8525B/8528 Technical Service Manual Technical Description 4-35
Publication No: 15-02036 Issue 6
4.13.2 Selective Call Scanning
With Option PS fitted and Codan 8422 or 8511 units connected to J301, the
scan facility of the transceiver can be used to scan up to eight channels for
selective calls. This mode of operation is initiated by selecting Scan on the
transceiver front panel and Mute on the external equipment. Selection of the
scan mode is indicated to the external equipment by the Scan output line
(pin 7) being taken high via V301 and V302, which convert the levels in the
transceiver to the +10V or 0V levels required by the external equipment.
The external equipment switches the Quiet Line input high in response. This
is inverted by V303 and applied to the Microprocessor Controller PCB. As a
result of this the speaker is muted, the S’call Mute indicator is lit and
scanning is restricted to the first eight channels of the scan program.
When any selective call is detected, the external equipment takes the PTT
line low. This does not cause a PTT function but stops the scan long enough
for the whole selective call message to be decoded. If it is found not to be
addressed to the transceiver, the PTT line is released and scanning is
resumed.
If, however, the selective call is found to be addressed to the transceiver the
alarm tones generated by the external unit are connected to pin 5 of J301.
The alarm tones are applied to the audio circuits so that they are reproduced
by the loudspeaker. They are also applied to the Microprocessor Control
PCB (04-02451) where they are integrated by diode D6 and capacitor C23.
The voltage on C23 falls rapidly to the low logic level and this is detected by
the microprocessor which initiates the transmit mode. The revertive tones
are produced by the external equipment and applied to the transceiver.
During this period the microprocessor ensures that scanning stops and the
Quiet Line input goes high to unmute the receiver. The microprocessor also
lights the Called indicator and the decimal point in the channel display on
the transceiver front panel.
If the call is answered immediately (by the operator pressing the microphone
PTT switch) the alarm tones will cease, the transceiver will leave the scan
mode, i.e. it will not scan again until the Scan pad is pressed and the Called
indicators will be extinguished.
If, however, the call is not answered within a short time the alarm tones will
cease and the audio will be muted again. Scanning will be resumed
approximately 2.5 minutes after this but the Called indicator and the decimal
point will remain lit until the PTT switch is pressed (refer to Fig 4.1.)
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