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5.2
.
10.2
ACM Circuit Description
The circuit
description
centers around the 8279
Programmable
Keyboard
/
Display
Matrix Encoder ( IC 5 )
.
This sets the audio
status buffers in an X
-
Y
pattern
or a matrix
.
Normally
,
this
device detects when either a
key
is
pressed
within the meter
housing
or when
a
command from the CPU enables a function
within the network
housing
.
In the
case
of
a keypress
,
the
8279 will
provide a unique
code to the CPU
indicating
which
•
of the
keys was pressed
.
This is of
course accompanied
with
an interrupt signal
( IRQ )
to be fed to the CPU
.
O
When the CPU reads the
8279
,
the READ ( RD ) and CHIP SELECT
( CS
) lines
go
LOW and receives the data from the 8279
.
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.
itches
can be external as on a channel status or a remote controller
unit
.
When
writing
to the 8279
,
the
WRITE line ( WR )
goes
LOW and
the
S
0
through S
2
outputs
of the 8279 are fed to two
74 LS
138
(
3
-
to
-
8
line decoders )
.
ICs
3
/ 4 in addition
,
an address line
is selected
causing
the chip select line ( CS
)
on
the 8279 to
go
to a LOW true state
.
The outputs of the 74 LS 138 drive two
sets of lines
.
This first set is
key seeking
the second LED
illustrations of those
keys
.
IC 3
drives the
keyboard scan
line for all of the
keys
located
on the meter housing
.
The return line for the
keyboard
is
then fed back to the 8279
.
IC 4
drives the LED
row lines
.
This is fed via a 74 HC 04
IC 6
to provide
proper
logic
level drive and isolate the
LED
drive
from the
output
of the 8279
.
The LED column lines are then
fed to the 8279
.
The mute
switching
lines for the monitor
speaker are
fed from
the
Mute
Switching
Board ( MSB ) to
a
flip
-
flop
located on the ACM
.
IC 2 ( MC 14013 or
4013 ) latches
the information for the MSB
.
The
output
of the 4013 is then
fed to a DG 212
Digital
Switch ( IC 1 )
.
These are the command
lines to turn the LEDs located within the mute
keys on
and
off
.
In addition
,
IC 1 also routes
a feed from the meter
drive
output
to the audio attenuator located in the meter
housing
.
The commands for both the Mute
Switching
Board
( MSB ) audio attenuator
are
in
parallel
.
5
-
14
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