Chapter 16 Timer and Counter Instructions Objectives Describe

Chapter 16 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. 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.

T 4, 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 • 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.

One Timer Element Is Made of Three 16 -bit Words

Timer Addressing • Sample timer element address. T 4: 2 – T 4 = 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. – T 4: 0. PRE – T 4: 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 (T 4: 2) are listed below: – T 4: 2/DN is the address for the done bit. – T 4: 2/EN is the address for the enable bit. – T 4: 2/TT is the address for the timer timing bit.

Timer File T 4

C 5, Counter File (1 of 2) • 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 three pieces or words. – Preset value – Accumulated value – Status bits

C 5, Counter File (2 of 2) • 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.

One Counter Element Is Made of Three 16 -bit Words

Counter Addressing • Sample counter element address C 5: 2 – C 5 = 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. – C 5: 0. PRE – C 5: 0. ACC

Counter Status Bits (1 of 2) • 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 (2 of 2) • 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 highspeed counter status bit for fixed SLC 500 PLCs.

Counter Status Bit Addressing • Status bit addresses for counter file 5, counter element 0 (C 5: 0) are listed below: – – – C 5: 0/DN is the address for the done bit. C 5: 0/CU is the address for the count up enable bit. C 5: 0/CD is the address for the count down enable bit. C 5: 0/OV is the address for the count up overflow bit. C 5: 0/UN is the address for the count down underflow bit.