HP 13255 Technical Information page 20

13255
Cartridge Tape MOdule
13255-91032/18
Rev
AUG-Ol-76
the serial data register at the appropriate time (corresponding to the
zone for data transitions). When the last data bit has been detected
at the decoder (there are 7 bits in the serial data register), the data
I/O but fer Is parallel loaded from the serial data register (shifted
right one oit). The data
I/O
but fer is sUbsequently output to the
terminal bus via drivers
uti
and
U32,
which are enabled by the
READ EN
signal.
3.0.4 ENCODER/DECODER LOGIC - CTU
INTERFAC~
peA.
3.0.4.1 This logic provides the timing and control functions for encoding or
decoding a data byte. The operation ot the encoder/decoder logic is
based upon the following theoretical analysis. The code that is re-
corded on the cartridge tape is Serial Biphase Mark COde (see below).
lheretore, the code shown below would be seen at the encoder output
(ut7,
Pin 10) when In Record mode or at the transition detector
(U28,
Pin 15) in Read mode. The code rules state that there is always a data
transition at mid cell, a 1-bit is a transition towards gap level while
a O-bit Is a transition away from gap level, and a phase transition
occurs at a cell boundary if adjacent data transitions are the same.
!
gap level__ ______ ____ ______ ___ _ ____ _
I t I I I I " I I
1 __ 1 1 ____ 1 1 __ 1 I _ I 1 ____ 1 1 _ _
1
o
1 1
o
o
1
o
*
*
*
*
*
* *
*Indicates cell boundaries
(125
microseconds)
3.0.4.2
Timing and control for the encode and decode operations is provided by
a cell decode counter
(U212)
and a variable modulus (divide
by
N) coun-
ter
(U213,
U214,
U312,
and
U313l
which are both synChronous with the
bus SYstem ClOCK. The variable modulus counter generates a carry Qllt-
put at time intervals eqUAl to one bipnase cell period divided by eight
which increments the cell decode counter. By initialIzing both coun-
ters at the beginning of a biphase cell, timing points (for enCOding
phase and data transitions in Read mode) and timing zones (for decoding
phase and data transitions In Head mode) can be decoded trom the cell
decode counter and variable modulus counter carry output.