Copyright Goodheart-Willcox Co., Inc. 200 Programmable Logic Controllers: Hardware and Programming Summary • PLC counter instructions are used to count parts in manufacturing plants or to count events in industrial control system applications. • A double-input counter has two inputs: one to enable the counter, and one to count the number of pulse signals. • A single-input counter has one input that both enables and counts the signals. • The two different types of counter instructions available in PLCs are the count up and count down instructions. • The count up instruction’s accumulated register value increments (counts up) whenever the counter input device changes state. • The count down instruction’s accumulated register value decrements (counts down) whenever the counter input device changes state. • The three 16-bit registers of Allen-Bradley SLC 500 series counter instructions are the preset register, the accumulated register, and the status register. • PLC counter instructions are connected when the content of an entire section of an operation needs to be known at one time. • Counters can be cascaded so that one counter increments or decrements when another counter is done. • Counter and timer instructions can be used together to create PLC ladder logic diagrams. Review Questions Answer the following questions using the information provided in this chapter. 1. How many types of counters are available in an Allen-Bradley SLC 500 series PLC? Name them. 2. How do you increment the content of an accumulated register in a count up instruction? 3. How do you reset a counter instruction in an Allen-Bradley SLC 500 series PLC? 4. What is the status of the done bit in an Allen-Bradley count down instruction when the content of its accumulated register is equal to the content of its preset register? 5. What is the status of the done bit in an Allen-Bradley count up instruction when the content of its accumulated register is equal to the content of its preset register? 6. How is the accumulated register addressed in an Allen-Bradley SLC 500 series counter instruction? 7. How is the preset register addressed in an Allen-Bradley SLC 500 series counter instruction? 8. How many registers does an Allen-Bradley PLC counter function have? Name them. 9. Describe the counter overflow and underflow conditions. Name the flags used to indicate these conditions. 10. Describe the terms leading-edge trigger and trailing-edge trigger as they relate to a counter input signal. 11. Name the flags used for PLC counter up (CTU), and PLC counter down (CD) functions.