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SYM53C876/876E Data Manual
4-57
Registers
SCSI Registers
Bit 2
UDC (Unexpected Disconnect)
This bit is set when the SYM53C876 SCSI
function is operating in initiator mode and the
target device unexpectedly disconnects from
the SCSI bus. This bit is only valid when the
SYM53C876 SCSI function operates in the
initiator mode. When the SCSI function oper-
ates in low level mode, any disconnect causes
an interrupt, even a valid SCSI disconnect.
This bit is also set if a selection time-out
occurs (it may occur before, at the same time,
or stacked after the STO interrupt, since this
is not considered an expected disconnect).
Bit 1
RST (SCSI RST/ Received)
This bit is set when the SYM53C876 SCSI
function detects an active SRST/ signal,
whether the reset is generated external to the
chip or caused by the Assert SRST/ bit in the
SCNTL1 register. This SCSI reset detection
logic is edge-sensitive, so that multiple inter-
rupts are not generated for a single assertion
of the SRST/ signal.
Bit 0
PAR (Parity Error)
This bit is set when the SYM53C876 SCSI
function detects a parity error while receiving
SCSI data. The Enable Parity Checking bit
(bit 3 in the SCNTL0 register) must be set for
this bit to become active. The SYM53C876
SCSI function always generates parity when
sending SCSI data.
Register 43h
SCSI Interrupt Status One (SIST1)
Read Only
Reading the SIST1 register returns the status of
the various interrupt conditions, whether they are
enabled in the SIEN1 register or not. Each bit
that is set indicates an occurrence of the corre-
sponding condition.
Reading the SIST1 clears the interrupt condition.
Bits 7-4 Reserved
Bit 2
STO (Selection or Reselection Time-
out)
The SCSI device which the SYM53C876
SCSI function is attempting to select or rese-
lect does not respond within the programmed
time-out period. See the description of the
STIME0 register, bits 3-0, for more informa-
tion on the time-out timer.
Bit 1
GEN (General Purpose Timer
Expired)
This bit is set when the general purpose timer
expires. The time measured is the time
between enabling and disabling of the timer.
See the description of the STIME1 register,
bits 3-0, for more information on the general
purpose timer.
RES
7
Default >>>
X
RES
6
RES
5
RES
4
RES
3
STO
2
GEN
1
HTH
0
X
X
X
0
0
0
0