Viper 60s Theory Of Operation And Maintenance Manual page 162

THEORY OF OPERA TION
The control nibble (Byte 1, Bits 0-4) is defined as follows:
3 2
o
o
o
o
o
o
1
o
o
o
o
o
o
2
The current block contains user data or file mark.
Reserved for
QIC·24/120
use.
The current block is a unique control block for use
only on track format.
3 Reserved
6.9.2.5 Cyclical Redundancy Check
The cyclical redundancy check (CRC) consists of two bytes calculated over the 512 bytes of
interchange data and the 4-byte block address starting with all ones CRC initial value. The
CRC
is
encoded into GCR bytes in accordance with Table 6-2. The CRC is generated using the
following polynomial:
CRC
=
X 16
+ X12 + Xs + 1
6.9.2.6 Postamble
A postamble is generated in two configurations: Normal and Elongated. Each of these
postamble configurations are shown below.
TYPE DESCRIPTION
Normal Postamble The normal postamble is recorded after the CRC as a guard band.
Elongated Postamble The elongated postamble follows the first data block recorded after an
underrun conditions occurs.
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