Viper 60s Theory Of Operation And Maintenance Manual page 160

THEORY OF OPERATION
6.9.2.1 Preamble
A preamble is generated in three configurations: Normal, Elongated, and Long. Each of these
preamble configurations are shown below.
TYPE
, DESCRIPTION
, -_ _ _ _ _ _ - - - l _ _ _ _ . _ _ _ _ _ . _ _ ._. _ _ . _ _ ... _ _ .... _ _ - ,
Normal Preamble
Elongated Preamble
Preamble
The normal preamble is used to synchronize the phase-locked-loop in
read electronics to the data frequency. It is also used to measure the
average Signal amplitude.
The elongated preamble precedes the first data block recorded after an
underrun conditions occurs.
The long preamble precedes the first data block for interchange recorded
at the beginning of a track.
6.9.2.2 Data Block Marker
The data block marker identifies the start of data and consists of the following GCR pattern:
G2
MS nibble
LS nibble
6.9.2.3 Data Block
The data block contains 512 bytes. Each 8-bit data byte is separated into two 4-bit groups
called nibbles. Each 4-bit data nibble (B3 - BO)
is
encoded into a 5-bit GCR nibble (G4 GO) for
recording onto the tape. Data bytes are encoded into GCR bytes in accordance with the code
translation shown in Table 6-2.
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