Zilog Z80230 - Manual (Page 107)

SCC/ESCC
User Manual
UM010903-0515 Data Communication Modes
100
Byte-Oriented Synchronous Transmit
Once Synchronous mode has been selected, any of three of the following sync character lengths
may be selected:
• 6-bit
• 8-bit
• 16-bit
The 6-bit option sync character is selected by setting bits 4 and 5 of WR4 to zeros and bit 0 of
WR10 to one. Only the least significant six bits of WR6 are transmitted.
The 8-bit sync character is selected by setting bits 4 and 5 of WR4 to zeros and bit 0 of WR10 to
zeros. With this option selected, the transmitter sends the contents of WR6 when it has no data to
send.
For a 16-bit sync character, set bit D4 of WR4 to 1 and bit D5 of WR4 and bit D0 of WR10 to 0. In
this mode, the transmitter sends the concatenation of WR6 and WR7 for the idle line condition.
Because the receiver requires that sync characters be left-justified in the registers, while the trans-
mitter requires them to be right justified, only the receiver works with a 12 bit sync character.
While the receiver is in External Sync mode, the transmitter sync length may be six or eight bits,
as selected by bit D0 of WR10.
Monosync and Bisync modes require clocking information to be transmitted along with the data
either by a method of encoding data that contains clocking information, or by a modem that
encodes or decodes clock information in the modulation process. See the Monosync message for-
mat displayed in Figure .
The Bisync mode of operation is similar to the Monosync mode, except that two sync characters
are provided instead of one. Bisync attempts a more structured approach to synchronization
through the use of special characters as message headers or trailers.
Character-oriented mode is selected by programming bits D3 and D2 of WR4 with zeros. This
selects Synchronous mode, as opposed to Asynchronous mode, but this selection is further modi-
fied by bits 5 and 7 of WR4 as well as bits 1 and 0 of WR10. During the sync character-oriented
modes, except in External Sync mode, the state of bits 7 and 6 of WR4 are always forced inter-
nally to zeros. In external sync mode, these two bits must be programmed with zeros ( Table ). The
combination, other than 00 in External Sync mode, puts the SCC in special synchronization
modes.
Registers Used in Character-Oriented Modes
Reg Bit No Description
WR4 3 (=0) select sync mode
2 (=0)