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S5-115U Manual
13
Integral Real-Time Clock
(only CPU 943/CPU 944 with two serial interfaces)
The integral real-time clock offers the following additional methods of controlling the process:
•
Alarm clock function
e.g. for monitoring the duration of a process
•
Operating hours counter
e.g. for monitoring inspection intervals
•
Real-time clock function
e.g. for establishing the time at which the CPU stopped in the event of a fault
The clock has an accuracy of±2 seconds per day at a temperature of 15 °C. This accuracy changes
with temperature according to the following formula:
Temperature dependancy (T
Example: Tolerance at 40 ° C: ±2 s - 3.5 · (40 - 15)
13.1
Setting the System Data Parameters
The hardware clock of the CPU 944 requires a clock data area and a status word to enable use of
the clock.
The following information must be stored in system data word 8 to 10:
•
The location of the clock data area
•
The location of the status word
You can initialize the integral clock via the integral DB1 (parameter block CLP) (see Section 11.3).
The following explains the other method of initializing the integral clock.
Initializing the Integral Clock in the System Data Area
You can initialize the clock function in a function block, programmed by you, which is best called
via one of the two restart organization blocks OB21 or OB22. The parameters are stored in the
relevant system data word in the function block using transfer operations (e.g. "T RS", "TNB").
System data words 8 to 10 are responsible for the location of the clock data area and the status
word. These data words determine whether you are dealing with a flag or a data block. They also
establish the precise location within the defined area.
The operating system does not implement a standard setting of the system data locations so the
clock is switched off under normal circumstances.
Table 13-1 gives details of the meanings of the individual bytes of system data words 8 to 10.
System data words 11 and 12 are explained after Table 13-1.
EWA 4NEB 811 6130-02b
in °C): t in ms/day=±2s - 3.5 · (T
amb
2
ms/day
Integral Real-Time Clock
2
- 15)
ms/day
amb
ca. 0 to - 4 s/day.
13-1
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