Motorola ASTRO Spectra Instruction Manual page 51

Input/Output Control
Definitions
6881086C22-B
Theory of Operation: Tone Remote Control Board
On the TRC board, A+ is converted to regulated 9.6V through U100. From this
9.6V source, several comparator voltages are produced. SWB+ is routed to a
5V regulator (U132) on the TRC board, which provides regulated 5V to all of
the logic ICs.
One exception to the IC power distribution is the Serial I/O IC (U131). This IC
needs to be powered at all times, regardless of the state of S101-1. A+ is
routed to U131, as well as a continuous 5V source from the regulated 9.6V
source.
AGC_INHIBIT*—This signal is generated by the microprocessor (U123)
through the latch (U124 pin 14). It is used to disable the AGC circuit of the TRC.
TONE_DETECT—This signal is generated by the microprocessor (U123)
through the latch (U124 pin 12). It is toggled from low(0V) to high(5V) when a
guard/function tone is successfully detected. This line provides a discharge
path between C503 and C506, which varies the gain of the AGC circuit.
LPTT*—This signal is generated by the microprocessor (U123) through the
latch (U124 pin 19). It is high during an LPTT to provide additional gain for
LLGT. This additional gain improves the accuracy of the tone decoding
process.
GT/FT*—This signal is generated by the microprocessor (U123) through the
latch (U124 pin 17). During the detection of guard tone, the bandpass filter is
switched into the path of the audio. This allows isolation of the 2175 Hz signal
and provides more accurate decoding. Once the guard tone has been
detected, the signal line goes low. This switches the bandpass filter out of the
path to allow the function tone(s) to pass to the limiter/decoder undisturbed.
RX_MUTE—This signal is generated by the microprocessor (U123 pin 36). It
is used to mute receiver audio (DET_AUD) when an HLGT has been detected
or an LPTT is in progress. DET_AUD is also muted during a local PTT to block
ASTRO noise from entering the wireline.
WL_MUTE—This signal is generated by the microprocessor (U123 pin 35). It
is used to mute the wireline during a remote PTT, and when S100-2 is in the
ON position (mute local audio and local trunking tones).
AGC_RESET*—This signal is generated by the microprocessor (U123)
through the latch (U124 pin 16). It is used to provide an additional discharge
path for C503 and C506 (changing the feedback path of U112A in the AGC
circuit). Upon detection of HLGT by the microprocessor, the AGC_RESET line
will go high. AGC_RESET will go low at the end of LPTT.
SPKR_UNMUTE—This signal is available at Accessory Connector 2 pin 25 as
an output to provide an indication to accessories when the radio is receiving
valid audio and the speaker is unmuted. This signal is configurable for either
active high or active low operation using Plug P102. See Table 6 on page 11.
When configured for active high, the emitter of Q117 is used to determine the
state of the speaker. (If the voltage at Q117's emitter is low, the speaker is
muted, and if the voltage here is high, the speaker is unmuted.) Similarly, if
active low configuration is selected, the collector of Q118 is used to determine
the state of the speaker. (If the voltage at Q118's collector is low, the speaker
is unmuted, and if the voltage here is high, the speaker is muted.)
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