Zilog Z80230 - Manual (Page 83)

SCC/ESCC
User Manual
UM010903-0515 SCC/ESCC Ancillary Support Circuitry
76
forced High on the falling edge of the TxC cycle after the falling edge of the last bit of the clos-
ing flag. Using SDLC Loop mode is independent of this feature.
This feature is used in combination with the automatic SDLC opening flag transmission fea-
ture, WR7' D0=1, to assure that data packets are properly formatted. Therefore, when these fea-
tures are used together, it is not necessary for the CPU to issue any commands when using the
force idle mode in combination with NRZI data encoding. If WR7' D0 is reset, like the SCC, it is
necessary to reset the mark idle bit (WR10 D2) to enable flag transmission before an SDLC
packet is transmitted.
FM1 (Bi-phase Mark).
In FM1 encoding, also known as biphase mark, a transition is present on
every bit cell boundary, and an additional 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 cen-
ter of the bit cell. FM1 encoded data contains sufficient information to recover a clock from the
data.
FM0 (Bi-phase Space).
In FM0 encoding, also known as bi-phase 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 cen-
ter of the bit cell. FM0 encoded data contains sufficient information to recover a clock from the
data.
Manchester (Bi-phase Level).
Manchester (bi-phase level) encoding always produces a transition
at the center of the bit cell. If the transition is Low to High, the bit is 0. If the transition is High to
Low, the bit is 1. Encoding of Manchester format requires an external circuit consisting of a ‘D’
flip-flop and four gates ( Figure on page 77). The SCC is used to decode Manchester data by using
the DPLL in the FM mode and programming the receiver for NRZ data (See Introduction on page
71).
Data Encoding Initialization.
The data encoding method is selected in the initialization proce-
dure before the transmitter and receiver are enabled, but no other restrictions apply. Note that in
NRZ and NRZI, the receiver samples the data only on one edge, as displayed in Figure on page
75. However, in FM1 and FM0, the receiver samples the data on both edges. Also, as displayed in