Zilog Z80230 - Manual (Page 82)

SCC/ESCC
User Manual
UM010903-0515 SCC/ESCC Ancillary Support Circuitry
75
synchronous. The data encoding selected is active even though the transmitter or receiver is idling
or disabled.
Data Encoding Methods
NRZ (Non-Return to Zero).
In NRZ, encoding a 1 is represented by a High level and a 0 is repre-
sented by a Low level. In this encoding method, 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.
NRZI (Non-Return to Zero Inverted).
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 clock-
ing 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 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.
ESCC:
TxD Pin Forced High in SDLC feature. When the ESCC is programmed for SDLC mode with
NRZI data encoding and mark idle (WR10 D6=0, D5=1, D3=1), the TxD pin is automatically
forced high when the transmitter goes to the mark idle state. There are several different ways
for the transmitter to go into the idle state. In each of the following cases the TxD pin is forced
high when the mark idle condition is reached: data, CRC, flag and idle; data, flag and idle;
data, abort (on under-run) and idle; data, abort (command) and idle; idle flag and command to
idle mark. The Force High feature is disabled when the mark idle bit is reset. The TxD pin is
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