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SCSI Interface Product Manual, Rev. B
9.3.2.2
Code description
The protection code (see Table 258) is a cyclic binary BCH code.
Table 258:
Protection code
Maximum data
Code
bits allowed
(21,15,4)
15
The BCH protection code is a cyclic code with a generator polynomial of x 6 + x 5 + x 2 + 1.
The canonical form of the code generator is shown in Figure 21. This is a serial implementation: the register is
initialized to zero, then the data is fed in one bit at a time, codeword bit 14 (as defined above) first, followed by
codeword bits 13, 12, 11, and so on until bit 0. As each data bit is input, the shift register is clocked. When all
15 bits have been clocked into the generator, the check bits are available in the registers, check bit 0 (codeword
bit 15) on the right in the diagram and check bit 5 (codeword bit 20) on the left. The + signs represent an XOR
operation.
Data In
(Bits 14
Down to 0)
Figure 21.
Protection code generator
Using this representation as a baseline, it is possible to construct logic to generate the six check bits from an
input data stream of n-bit width, including all 15 bits simultaneously, which is the expected implementation,
wherein:
Redundant bit 0 is the XOR of code word bits 0, 1, 2, 3, 5, 6, 7, 10, 11, and 13.
Redundant bit 1 is the XOR of code word bits 1, 2, 3, 4, 6, 7, 8, 11, 12, and 14.
Redundant bit 2 is the XOR of code word bits 0, 1, 4, 6, 8, 9, 10, 11, and 12.
Redundant bit 3 is the XOR of code word bits 1, 2, 5, 7, 9, 10, 11, 12, and 13.
Redundant bit 4 is the XOR of code word bits 2, 3, 6, 8, 10, 11, 12, 13, and 14.
Redundant bit 5 is the XOR of code word bits 0, 1, 2, 4, 5, 6, 9, 10, 12, and 14.
9.3.2.3
Error detection properties
This protection code was selected to have adequate detection properties for asynchronous information transfer
phases, given that these transfers are inherently less prone to errors and that these transfers have short code
words (approximately 20 bits as compared to thousands of bits during a DT data phase). The BCH protection
code Hamming distance is a minimum of four, the same as achieved by the data CRC for transfers of less than
eight kilobytes. The protection code will detect all errors of three bits or fewer, all errors of an odd number of
bits, and 98.4% of all possible errors.
9.3.3
Protection code usage
Protection code checking is enabled or disabled on an I_T nexus basis. All Command, Message, and STATUS
phase information is checked for an I_T nexus while checking is enabled. Protection code checking is disabled
after a power cycle, after a hard reset, after a SCSI target port Reset message, and after a change in the trans-
ceiver mode (e.g., LVD mode to MSE mode). Protection code checking is always disabled for information unit
transfers.
9.3.3.1
Protection code transmission
SCSI devices supporting this protection code transmit the protection code check data during all Command,
Message, and STATUS phases. The protection code byte is transferred on the upper eight bits of a wide bus
simultaneously with the information day byte on the lower eight bits of the bus using the same clock for the
transfer. Thus the transfer of the information byte and the protection code byte is performed exactly like a nor-
mal wide transfer. The check data is transmitted even if detection is not enabled.
Number of
Minimum distance
redundant bits
of the code
6
4
333
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