GE PACSystems RX7i, Cpu Programmer'S Reference Manual

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Chapter 4. Ladder Diagram (LD) Programming
216 PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual GFK-2950C
4.13 Timers
This section describes the PACSystems timed contacts and timer function blocks that are
implemented in the LD language.
4.13.1 Timed Contacts
The PACSystems has four timed contacts that can be used to provide regular pulses of power flow to
other program functions. Timed contacts cycle on and off, in square-wave form, every 0.01 second,
0.1 second, 1.0 second, and 1 minute. Timed contacts can be read by an external communications
device to monitor the state of the CPU and the communications link. Timed contacts are also often
used to blink pilot lights and LEDs.
The timed contacts are referenced as T_10MS (0.01 second), T_100MS (0.1 second), T_SEC (1.0
second), and T_MIN (1 minute). These contacts represent specific locations in %S memory:
#T_10MS 0.01 second timed contact %S0003
#T_100MS 0.1 second timed contact %S0004
#T_SEC 1.0 second timed contact %S0005
#T_MIN 1.0 minute timed contact %S0006
These contacts provide a pulse having an equal on and off time duration. The following timing
diagram illustrates the on/off time duration of these contacts.
X
X/2
SEC
T XXXXX
X/2
SEC
SEC
Caution
Do not use timed contacts for applications requiring
accurate measurement of elapsed time. Timers, time-
based subroutines, and PID blocks are preferred for
these types of applications.
The CPU updates the timed contact references based
on a free-running timer that has no relationship to the
start of the CPU sweep. If the sweep time remains in
phase with the timed contact clock, the contact will
always appear to be in the same state. For example, if
the CPU is in constant sweep mode with a sweep time
setting of 100ms, the T_10MS and T_100MS bits will
never toggle.