ZiLOG Z-80 SIB User Manual page 42

Bit
3
is
the send break command bit
(SBRK)
.
When SBRK
is
set, the
transmitter ouput
(TxD)
is
interrupted
and
a
continuous binary
"0"
level (spacing)
,
is
applied
to the
TxD
output signal. The break will continue until
a
subsequent
command word
is
sent to the USART to remove SBRK,
Bit
4
is
the error reset bit
(ER)
,
When
a
command word
is
transmitted with the ER bit set, all three error flags
in
the
status register are reset. Error reset occurs when the
command word
is
loaded into
the USART.
No latch
is
provided
in
the command register
to
save the ER command bit.
Bit
5,
the request
to
send command bit
(RTS)
,
sets
a
latch
to
reflect the RTS signal level.
The output
of this latch
is
created independently
of
other signals in the USART. As
a
result, data transfers may be made by the
microprocessor
to
the
transmit register,
and data may
be
actively transmitted
to the
communication line through TxD regardless
of the
status of
RTS.
Bit
6,
the internal reset (IR),
causes the
USART
to
return
to
the idle mode. All
functions within
the
USART cease,
and no
new operation can
be
resumed until
the
circuit
is
reinitiallized.
If the
operating mode
is to
be altered
during
the
execution
of
a
microprocessor program,
the
USART
must first be reset. Either the external reset connection
can be activated,
or
the
internal reset command can
be sent
to the
USART, Internal reset
is
a
momentary function
performed only when
the
command
is used.
Bit
7
is
the
enter hunt command bit
(EH)
,
The enter hunt
mode command
is
only effective
for the
USART when
it is
operating
in the
synchronous mode.
EH
causes
the
receiver
to
stop assembling characters
at the
RxD input and start
searching for the
prescribed
sync pattern. Once
the "enter
hunt" mode has been initiated, the search
for the
sync
pattern will continue indefinitely until
EH
is
reset when
a
subsequent command word
is
sent,
when
the IR
command
is
sent
to the
USART,
or
when sync characters are recognized.
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