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5
CIRCUIT DESCRIPTION
5.8
External Squelch
The audio output can be muted through pin 8 of the receiver system connector P1.
When pin 8 is pulled to less than 1 volt above ground, the micro-controller U15 will
mute the audio output.
This facility can be used to mute the audio during transmission, as is required in
single frequency systems, by simply connecting pin 8 of the receiver to the
transmitter T/R relay driver output (pin 16 on Eclipse transmitters).
5.9
Microprocessor Controller
The microprocessor controller circuit uses an advanced eight bit processor and
several support chips. The processor U15 includes EE memory for channel
frequencies, tones, and other information. It also has an asynchronous serial port and
an analog to digital converter.
The program is stored in U12, a CMOS EPROM. U13 is an address latch for the low
order address bits. U11 is used to read the channel select lines onto the data bus. U7
is an address decoder for U11 and U12. U14 is a supervisory chip which keeps the
processor reset unless the +5 Volt supply is within operating limits. U16 translates
the asynchronous serial port data to standard RS232 levels.
The analog to digital converter is used to measure the received signal strength, tuning
voltage, dc supply voltage and the carrier squelch setting.
5.10
Carrier Operated Switch
The carrier operated switch is an opto-coupled (ISO1) output. Internal jumpers
(JP4,7,8,9) can be connected to provide loop source, loop switch, free switch and
various other configurations.
The COS can be set to be active (switch closed) on carrier or active in the absence of
carrier.
The generic term ``Carrier Operated Switch'' may be misleading in this case. SINCE,
if a sub-audible tone has been programmed for the channel in use, the COS is
controlled by carrier and tone detection.
RF Technology R220
5.8 External Squelch
Page 15
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