M8SL2-00-002-812 Circuit Operation D2.5
© Copyright Tait Electronics Limited August 2004. All rights reserved.
nally to the loudspeaker. It will therefore also reach the guard tone detector. Rx audio is
therefore notch filtered at the guard tone frequency to prevent false guard tone detec-
tion. If the line interface is 4-wire this filtering is unnecessary but the talk-through gate
must be turned on to send Rx audio to the loudspeaker. If the T803 is in repeater mode
the transmitter is keyed and will remain so until the RX_MUTE line goes high. Note that
the T803 will not key the transmitter if the receiver’s CTCSS is defeated (CD_OUT is
low) to prevent receiver channel noise being transmitted.
From the description above of the keytone/function tone detection it can be seen that
keying from line has priority over repeater mode keying and audio gating.
2.3 Alarm Monitoring
The T803 microprocessor monitors eight alarm sources:
• Transmitter forward power
• Transmitter reverse power
• low battery
• line fail
• four external closure alarms
When an alarm is triggered it is latched as indicated by the corresponding LED on the
front panel. If the T803 is keying the transmitter or sending receiver audio to the line the
latched alarm is ignored. Once the T803 re-enters the idle state the latched alarms are
actioned. The microprocessor reads the EEPROM to determine what tone sequence the
user programmed and instructs the DSP accordingly. If the tone sequence is to be sent to
the transmitter, it is keyed 500ms before the tones are generated. Additionally the user
may program any of the 4 auxiliary output ports as a response to an alarm event. The
micro will activate (pull low) these outputs if enabled. If several alarms are latched
together they are actioned in the order of priority:
1. External closure alarms 0 to 3
2. Reverse power
3. Forward power
4. Low battery
5. Line fail
If the programmed alarm action is a pip tone, the micro will instruct the DSP to append
200ms burst of 600Hz tone to each transmission (to line and/or radio).
Note: Sub-rack alarms (Fwd and Rev Power, Low battery or Line fail) cannot be
re-triggered until they are cleared. The 4 external closure alarms however
will re-send any user programmed tone sequence if re-triggered when
already latched.
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