Control Systems: PLCs & Variable Speed Drives is one of 9 topics in the Grade 12 Electrical Technology CAPS syllabus, covered across 3 lessons on Study Vault. Below are real practice questions from this topic with full worked answers.
Sample practice questions
Easy
1. What is the main disadvantage of hard-wired control logic compared to a PLC's soft-wired approach?
Changing the control logic requires physically rewiring the panel, a slow and labour-intensive process
Hard-wired logic can be changed instantly by editing software
Hard-wired logic has no disadvantages compared to soft-wired control
Hard-wired panels require no physical wiring at all
Answer: Changing the control logic requires physically rewiring the panel, a slow and labour-intensive process — Hard-wired control logic requires physically rewiring relays, contactors, timers and switches to change the control scheme, a slow, labour-intensive and error-prone process compared to editing a PLC program.
Easy
2. What are the three stages of a PLC's repeating scan cycle, in order?
Input scan, program scan, output scan
Output scan, input scan, program scan
Program scan only, repeated three times
Input scan and output scan only, with no program scan
Answer: Input scan, program scan, output scan — The PLC scan cycle consists of the input scan (reading physical inputs), the program scan (executing the stored program), and the output scan (updating physical outputs), repeated continuously.
Medium
3. Name the stage of the PLC scan cycle during which the PLC reads the current state of all its physical inputs.
Answer: input scan — The input scan is the stage during which the PLC reads the current state of all its physical inputs and stores these values in internal memory.
