Chapter 9 of 12: show all chapters
Chapter 9 of 12Part 3: Writing the rules · 4 min
Your first line of PLC code
Code is just a rule written in a short, strict way that a PLC can read. Here is your first line, one piece at a time.
From words to code
In words: “Run the pump while Start is held.” Put another way: “The pump should copy whatever the Start button is doing.”
As code, that is one line:
PumpOn := Start;
Siemens and Rockwell write it slightly differently
Siemens puts a # in front of each name. Rockwell leaves it out. It is the same rule with the same result. You will meet both companies properly in the last part.
| Siemens | Rockwell |
|---|---|
| #PumpOn := #Start; | PumpOn := Start; |
Adding a “but not if”
“Run the pump while Start is held, but not if the tank is full” becomes:
PumpOn := Start AND NOT TankFull;
Try it
Click each piece of the code to see what it means.
Your first line
Read it aloud: “PumpOn becomes Start.”
With a “but not if”
Read it aloud: “PumpOn becomes Start, and not TankFull.”
PumpOnThe tag for the pump. When it is TRUE the pump runs; when it is FALSE the pump stops.
Try the second line
Pump stopped
Start FALSE AND NOT TankFull FALSE → PumpOn FALSE
New words
- Code
- Rules written in a short, strict way that a PLC can read.
- :=
- “Becomes”. The tag on the left gets the value worked out on the right.
- AND NOT
- “But not if”. Both parts must agree before the result is TRUE.
Quick check
In PumpOn := Start; what happens when Start is FALSE?