CH.9 | PLC COUNTER INSTRUCTIONS

INTRODUCTION

  • Counter Instructions are used to count parts in manufacturing plants or to count events in industrial control system applications.

9.1 PLC COUNTER INSTRUCTIONS

  • Like Timer instructions, Counter’s can have a double or single input.

  • Double_input Counter requires two inputs

    • One input signal to enable the counter

    • The other input counts the number of pulse signals

    • Signals can be either

      • Trailing-Edge trigger, which is high-to-low

      • Leading_Edge Trigger, which is low-to-high

  • Single Input Counter requires one input

    • One input signal both enables and counts the signals or pulses.

  • There are two types of counter instructions in PLCs

    • Count Up

      • Causes the accumulated register’s value to increment whenever the counter input device changes state.

    • Count Down

      • Causes the accumulated register’s value to decrement whenever the counter input device changes state.

9.2 PLC COUNTER INSTRUCTION REGISTERS

  • Three registers just like Timers

    • Preset Register (Holds the preset number)

    • Accumulated Register (Holds the present count)

    • Status Register (Holds two flag bits)

      • Count Bit

        • enabled every time the counter counts up (CU) or counts down (CD)

      • Done Bit

        • Enabled whenever the accumulated register is equal to or greater than the Preset Register.

    • If the counter exceeds the maximum number of 7FFFh (+32767), then it starts over at 8000h (-32767) and its overflow bit turns on.

      • Overflow Bit (OV) signals an error state in which the number does not fit in the 16 bit register.

    • If the counter exceeds the minimum number of 8000h (-32767), then it starts over at 7FFFh (+32767) and the underflow bit turns on.

      • Underflow Bit (UN) signals an error state in which the number to be stored is smaller than the 16-bit register can specify.

    • There is an addition status bit called the Update Accumulator Bit (UA)

      • Used with high-speed counter (HSC) instructions.

      • Not used in Allen-Bradley 500 series.

9.3 PLC COUNT UP INSTRUCTIONS

  • Count Up Instruction

    • Accumulated register increments whenever the counter input device changes state.

    • Done bit remains enabled once accumulator is equal to or greater than preset until counter is reset.

    • Reset (RES) Instruction is used to reset the accumulated register to zero.

9.4 PLC COUNT DOWN INSTRUCTIONS

  • Count Down Instruction

    • The accumulated register decrements whenever the counter input device changes state.

    • Preset Value is usually a negative number

    • Therefore the content of the accumulated register decrements from its initial value of ZERO to a negative number.

9.5 CONNECTING PLC COUNTER INSTRUCTIONS

  • Sometimes the content of an entire section of a conveyor needs to be known.

    • Accomplished by placing TWO proximity sensors.

      • One sensor is connected to COUNT UP

      • Another sensor is connected to COUNT DOWN

9.6 CASCADING COUNTERS

  • In some industrial applications, counters are cascaded so that one counter increments or decrements when another is done.

    • Used mainly in material handling and process control environments.

    • EX. Six bottles of water are counted and then packed in a container.

      • One counter counts the bottles then once finished a second counter counts the completed container.