ZiLOG Z-80 SIB User Manual page 40

Bit
of the
command word
is
the transmit enable bit
(TxEN)
Data transmission from
the
USART cannot
take
place unless
TxEN
is set in the
command register. Figure
5
defines
the
way
in
which TxEN, TxE, and TxRDY combine
to
control
transmitter operations
Bit
1
is the
data terminal ready
(DTR) bit.
When
the
DTK
command bit
is
set, the
DTR- output connection
is
active
(low).
DTR
is used to
advise
a
modem that the data terminal
is
prepared
to
accept
or
transmit data.
Bit
2
is the
receiver enable command bit
(RxE),
RxE
is
used
to
enable
the
RxRDY output signal.
RxE
prevents the RxRDY
signal from being generated
to
notify the processor that
a
complete character
is
framed
in
the
receive character buffer.
It does not inhibit the assembly of data characters
at the
input, however. Consequently,
if
communication circuits are
active, characters will be assembled by the receiver and
transferred
to the
receiver character buffer.
If
RxE
is
disabled,
the
overrun error
(OE)
will probably
be set; to
insure proper operation, the overrun error
is
usually reset
with the same command that enables RxE
32