CONTROL UNIT

PURPOSE:To decide the abnormality of a user program according to the setting state of a flag by setting the data to a timer area of a memory via the user program to periodically update this data and turning off the flag in a time-up state. CONSTITUTION:A basic program 1 writes the timer set value into the 1st word of a timer area 21 as the initialization. Then one is subtracted from the timer set value and this subtraction result (the current value) is written into the 1st word. While the unsubtracted value written into the 1st word is written into the 2nd word. If the current value and then the previous value are not equal to '0', the current value is defined as the preceding value and -1 is defined as the current value. If the current value is not equal to '0' and the preceding value is equal to '0', the initialization is carried out and a flag SW65 is turned on. No operation is carried out if the current value and then the preceding value are equal to '0'. If the current value is equal to '0' and the preceding value is not equal to '0', a time-up state is decided and the SW65 is turned off with the preceding value set at '0' respectively. When an OFF state of the SW65 is detected during a control operation, a control program 3 judges the abnormality of the program 1 and starts the corresponding error process.