4-34 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.13 Option PS—External Signalling Interface
Connector J301 can be fitted to the transceiver rear panel to provide
connection for Codan 8422 or 8511 Selective Call Controllers, radio
teletype (RTTY) or other signalling equipment. When the option is fitted,
J301 is connected to P301 on the Motherboard (refer to 04-02453) and
components associated with the interface are installed on the board. J301 pin
allocations and signal levels are given in Table 1.2.4.
4.13.1 External Selective Calling
Power is applied to the external unit via J301 pins 8 (A rail) and 1(0V).
Audio received on the channel to which the transceiver is tuned is applied
via pin 2 to the external unit. Any FSK signals in this audio are demodulated
and examined for the selective call code preset into the external unit. While
no selective calls are detected, the external unit holds the Quiet Line input to
the transceiver (pin 4) Hi (+10V). In converting this 10V to the levels
required by the transceiver, V303 inverts the Quiet Line input to hold low
the corresponding input to the Microprocessor Controller PCB so that the
audio is not reproduced by the loudspeaker.
On receipt of a selective call code addressed to the transceiver, the external
unit generates tones forming the revertive message to be sent to the calling
station. These are applied to the transceiver via pin 3 while the PTT line
(pin 6) is taken low. The PTT is applied to the Microprocessor Control PCB
and if other conditions are satisfied this switches the transceiver into the
transmit mode so that the tones are transmitted on the channel to which the
transceiver is set. At the same time, the Quiet Line input goes low so that
audio is reproduced as soon as the transceiver returns to the receive mode
and the alarm tones are applied via pin 5 to the auxiliary L/S input so that
the alarm sounds.
When a selective call is initiated at the external unit, the appropriate FSK
signal is generated. The signal is applied to the transceiver at J301 pin 3
while the PTT line is taken low. Consequently if all conditions at the
microprocessor are satisfied the FSK signal is transmitted.
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