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the 12-bit binary counter end-of-frame for character
display. Also provided is signal LINE 1 which signals
the dual-display control on the Scope/DVM Control
board that the first character line has been traced.
14.4.5 DTMF ENCODE/DECODE
ICs U36 and U37 contain all circuitry necessary to
encode
and decode dual tone multi
frequency
(DTMF)
signals.
The processor uses four outputs
(PA4-
PA7) from
PIA U38 to instruct the DTMF encoder which tones
to
generate.
When
control
line CB2 of PIA U38 is
brought to a logic 1, the selected DTMF tone is trans-
mitted.
Incoming DTMF
signals
appear at the decoder via
the VERT FROM RNG
SW
line. Each received
tone
must have an amplitude of 19.5 to 615 m V rms with a
minimum duration
of
40 ms.
Consecutive
DTMF tones
must be separated by at least
40
ms. The difference
between high and
low
level
tones
must
not exceed + 10
dB.
After
a
valid
tone
has been received and decoded,
U37
outputs
a hexadecimal number representing the
decoded
signal,
and brings the STROBE output
to
a
logic
1.
PIA U38 then reads the hexadecimal number
and
clears
U37
by
applying
a logic 1
to
its
CLEAR
line.
14.4.6 PRINTER PORT
U44
contains
the circuitry to output
data
to a
printer
in accordance with the RS-232
standard protocoL
The processor transfers data
to U44, where
it is con-
verted
from parallel to
serial
format. U35 then con-
verts the serial
data from TTL levels
to
bipolar levels
( +5 V
/
-5
V),
as long as
a
logic 0 appears on pin 16
of
U44.
14-4 A
/(
14-48 blank)
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