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10.4.2 Dial Tone Detect
The dial tone detect circuitry prevents the dial
tone received by a modem from being decoded
as random data.
The dial tone detect circuit should be activated
only if both of the following conditions are
present:
1.
If the 2400 bps modem automatically
answers incoming calls, and
2.
If the DTE does not time out and does not
automatically disconnect when the dial
tone is detected, thereby causing false data
to be decoded.
For example, assume the DTE is set up for unat-
tended operation and an incoming call is a wrong
number. When the calling party hangs up, the
modem will decode the dial tone as data. Dial
tone detect will prevent this error but it will not
disconnect the line. Disconnecting the line must
be done through programming.
Note:
Normally, the dial tone detect circuit
will not be activated in the modem because if it is
adjusted incorrectly, the 'receive data' line may
be incorrectly clamped to a mark level. Further,
this clamping may be intermittent and cause con-
fusion during troubleshooting. Such clamping
could occur if the dial tone detect potentiometer
is adjusted to be too sensitive (too far clockwise),
which could cause the 'receive data' line to be
clamped when valid data is being received.
Adjustment
To activate the dial tone detect circuit, attach a
jumper from H3D09 to G2B04 on the B-A 1 board.
This adjustment is very sensitive and could need to
be repeated before proper operation is attained.
The dial tone adjustment procedure is as follows:
If
no:
The dial tone detect adjustment must be made.
Note:
If the OPERATE indicator stops blinking
in less than three seconds, then 'receive data'
courd be intermittently clamped to a mark level
during normal data transfer. (The potentiometer
is adjusted too far clockwise and the dial tone
1.
Set the test/operate switch to the T4 position.
detect adjustment must be made.)
2.
Set the TALK/DATA switch to the data
position.
3.
Lift the telephone handset, pull up the exclu-
sion key, and listen for the dial tone.
Service Check
If
there is a dial tone:
Cradle the handset. Check to see that the
SIGNAL indicator comes on and the OPER-
ATE indicator starts blinking. (The OPE R-
ATE indicator should stop blinking [comes
on steady] within three to 10 seconds.)
If
there is no dial tone:
The previous call has not been disconnected.
Either hang up and try again or call last caller
and ask them to disconnect.
Does the OPERATE indicator stop blinking (comes
on steady) within three to 10 seconds after the
SIGNAL indicator comes on?
If
yes:
Dial tone detect is working correctly.
Adjustment
Important: The potentiometer adjustment must
be made within 20 to 25 seconds because on
some telephone systems, a prerecorded message is
put on the line after an inactivity period of from
20 to 30 seconds.
To adjust the dial tone detect circuit do the
following:
1.
If the OPERATE indicator is not blinking,
turn the potentiometer on card H3 counter-
clockwise (CCW) until the OPE RATE indi-
cator starts to blink.
2..
Adjust the potentiometer clockwise (CW)
very slowly, until the OPERATE indicator
stops blinking (comes on steady), then
3.
turn the potentiometer 1/8 of a turn farther,
clockwise.
Lift the handset, momentarily, and replace
it in the cradle. The OPERATE indicator
should stop blinking within three to 10
seconds. Since this adjustment is very sensi-
tive, the adjustment procedure might have
to be repeated.
If satisfactory results cannot be obtained from the
adjustment procedure, replace card H3.
Data Communications
10-19
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