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Port Addresses
The
Radio Shack RS-232-C
Interface
is
an
1/O-mapped
device
which
uses
four port addresses
(E8H.
E9H.
EAH
and
EBH)
for
communica-
tion
with
the
CPU. I/O-mapped
devices
in a
Z-80 system
use the
lower
address
bits
(A0-A7)
in
conjunction with
the
IN* and
OUT*
signals to
address the desired
ports.
Figure
5
shows
the
timing
associated
with
this
addressing
scheme.
SYSTEM CLOCK
A0-A7
IN*
DATA
BUS
OUT*
DATA
BUS
T2
I
TW
PORTADRESS
OUT
J-
-GHh
1.8
Mhz
READ
CYCLE
WRITE CYCLE
FIGURE
5.
I/O
TIMING
DIAGRAM
Decode
Logic
The
block
labeled
DECODE
LOGIC
in
Figure
4
provides
eight
low-going
strobes that
are
used
by
the
RS-232-C. These
strobes
allow
the
CPU
to select
any of
the
registers
and
latches
in
the
RS-232-C
Interface for
an input or
output
operation.
Table 4
summarizes
the
I/O port address
allocation
as
implemented
in
the
RS-232-C
Interface
and
the function
performed.
ADDRESS
OUT
(I/O
PORT
WRITE)
IN
(I/O
PORT READ)
E8H
Master Reset
(Any
Data)
Modem
Status Register
E9H
Baud
Rate
Select
Configuration Sense
Switches
EAH
UART
Control
Register
and
Handshake
Latch
UART
Status Register
EBH
Transmit Data
Register
Received Data
Register
TABLE
4.
I/O
MAPPED
MEMORY
ALLOCATION
14
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