Siemens EH.75M.11. Spezifikationen PDF herunterladen (Seite 126)

T400 Controller Facilities Manual

667/EB/20200/000 Page 11-1 Issue 9

11. THE MASTER TIME CLOCK SYSTEM

11.1 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 12).

- Plan timings for cableless linking in software (see section 9.9).

- Plan influence definitions in software (see section 9.9).

The software real time digital clock which controls the system has a resolution of one

second and records seconds, minutes, hours, days and weeks. The clock is normally

triggered by 50Hz signals derived from the a.c. mains supply, i.e. a convenient source

available to all controllers in a linked system permitting synchronism to be maintained

without inter-connection.

In areas where the stability of the mains frequency is unsatisfactory, a high precision

crystal oscillator signal can be used in place of the mains derived signal. This may be

introduced via the handset using the CTS command.

The handset command MST may be used to specify a time when a changeover from

crystal to mains is to occur.

11.2 REAL TIME CLOCK (RTC)

The RTC maintains the current time of day in hours, minutes, seconds and week

numbers.

The clock is set up via the handset using the WEK, STM & CKL commands.

The current time of day may be read via the handset using the TOD command.

The clock is normally driven from the mains frequency. Short mains breaks (50mS or

less) which do not cause the controller to shut down are compensated for in the clock

software module. If a longer break occurs, the standby system on the RTC circuit

counts the period for which the power is off. These circuits derive their power from a

lithium battery which is switched and mounted on the Main Processor PCB.

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 PCB is removed from the PCB module.

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Siemens EH.75M.11. Spezifikationen Seite 126