T801-4 Installation and Operation Manual
Operation
33
© Tait Electronics Limited September 2007
Fault Output Delay
Activation of the alarm outputs generated by the Satsync 7 module can be
delayed by a programmable time period. The delay applies directly to the
GPS and OSC alarms, but will also delay the GPS RX IND and 1PPS
VALID OUT alarms, except when
PORT B
is enabled (DIP switch 1) for the
provision of external 1PPS VALID signals.
The fault delay is useful for masking alarms caused by short and temporary
variations such as:
■
The time it takes to acquire a GPS fix during power-up.
■
A momentary loss of the GPS fix. For example, there may be periods
during the day when the GPS goes in and out of fix.
■
The oscillator accuracy briefly dropping below the ±1 part in 10
8
threshold. This can occur during the initial 30-day ageing period of the
oscillator.
The delay also applies to the associated front panel status LEDs. The LEDs
will be flashing if the alarm is within the delay period.
The factory default delay setting is 60 seconds. The maximum setting is
3600 seconds.
2.9
1PPS Outputs
The T801-4 1PPS outputs (TTL and RS-485) are derived directly from the
active GPS receiver. These outputs are controlled by the GPS alarm(s) via
DIP switch 3 (RS-485 output) and DIP switch 4 (TTL output).
If the T801-4 is using the GPS receiver of a QS
2
RSU as the 1PPS source,
the VALID signal from the RSU becomes the control for the T801-4 1PPS
outputs (along with DIP switches 3 and 4).
When a GPS alarm is activated, indicating that the 1PPS signal is invalid
(e.g. no GPS fix), the T801-4 1PPS outputs can be disabled if DIP switches
3 and 4 are in the off position (factory default). If a fault output delay period
has been applied to the GPS alarm (e.g. to mask a momentary loss of the
GPS fix), the disabling of the 1PPS outputs will also be delayed, and an
invalid 1PPS signal will be supplied during the delay period (refer to
“Fault
Output Delay”
above).
A fault delay period cannot be applied to the RSU VALID signal when the
T801-4 is operating with a QS
2
RSU GPS receiver with firmware earlier
than version 3.00.00.00. However, with version 3.00.00.00 and later
firmware, there will be a 28 second delay (approximately) applied before the
T801-4 1PPS outputs are disabled. This delay is created by the pulse-
stretcher added to the T801-4 VALID inputs for compatibility with the
pulsed VALID signal from the QS
2
RSU with version 3 and later firmware.
With DIP switches 3 and 4 in the on position, the T801-4 1PPS outputs
will not be disabled by GPS alarm(s) or RSU VALID alarms. Any 1PPS
