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An
output
to the Port
Address
EAH
loads the
UART
Control
Register
and Handshake
Latch.
These
two
functions
are
separate
and
distinct,
with each
using
different
bits
of
the
register for
control.
The
upper
five bits
(D7
-
D3)
of
this
Port
Address
determine
the
word
length,
stop
bits,
and
parity
convention of
the
serial
channel.
The
lower
three
bits
(D2
-
DO)
are
latched
control outputs,
one
of
which
disables
Transmit Data
when
low,
and
the
remaining
two
control
Data Terminal
Ready
and
Request-To-Send.
A
certain
degree
of caution should be used
when
outputting
to
this
Port
Address
to avoid
an
unwanted
interaction.
Table
6
summarizes
the
registers
and
bit
allocations
of each
for
the
RS-232-C
Interface.
DATA
BIT
MODEM
STATUS
REGISTER
CONFIGURATION
SENSE SWITCHES
UART CONTROL
REGISTER
AND
HANDSHAKE LATCH
UART
STATUS
REGISTER
D7
Clear to
send
Pin 5
DB-25
Even
Parity
Enable
1
= even,
=
odd
Even
Parity
Enable
1
=
even,
=
odd
Data Received
1
= Condition
true
D6
Data
Set
Ready
Pin 6
DB-25
Word
length Select
1
Word
length Select
1
Transmitter Holding
Register
Empty
1
=
Condition
true
D5
Carrier
Detect
Pin
8
DB-25
Word
length Select 2
Word
length Select 2
Overrun
Error
1
=
Condition
true
D4
Ring
Indicator
Pin
22
DB-25
Stop
Bit Select
1
=
2
bits,
=
1
bit
Stop
Bit Select
1
=
2
bits,
=
1
bit
Framing
Error
1
=
Condition
true
D3
Unused
Parity Inhibit
1
disables parity
Parity Inhibit
1
disables parity
Parity
Error
1
=
Condition
true
D2
Unused
Baud
Rate
3
Break
Disables
Transmit
data
Unused
Dl
Receiver Input
UART
P20
Baud
Rate
1
Request
to
send
Pin
4
DB-25
Unused
DO
Unused
Baud
Rate
2
Data Terminal
Ready
Pin
20
DB-25
Unused
IN
0E8H
IN
0E9H
OUT
0EAH
IN
0EAH
TABLE
6.
BIT
ALLOCATIONS FOR
REGISTERS
AND
LATCHES
17
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