A PLC down-counter (CTD) counter counts:
A. scan transitions.
B. true-to-false transitions.
C. false-to-true transitions.
D. both B and C.
Show Answer
Answer: C
Share your understanding of this question with the correct explanation.
Explanation:
A PLC (Programmable Logic Controller) down-counter (CTD) is a type of counter that counts false-to-true transitions. In the context of digital signals, a “false-to-true” transition refers to a change from a logic low state (0) to a logic high state (1). The counter is triggered or decrements when it detects this specific transition on its input.
Here’s a step-by-step explanation of how a PLC down-counter (CTD) operates:
Initial State: When the PLC program starts or the counter is reset, the down-counter’s preset value is set to a specific target count value. For example, if the preset value is set to 5, it means the counter will count down from 5 to 0.
Monitoring Input: The down-counter continuously monitors its input, which is typically a digital signal from an external device or sensor. This input is often connected to a field device, such as a sensor or a limit switch, which provides the signal to the PLC.
False-to-True Transition: As the PLC scans through its program, it checks the input signal’s status. When the input signal changes from false (0) to true (1), indicating a false-to-true transition, the down-counter decrements its count by one.
Counting Down: The down-counter keeps decrementing its count each time it detects a false-to-true transition on the input signal. For example, if the preset value was 5, and the PLC detects five false-to-true transitions on the input, the counter’s value will go down from 5 to 4, 3, 2, 1, and finally to 0.
Output or Action: When the down-counter reaches its preset value of 0, it may trigger specific actions or logic in the PLC program. This could be activating a specific output, initiating a process, or any other action based on the application’s requirements.
It’s essential to understand the distinction between up-counters (CTU) and down-counters (CTD). While both are types of counters used in PLC programming, they count transitions in different directions:
Up-Counters (CTU): Count true-to-false transitions. Increment the count when the input signal changes from false to true.
Down-Counters (CTD): Count false-to-true transitions. Decrement the count when the input signal changes from false to true.
A PLC down-counter (CTD) counts false-to-true transitions on its input, decrementing its count each time it detects this specific transition. Once it reaches its preset value (0 in this case), it can trigger specific actions or logic in the PLC program.