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Chapter 15. Timer and Counter Instructions. Objectives. Describe the function of an on-delay timer. Describe the function of an off-delay timer. Describe in what instances one would use a retentive timer. Describe the function of an up counter. Describe the function of a down counter.
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Chapter 15 Timer and Counter Instructions
Objectives • Describe the function of an on-delay timer. • Describe the function of an off-delay timer. • Describe in what instances one would use a retentive timer. • Describe the function of an up counter. • Describe the function of a down counter.
Objectives (cont’d.) • Describe in what instances one would use an up counter versus a down counter. • Define preset, accumulative value, and the timer or counter address. • Explain how the various timers and counters are reset.
T4, Timer File • The timer file stores only timer elements. • An element is a word or group of words that work together as a unit. • A timer is made of of three pieces or words: • Preset value • Accumulated value • Status bits
T4, Timer File (cont’d.) • The preset value and accumulated value are 16-bit signed integers. • Status bits are single bits that make up one 16-bit word. • These three words work together as a unit.
Timer Addressing • Sample timer element addressT4:2 • T4 = timer file 4 • :2 = timer element #2 (0-255 timer elements per file)
Sub-Element • A sub-element is part of an element addressable as a unit. • The preset value and accumulated value are sub-elements of a timer: • T4:0.PRE • T4:0.ACC
Timer Status Bits • Timers have three status bits. • Done bit (DN) is true when the accumulated value and preset are equal. • Timer timing bit (TT) is true when the timer is timing. • Enable bit (EN) is true when the timer instruction is enabled or true.
Timer Bit Addressing • Status bit addresses for timer file 4, timer element 2 (T4:2) are listed below: • T4:2/DN is the address for the done bit. • T4:2/EN is the address for the enable bit. • T4:2/TT is the address for the timer timing bit.
C5, Counter File • The counter file stores only counter elements. • An element is a word or group of words that work together as a unit. • A counter is made of of three pieces or words: • Preset value • Accumulated value • Status bits
C5, Counter File (cont’d.) • The preset value and accumulated value are 16-bit signed integers. • Status bits are single bits that make up one 16-bit word. • These three words work together as a unit.
Counter Addressing • Sample counter element address C5:2 • C5 = timer file 5 • :2 = counter element #2 (0-255 timer elements per file)
Sub-Element • A sub-element is part of an element addressable as a unit. • The preset value and accumulated value are sub-elements of a counter: • C5:0.PRE • C5:0.ACC
Counter Status Bits • Counters have five status bits. • Done bit (DN) is true when the accumulated value and preset are equal. • Count up enable bit (CU) is true when the up counter is true or enabled. • Count down enable bit (CD) is true when the count down counter is enabled or true.
Counter Status Bits (cont’d.) • The overflow bit (OV) is true when the up counter has overflowed above +32767. • The underflow bit (UN) is true when the down counter has underflowed below -32768. • The update accumulator bit (UA) is a high-speed counter status bit for fixed SLC 500 PLCs.
Counter Status Bit Addressing • Status bit addresses for counter file 5, counter element 0 (C5:0) are listed below: • C5:0/DN is the address for the done bit. • C5:0/CU is the address for the count up enable bit.
Counter Status Bit Addressing (cont’d.) • C5:0/CD is the address for the count down enable bit. • C5:0/OV is the address for the count up overflow bit. • C5:0/UN is the address for the count down underflow bit.
