Radio Shack TRS-80 Service Manual page 17

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2! Buzzer circuit
SDO
CMT
OUT 10
i-0
rc6
i 9
p
O PB2 1:-
1
,-
1
-~-t<}-
..
R
T
CMT IN
Yee
Y ee
The control signal CMT OUT is sent out from the pin SOO o f the
1 /0
port which sounds the buzzer in
combination with the
low m 1e or either the programmed output from the
1 /0
port or CMT IN sent from the cassette tape deck.
3) RAM R/W signal circuit
R/W RAM I
R/W
CPU
ADtO
4) ON key double action p reventive circuit
Y ee
DAP
202
This circuit prevents writing if Al} IO Is in high state during
the write mode (when R{W is
' ° i:ri
This is
to prevent wrong
operation when a specific key. or
m
is
pushed with-
out perfonning "NEW 0
E!llTER" after battery
replacement.
This circuit consists of the Schmitt circuit that prevents the
possibility of setting the input flag or the Ui58 I I
1 /0
p0rt
which depends on how the ON key i.s pushed.
BF
I
--N'l'-.....__~ s'°i
)'
4
'--._ _ _,, I /0 PORT
TC50H OOI
I ~36PF-89PF
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PB7
-14-