check byte are received from the SCSI bus (all signals are shown active HIGH):

Data Bytes

Running SLPAR

 

 

 

00000000

 

 

 

 

1. 11001100

11001100

(XOR of word 1)

 

 

 

2. 01010101

10011001

(XOR of word 1 and 2)

 

 

 

3. 00001111

10010110

(XOR of word 1, 2 and 3) Even Parity

 

 

 

4. 10010110

00000000

 

 

 

 

A one in any bit position of the final SLPAR value would indicate a transmission error.

The SLPAR register is also used to generate the check bytes for SCSI send operations. If the SLPAR register 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 clears it to zero.

The longitudinal parity checks are meant to provide an added measure of SCSI data integrity and are entirely optional. This register does not latch SCSI selection/reselection IDs under any circumstances. The default value of this register is zero.

The longitudinal parity function normally operates 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 SCSI Control Two (SCNTL2) register, the longitudinal parity function is made to operate as a word-wide function. During 16-bit transfers, the high byte of the SCSI bus is XORed with the high byte of the current longitudinal parity value, and the low byte of the SCSI bus is XORed with the low byte of the current longitudinal parity value. In this mode, the 16-bit longitudinal parity value is accessed a byte at a time through the SCSI Longitudinal Parity (SLPAR) register.

4-80Registers

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Image 172
LSI 53C875A technical manual Data Bytes Running Slpar