Chapter 6 of 12: show all chapters
Chapter 6 of 12Part 2: Inside a control system · 3 min
TRUE and FALSE: most PLC signals are ON or OFF
A button is pressed or it is not. A pump is running or it is not. PLCs mostly work with simple yes-or-no signals.
Yes or no
Most inputs and outputs only have two states. Pressed or not pressed. Open or shut. Running or stopped.
PLC people say TRUE and FALSE
In a PLC, ON is called TRUE and OFF is called FALSE. You will also see 1 and 0. They all mean the same thing.
| In everyday words | In a PLC | As a number |
|---|---|---|
| ON, yes, pressed, running | TRUE | 1 |
| OFF, no, released, stopped | FALSE | 0 |
Every signal has a name
So the rules can talk about each signal, it gets a short name, such as Start or PumpOn. PLC people call a named signal a tag.
A few tags hold a number instead, such as how full a tank is or how hot the water is. You will meet those later. For now, almost everything is TRUE or FALSE.
Try it
Hold the Start button, then let go. Watch its tag change. Then drag the tank level.
On the machine
Inside the PLC
| Tag | Value | What it means |
|---|---|---|
Start | FALSE = 0 | Not pressed |
TankLevel | 40 | 40% full. A number tag. |
TankFull | FALSE = 0 | Not full yet |
New words
- TRUE
- ON. The button is pressed, or the pump is running. Also 1.
- FALSE
- OFF. The button is not pressed, or the pump is stopped. Also 0.
- Tag
- A named signal, such as Start or PumpOn.
Quick check
Nobody is pressing the Start button. What is the Start tag?