Each
of
the above registers
performs
a
specific
function
related to
the RS-232-C Interface. An
output to the Master Reset
Port
(E8H)
resets the
UART into
a
known state. This
operation should
be
performed once before attempting to
load or read any
of
the
UART's registers. This
operation
is
performed by
executing
the
following
Z80 instruction:
OUT
(0E8H), A
;
OUTPUT
TO
MASTER
RESET
LOCATION
The contents of A
are
not
important and do not
relate in any
way to
the result of the operation. An input operation from
this port
address
(E8H)
causes the contents of the
Modem Status Register
to be loaded into a register of the
CPU. This is accomplished by
executing the following
Z80
instruction:
IN
A,
(0E8H)
;
READ
MODEM
STATUS
REGISTER
Execution of this instruction loads
the contents of the
Modem
Status Register into A.
The information now in A represents
information
relevant
to
the status of external equipment
connected
to the
Interface.
Any output operation to
Port E9H
loads
the Baud Rate
Generator
with a
constant
which determines the receive and
transmit baud
rates of the serial
channel. This is accomplished by
executing the
following
Z80 instructions:
LD A,
22H
;
LOAD
CONSTANT IN
A
OUT
(0E9H),
H ;
LOAD
BRG
WITH
CONSTANT
Execution
of this sequence results in
the serial channel
being con-
figured to
transmit and receive data at 1 10
baud. Table 5
summarizes
the relationship between the
constants
loaded into the
BRG and the resultant
baud rates
selected. The high-order
nibble
(D7-D4) loaded
into the
BRG determines the transmit
baud rate,
while the
low-order nibble (D3-D0)
determines
the
receive baud rate
of the
serial
channel.
15
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