Zilog Z16C35 - Manual (Page 38)

ISCC
User Manual
UM011002-0808
32
Figure 3–6. Data Encoding Methods
In NRZ, encoding a “1” is represented by a HIGH level and a “0” is represented by a
LOW level. In this encoding meth-od, only a minimal amount of clocking information is
available in the data stream in the form of transitions on bit-cell boundaries. In an arbitrary
data pattern, this may not be sufficient to generate a clock for the data from the data itself.
In NRZI, encoding a “1” is represented by no change in the level and a “0” is represented
by a change in the level. As in NRZ, only a minimal amount of clocking information is
available in the data stream, in the form of transitions on bit cell boundaries. In an arbi-
trary data pattern this may not be sufficient to generate a clock for the data from the data
itself. In the case of SDLC, where the number of consecutive “1s” in the data stream is
limited, a minimum number of transitions to generate a clock are guaranteed.
In FM1 encoding, also known as biphase mark, a transition is present on every bit cell
boundary, and an addition transition may be present in the middle of the bit cell. In FM1 a
“0” is sent as no transition in the center of the bit cell and a “1” is sent as a transition in the
center of the bit cell. FM1 encoded data contains sufficient information to recover a clock
from the data.
In FM0 encoding, also known as biphase space, a transition is present on every bit cell
boundary and an additional transition may be present in the middle of the bit cell. In FM0,
a “1” is sent as no transition in the center of the bit cell and a “0” is sent as a transition in
the center of the bit cell. FM0 encoded data contains sufficient information to recover a
clock from the data.
DATA
NRZ
1 1 0 0 1 0
NRZI
FM1
(Biphase Mark)
FM0
(Biphase Space)
MANCHESTER
Bit Cell Level:
High = 1
Low = 0
No Change = 1
Change = 0
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