Watchdog Timer
A watchdog timer in the Series 90-30 PLC is designed to catch catastrophic failure conditions that result in an unusually long sweep. The timer value for the watchdog timer is 200 milliseconds (500 milliseconds in the 350 and 360 series of PLC CPUs); this is a fixed value that cannot be changed. The watchdog timer always starts from zero at the beginning of each sweep.
For 331 and lower model 90-30 CPUs, if the watchdog timeout value is exceeded, the OK LED goes off; the CPU is placed in reset and completely shuts down; and outputs go to their default state. No communication of any form is possible, and all microprocessors on all boards are halted. To recover, power must be cycled on the rack containing the CPU. In the 90-20, Series 90 Micro and 340 and higher 90-30 CPUs, A watchdog timeout causes the CPU to reset, execute its powerup logic, generate a watchdog failure fault, and change its mode to STOP.
Constant Sweep Timer
The constant sweep timer controls the length of a program sweep when the Series 90-30 PLC operates in CONSTANT SWEEP TIME mode. In this mode of operation, each sweep consumes the same amount of time. Typically, for most application programs, the input scan, application program logic scan, and output scan do not require exactly the same amount of execution time in each sweep. The value of the constant sweep timer is set by the programmer and can be any value from 5 to the value of the watchdog timer (default is 100 milliseconds).
If the constant sweep timer expires before the completion of the sweep and the previous sweep was not oversweep, the PLC places an oversweep alarm in the PLC fault table. At the beginning of the next sweep, the PLC sets the OV_SWP fault contact. The OV_SWP contact is reset when the PLC is not in CONSTANT SWEEP TIME mode or the time of the last sweep did not exceed the constant sweep timer.
Time-Tick Contacts
The Series 90 PLC provides four time-tick contacts with time durations of 0.01 second, 0.1 second, 1.0 second, and 1 minute. The state of these contacts does not change during the execution of the sweep. These contacts provide a pulse having an equal on and off time duration. The contacts are referenced as T_10MS (0.01 second), T_100MS (0.1 second), T_SEC (1.0 second), and T_MIN (1 minute).
The following timing diagram represents the on/off time duration of these contacts.
