or reselection due to the
changing of target or initiator modes.