2. 9
CHAPTER
PLC Counter Instructions

3. Objectives
Use count up instructions to create PLC ladder logic diagrams.
Use count down instructions to create PLC ladder logic diagrams.
Reset counter instructions.
Connect different counter instructions and cascade counter instructions.
Use timer and counter instructions to create PLC ladder logic diagrams.

4. PLC Counter Instructions
Double-input counter
Single-input counter

5. PLC Counter Instructions (Cont.)
Similar to the timer instruction
Displayed in block format or coil format
Single-input in block format
Allen-Bradley SLC 500 series programmable logic controller uses one input
Addresses can be C5:0 to C5:255
Can have up to 256 instructions

6. PLC Counter Instructions (Cont.)
Count up:
Accumulated register value increments (counts up) whenever the counter input device changes state
Count down:
Accumulated register value decrements (counts down) whenever the counter input device changes state

7. PLC Counter Instruction Registers
Preset register (C5:0.PRE):
Used to hold the preset counter number
Accumulated register (C5:0.ACC):
Holds the accumulated counter number
Status register:
Holds flag bits that are used by the PLC to keep track of the operation of counter C5:0

8. PLC Counter Instruction Registers (Cont.)
Two flag bits:
Count bit
Done bit C5:0/DN

9. PLC Counter Instruction Registers (Cont.)
Overflow bit (OV)
Underflow bit (UN)
Update accumulator bit (UA):
Used with high-speed counter (HSC) instructions

10. SLC 500 Counter Instruction Registers and Status Bits
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11. PLC Count Up Instructions
Accumulated register increments when counter input device changes state
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12. PLC Ladder Logic Diagram Using Count Up Instruction
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13. PLC Count Down Instructions
Accumulated register decrements whenever the counter input device changes state
Preset value for the count down usually is a negative number
Content of the accumulated register decrements from its initial value of zero to the negative preset number

14. Count Down Instruction Connected to a NO Input
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15. PLC Count Instructions Comparison
Count down instruction:
Done bit (C5:0/DN) resets to zero
Done coil de-energizes when the content of the accumulated register decrements to the preset number
Count up instruction:
Done bit (C5:0/DN) is enabled
Done coil energizes when the content of the accumulated register is equal to the content of the preset register

16. Connecting PLC Counter Instructions
In some manufacturing operations:
Content of an entire section of a conveyor needs to be known at one time
Place two proximity sensors on the conveyor
One sensor connected to count up instruction
One sensor connected to count down instruction

17. Connecting PLC Counter Instructions (Cont.)
Conveyor and proximity sensors and how counting is accomplished
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18. Connecting PLC Counter Instructions (Cont.)
Infeed proximity switch:
Connected to the count up instruction
Outfeed proximity switch:
Connected to the count down instruction

19. Cascading Counters In some industrial applications:
Counters are cascaded so that one counter increments or decrements when another counter is done
Used mainly in material handling and process control environments

20. PLC Ladder Logic Diagram Using Cascading Counters
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21. PLC Ladder Logic Diagram with Counters and Timers
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