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SYM53C876/876E Data Manual
Registers
SCSI Registers
Bit 0
HTH (Handshake-to-Handshake
Timer Expired)
This bit is set when the handshake-to-hand-
shake timer expires. The time measured is the
SCSI Request to Request (target) or
Acknowledge to Acknowledge (initiator)
period. See the description of the STIME0
register, bits 7-4, for more information on the
handshake-to-handshake timer.
Register 44h
SCSI Longitudinal Parity (SLPAR)
Read/Write
This register performs a bytewise longitudinal
parity 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 cor-
rect even parity bits, SLPAR should go to zero
(assuming it started at zero). As an example, sup-
pose that the following three data bytes and one
check byte are received from the SCSI bus (all sig-
nals are shown active high):
A one in any bit position of the final SLPAR value
would indicate a transmission error.
The SLPAR register also generates the check
bytes for SCSI send operations. If the SLPAR reg-
ister contains all zeros prior to sending a block
move, it contains the appropriate check byte at
the end of the block move. This byte must then be
sent across the SCSI bus.
Note: Writing any value to this register resets it
to zero.
The longitudinal parity checks are meant to pro-
vide an added measure of SCSI data integrity and
are entirely optional. This register does not latch
SCSI selection/reselection IDs under any circum-
stances. The default value of this register is zero.
The longitudinal parity function normally oper-
ates as a byte function. During 16-bit transfers,
the high and low bytes are XORed together and
then XORed into the current longitudinal parity
value. By setting the SLPMD bit in the SCNTL3
register, the longitudinal parity function is made
Data Bytes
Running SLPAR
---
1. 11001100
2. 01010101
3. 00001111
00000000
11001100 (XOR of word 1)
10011001 (XOR of word 1 and 2)
10010110 (XOR of word 1, 2 and 3)
Even Parity >>>10010110
00000000
4. 10010110