Chapter 4. Ladder Diagram (LD) Programming216 PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual GFK-2950C4.13 TimersThis section describes the PACSystems timed contacts and timer function blocks that areimplemented in the LD language.4.13.1 Timed ContactsThe PACSystems has four timed contacts that can be used to provide regular pulses of power flow toother 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 communicationsdevice to monitor the state of the CPU and the communications link. Timed contacts are also oftenused to blink pilot lights and LEDs.The timed contacts are referenced as T_10MS (0.01 second), T_100MS (0.1 second), T_SEC (1.0second), 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 %S0006These contacts provide a pulse having an equal on and off time duration. The following timingdiagram illustrates the on/off time duration of these contacts.XX/2SECT XXXXXX/2SECSECCautionDo not use timed contacts for applications requiringaccurate measurement of elapsed time. Timers, time-based subroutines, and PID blocks are preferred forthese types of applications.The CPU updates the timed contact references basedon a free-running timer that has no relationship to thestart of the CPU sweep. If the sweep time remains inphase with the timed contact clock, the contact willalways appear to be in the same state. For example, ifthe CPU is in constant sweep mode with a sweep timesetting of 100ms, the T_10MS and T_100MS bits willnever toggle.
