EXPANDED MCS-48 SYSTEM
A high to low transition of the
PROG
line indi-
cates that address
is
present while a low to
high transition indicates
the
presence of data.
Additional 8243's may
be
added to the four bit
bus and chip
selected using additional output
lines from the 8048/8748.
I/O
Port
Characteristics
Each
of the four 4-bit ports of
the
8243 can
serve
as
either input or output
and
can pro-
vide high drive
capability
in
both
the high
and
low state.
3.3.2
I/O Expansion
with
Standard
Peripherals
Standard MCS-80/85 type
I/O
devices may
be
added to the MCS-48 using the same bus
and timing used
for
Data Memory expan-
sion.
I/O devices reside on the Data
Memory bus and in the data memory
address space and are accessed with the
same
MOVX instructions.
See
the
previous
section on data memory expansion
for
a
description
of
timing. The following are a
few
of
the Standard MCS-80 devices which
are very
useful in MCS-48 systems.
8214
Priority Interrupt Encoder
8251
Serial Communications Interface
8255 General Purpose Programmable I/O
8279 Keyboard/Display Interface
8253 Intervallimer
See
Chapter 7 for detailed data sheets on
these and other components.
INT
P20
8048
RD
WR
BUS
KEYBOARD/DISPLAY
INTERFACE
3.3.3 Combination Memory and I/O
Expanders
As
mentioned
in
the sections
on
program
and
data memory expansion the 8355/8755
and
8155
expanders also contain I/O capability.
8355/8755: These two parts are
ROM
and
EPROM equivalents and therefore contain
the same
I/O structure. I/O consists of two
8-bit ports which
normally reside
in
the exter-
nal
data memory address space and are ac-
cessed with
MOVX instructions. Associated
with each port
is
an
8-bit Data Direction Reg-
ister which defines each bit
in
the port
as
either
an
input or
an
output. The data direction
registers are
directly addressable thereby al-
lowing
the user to define under software con-
trol each individual bit of
the
ports
as
either
input or output.
All outputs are statically
latched
and double buffered. Inputs are not
latched.
8155/8156: I/O on the 8155/8156
isconfig-
ured
as
two 8-bit programmable
I/O
ports
and one 6-bit
programmable port. These
three registers and a
Control/Status regis-
ter are
accessible
as
external data memory
with the
MOVX instructions. The contents
of
the control register determines the mode
of
the three ports. The ports can
be
programmed
as
input or output with
or
without associated handshake communi-
cation
lines.
In
the handshake mode, lines
of
the six,..bit port become input and
out-
put strobes for the two 8-bit ports.
See
the
8
KEYBOARD
INPUTS
INT
CfD
8279
SCAN
KEYBOARD
OUTPUTS
RD
DISPLAY
WR
(A) DISPLAY
OUTPUT
(B)DISPLAY
OUTPUT
CS
-
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