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Operating Registers
5-40
SYM53C810A Data Manual
Register 43 (C3)
SCSI Interrupt Status One (SIST 1)
Read Only
Reading the SIST 1 register returns the status of the
various interrupt conditions, whether or not they
are enabled in the SIEN1 register. Each bit that is
set indicates the corresponding condition has oc-
curred.
Reading the SIST 1 register will clear the interrupt
condition.
Bits 7-3 Reserved
Bit 2
ST O (Selection or Reselection
T ime-out)
T his bit is set when the SCSI device which the
SYM53C810A53C810A was attempting to
select or reselect did not respond within the
programmed time-out period. See the descrip-
tion of the ST IME0 register, bits 3-0, for more
information on the time-out timer.
Bit 1
GE N (General Purpose T imer
E xpired)
T his bit is set when the general purpose timer
has expired. T he time measured is the time
between enabling and disabling of the timer.
See the description of the ST IME1 register,
bits 3-0, for more information on the general
purpose timer.
Bit 0
HT H (Handshake-to-Handshake
T imer E xpired)
T his bit is set when the handshake-to-hand-
shake timer has expired. T he time measured is
the SCSI Request to Request (target) or
Acknowledge to Acknowledge (initiator)
period. See the description of the ST IME0
register, bits 7-4, for more information on the
handshake-to-handshake timer.
Register 44 (C4)
SCSI Longitudinal Parity (SLPAR)
Read/Write
T his register performs a bytewise longitudinal par-
ity check on all SCSI data received or sent through
the SCSI core. If one of the bytes received or sent
(usually the last) is the set of correct even parity
bits, SLPAR should go to zero (assuming it started
at zero). As an example, suppose that the following
three data bytes and one check byte are received
from the SCSI bus (all signals are shown active
high):
A one in any bit position of the final SLPAR value
would indicate a transmission error.
T he SLPAR register can also be used to generate
the check bytes for SCSI send operations. If the
SLPAR register contains all zeros prior to sending
a block move, it will contain the appropriate check
byte at the end of the block move. T his byte must
then be sent across the SCSI bus.
Note: writing any value to this register resets it to
zero.
T he longitudinal parity checks are meant to provide
an added measure of SCSI data integrity and are
entirely optional. T his register does not latch SCSI
selection/reselection IDs under any circumstances.
T he default value of this register is zero.
RES
7
RES
6
RES
5
RES
4
RES
3
STO
2
GEN
1
HTH
0
Default>>>
X
X
X
X
X
0
0
0
Data Bytes
Running SLPAR
---
1. 11001100
2. 01010101
3. 00001111
00000000
11001100 (X OR of word 1)
10011001 (X OR of word 1 and 2)
10010110 (X OR of word 1, 2 and 3)
Even Parity >>>10010110
00000000
4. 10010110