Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 11 Page 191 of 265
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25.1 Introduction
The master time clock system (MTCS) provides the following facilities:
- A software clock.
- A crystal oscillator and standby RTC circuit which is battery supported.
- A software timetable (see section 26).
- Plan timings for Cableless linking in software (see section 21).
- Plan influence definitions in software (see section 21).
The software real time digital clock that controls the system has a resolution of one
second and records seconds, minutes, hours, day, month, year and day of week.
The clock is normally triggered by the 50Hz or 60Hz signals derived from the AC
mains supply, i.e. a convenient source available to all controllers in a linked system
permitting synchronisation to be maintained without inter-connection.
In areas where the stability of the mains’ frequency is unsatisfactory, the controller
can be fitted with a GPS unit from which the controller can obtain the current time,
keeping its clock synchronised.
25.2 Real Time Clock (RTC)
The clock is normally driven from the mains’ frequency. Short mains breaks (50mS
or less) that do not cause the controller to shut down are compensated for. If a
longer break occurs, the standby system on the RTC circuit times the period for
which the power is off. These circuits derive their power from a lithium battery that is
mounted on the Main Processor card.
On restoration of the power, the duration of the power failure is read from the RTC
circuit. This period is then added to the time value saved when the power went off,
to establish the new time.
The battery maintains the circuit operation and powers the RAM so that the time of
day is not lost even if the card is removed from the PCB module.
If the mains fails for more than 30 days (this threshold is alterable via handset PFT
command), or the battery support appears to have failed, an entry is made in the
fault log and the RTC time is considered invalid. The timetable is then not available
and the timetable settings are set to their default state.
IMPORTANT: It is recommended that the controller is switched from its default
mains synchronised clock to its crystal synchronised clock (using the handset
command CTS=2) if the controller is configured to be synchronised to an external
clock (e.g. via a UTC system) that is not mains synchronised.