T801-4 Installation and Operation Manual
System Configuration
69
© Tait Electronics Limited September 2007
Setting DIP Switch 4
In this configuration, when the primary 1PPS signal becomes invalid, an
alarm is triggered on the T801-4 which causes a changeover to the standby
1PPS signal. When setting DIP switch 4, please consider the following
points:
■
If switch 4 is on (recommended setting), the 1PPS TTL outputs will
remain enabled during the changeover, and the integrity of the outputs
is quickly restored by the valid standby 1PPS signal.
Important
With this setting, the 1PPS TTL outputs will remain ena-
bled even if the standby 1PPS signal also becomes invalid.
This is because the 1PPS output from the RSU GPS
receiver is not disabled when the receiver loses GPS track-
ing. The maximum drift of the invalid 1PPS pulse could
be quite large, therefore it should not be used for precise
synchronisation. For this reason, it is extremely important
that you monitor the T801-4 effectively. Refer to
“Alarms
and Monitoring”
below for more details.
■
If switch 4 is off, the 1PPS TTL outputs will be disabled when the
changeover is triggered, regardless of the validity of the standby 1PPS
signal. The outputs are only re-enabled when
both
1PPS signals are
valid.
4.6.2
Alarms and Monitoring
Alarms During
Power-up
Because of the time it takes the GPS receivers to acquire a fix (refer to
“GPS
Fix Acquisition Times” on page 31
), it is very probable that an alarm
condition will occur after power-up, but before a fix is acquired. This is
quite normal. If DIP switch 5 is set on during this alarm period, a “false”
changeover event will occur. If this does happen, set DIP switch 5 off, wait
until the oscillator alarm has been released, then switch it back on.
Note
To avoid confusion, we recommend that you set DIP switch 5 on
only after the alarms have been released.
Monitoring the
1PPS Signals from
the RSU
If one of the 1PPS signals becomes invalid, the T801-4 will effect a
changeover to the other input (assuming DIP switch 5 has been set on), and
the Satsync 7 module(s) will continue to have a valid 1PPS signal as a
reference.
However, an important characteristic of this system is that the RSU GPS
receivers do
not
disable the 1PPS signal when they lose GPS tracking. This
has implications for the Satsync 7 oscillator and GPS alarms when both
1PPS signals become invalid.
