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FX Series Programmable Controllers Devices in Detail 4
4-18
4.9.4 Timers Used in Interrupt and ‘CALL’ Subroutines
If timers T192 to T199 and T246 to T249 are used in a CALL subroutine or an interruption
routine, the timin
action is updated at the point when an END instruction is executed. The
output contact is activated when a coil instruction or an END instruction is processed once the
timers current value has reached the preset (maximum duration) value.
Timers other than those specified above cannot function correctl
within the specified
circumstances.
When an interrupt timer (1 msec resolution) is used in an interrupt routine or within a ‘CALL’
subroutine, the output contact is activated when the first coil instruction of that timer is
executed after the timer has reached its preset (maximum duration) value.
4.9.5 Timer Accuracy
Timer accurac
can be affected b
the pro
ram confi
uration. That is to sa
, if a timer contact
is used before its associated coil, then the timer accurac
is reduced.
The followin
formulas
ive maximum and minimum errors for certain situations.
However, an avera
e expected error would be approximatel
;
1.5
×
The pro
ram scan time
Condition 1:
The timer contact appears after the timer coil.
Maximum timin
error:
2
×
Scan time + The input filter time
Minimum timin
error:
Input filter time - The timer resolution
Condition 2:
The timer contact appears before the timer coil.
Maximum timin
error:
3
×
Scan time + The input filter time
Minimum timin
error:
Input filter time- The timer resolution
Internal timer accuracy:
• The actual accurac
of the timin
elements within the PLC hardware is;
± 10 pulses per million pulses. This means that if a 100 msec timer is used to time a sin-
le da
, at the end of that da
the timer will be within 0.8 seconds of the true 24 hours or
86,400 seconds. The timer would have processed approximatel
864,000; 100 msec
pulses.