RFL 9780 RFL Electronics Inc.
April 8, 2003 6-20 (973) 334-3100
6.5.1.7 GUARD-BEFORE-TRIP AND TRIP-AFTER-GUARD TIMER
The guard-before-trip/trip-after-guard (GBT/TAG) timer ensures that a noise-free guard input has been
received for a preset time interval before the trip channel is opened. The guard can disappear because of
excessive noise on the communications channel, a disconnected line, equipment failure, or other
reasons. The only valid reason for the guard to disappear is when the transmitting 9780 shifts from
guard to trip; when this happens, the guard should be replaced by a trip input. If a trip input is not
detected within a preset time interval after the guard input ceases, the GBT/TAG timer assumes that the
condition was not valid, and the trip channel is disabled.
If the pre-guard timer has been satisfied, the logic-high GUARD_VAL signal will activate the
GBT/TAG timer. Once guard before trip requirements are satisfied, the trip channel is opened by the
INHIBIT signal, and will remain open for a period including a preset time after guard is no longer
valid. If the trip after guard requirements are not satisfied, the trip channel will be closed by the
INHIBIT signal and will remain closed until the pre-guard and GBT timers are once again satisfied.
When a logic-high TRIPPED input is applied to the GBT/TAG timer (indicating a valid trip is
received), the GBT/TAG timer is defeated, keeping the trip channel open. The GBT/TAG can only be
re-enabled by a logic-high GUARD_VAL signal, indicating that a solid guard signal is received for a
minimum of 100ms.
When neither trip nor guard are present, an abnormal condition exists which will trigger the alarm
timer.
6.5.1.8 BIPOLAR NOISE DETECTOR AND TIMER
The bipolar noise detector allows the 9780 Logic Module’s security and speed to adapt to the
environment; fast for clear-channel conditions, or slower and more secure when subjected to high noise
levels. Bipolar noise is defined as a noise that causes a bipolar system to change from present state to
the alternate state and then back to its original state. In the 9780, this would be a change from guard to
trip to guard, or from trip to guard to trip. The bipolar detector (if enabled) recognizes this condition
and enables the bipolar timer; this effectively lengthens the time-out period of the pre-trip timer when
this abnormality occurs. The noise detector will not be active when a valid trip is received, because
after noise has disappeared, a trip will be detected with no return to guard.
If either a valid trip or guard output is being produced (which is the normal state), the logic-high
NORMAL signal will disable the bipolar detector. The bipolar detector’s output signal, BNOISE, will
be a logic-high preventing the bipolar timer from adding the additional time requirement to the to open
the trip channel.
When the received signal goes from guard to trip, the guard output will disappear. Because there is
momentarily no valid guard or trip output, the NORMAL signal will be initially logic-low. The logic-
high TRIP_INPUT command from the receiver along with a logic-low GUARD_INPUT (absence of
guard) will drive the NORMAL signal high if the trip satisfies the pre-trip timer without a disturbance.
The noise detector circuit is disabled, its output BNOISE will remain logic-high, and the additional
time requirement is not required to open the trip channel.
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