Construct the flow diagram by converting the system’s intended operation into an ordered sequence of inputs, processing instructions, decisions, and outputs. Use standard flowchart symbols and arrows to show precisely how the programmable system responds under different conditions.
Begin by defining the system’s purpose and identifying its input sensors, processor-controlled operations, and output devices. Then express the control logic as a step-by-step algorithm before drawing the diagram.
| Symbol | Function in the flow diagram |
|---|---|
| Oval or rounded rectangle | Marks the start or end of the program. |
| Parallelogram | Represents an input from a sensor or an output to a display or actuator. |
| Rectangle | Shows a process, such as calculating a value or switching a motor on. |
| Diamond | Represents a decision with labelled branches, normally Yes/No or True/False. |
| Arrow | Shows the direction and sequence of control. |
For example, consider a temperature-controlled fan:
- Start the program.
- Read the temperature sensor.
- Ask: “Is the temperature above the set value?”
- If Yes, switch the fan on; if No, switch it off.
- Return to the sensor-reading step to create a feedback loop.
Every decision branch must be labelled and lead somewhere meaningful. The completed diagram should be checked using normal, boundary, and unexpected input values to confirm that the logic produces the required output.
A common misconception is that a flow diagram shows the physical wiring between electronic components. It does not: a system block diagram represents component relationships and signal flow, whereas a program flow diagram represents the sequence of control instructions.
Exam technique: For B3.4 questions, identify the input, process, and output before drawing. Examiners expect correct symbols, directional arrows, labelled decision branches, and loops where the system repeatedly monitors an input.
