The big idea: A flow diagram models what a programmable system DOES, using a small set of fixed symbols.
| Symbol | Used for |
|---|---|
| Rounded box | Start and stop |
| Parallelogram | Input and output |
| Rectangle | A process |
| Diamond | A decision |
Loop back: A monitoring program must loop back, or it reads its sensor once and stops for ever.
The four symbols, then a complete automatic-light program drawn with them.
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| Symbol | Shape | Rule |
|---|---|---|
| Terminal | A rounded box | START at the top; a STOP only if the program actually ends. A control program that runs for ever has no stop symbol |
| Input / output | A parallelogram | Reading a sensor or sending a value to an output device. Name the device, not the quantity |
| Process | A rectangle | Something the program does: set a variable, wait, switch something on |
| Decision | A diamond | A question with exactly TWO outputs, both labelled — YES and NO. Three lines out of a diamond is always wrong |
| Flow line | An arrow | Every path must lead somewhere, and the loop back is what makes it a control program |
The three mistakes examiners see: No loop back, so the program runs once. An unlabelled decision, so a reader cannot tell which path is which. And a path that goes nowhere, leaving the program with no defined behaviour on one branch.
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From a requirement to a diagram
Write the requirement as a sentence
"Switch the lamp on when it is dark, off when it is light, and do not flicker at dusk." Everything in the diagram comes from that sentence.
Find the decision
Usually one question with two answers. If there are two questions, there are two diamonds — not one with three outputs.
Add what stops it misbehaving
A dead band so the lamp does not flicker at the threshold, and a delay so a passing headlight does not trigger it. Both are extra boxes, not extra components.
Close the loop
Every branch returns to the read step, so the system keeps monitoring for as long as it has power.
How this is tested — constructing flow diagrams to model a programmable system. It comes up two ways:
Paper 1 — multiple choice
- Identify the symbol used for a stated step.
- Find the error in a given flow diagram.
Paper 2 — analysing a product
- Construct a flow diagram for a described control system.
- Explain what a given flow diagram does.
The trap: Leaving out the loop back. A monitoring program that does not return to the read step has checked once and stopped.
Construct a flow diagram for a greenhouse vent that opens when the temperature rises above 25 °C and closes below 22 °C.
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