Read a rung
I can read a rung and keep a pump running until Stop.
Practise it now: Make it stay running Free, no sign-up. New to this? Read the lesson first.
1. Why it matters
An operator presses Start once and walks away. Without memory, the pump stops as soon as they let go. With the memory built wrongly, Stop cannot stop it.
2. The idea in plain words
is a way to write logic as a drawing. Each horizontal line is a ; Siemens calls it a network. Read it from left to right: the conditions come first, then the command they control.
A contact tests one on/off . A normally-open contact passes when its tag is . A normally-closed contact passes when its tag is . Rockwell calls these XIC and XIO.
Contacts in a row must all pass, like AND. Paths side by side, called parallel paths, are alternatives, like OR.
These symbols are tests in software. They do not tell you how the real is wired.
A coil sits at the right-hand end and writes an on/off value. A normal coil, called OTE in Rockwell, writes TRUE when the path passes and FALSE when it does not.
Example: a Start contact followed by a PumpOn coil makes the pump copy Start. Let go of Start and the pump stops.
To keep it running, add a , where the pump remembers it was started. It is a second path, in parallel with Start, with a PumpOn contact. Put the Stop condition after both paths, so Stop can break either one.
Predict three . Scan 1: Start TRUE writes PumpOn TRUE. Scan 2: Start is FALSE, but the PumpOn contact still sees TRUE from the last scan, so the pump holds. Scan 3: Stop TRUE writes PumpOn FALSE, and the hold is gone from the next scan.
This is OR at work: either the new Start request or the remembered command can ask the pump to run.
Later exercises add set () and reset (unlatch) instructions. These change a value only when their rung passes.
3. See it happen
Scroll sideways to see the full rung.
After a quick press of Start, which path keeps the output on?
Decide your answer first. Then step through the scans to check it.
| Input or event | Result |
|---|---|
| Start TRUE, Stop FALSE, Permit TRUE | ? |
| Start FALSE | ? |
| Stop TRUE | ? |
A mistake to look for
Putting Stop only on the Start path. The pump then keeps itself on through its own contact, and Stop does nothing. Always test Stop while the pump is already running.
4. Vendor notes
Siemens notes
LAD networks use contacts and ordinary coils.
In SCL, the same logic is one line: #PumpOn := (#Start OR #PumpOn) AND NOT #Stop AND #Permit;
The right-hand side reads the previous command first. Then the line writes the new one.
5. On site
In a building's pump room, an operator presses Start once and walks away. The pump should keep running until someone presses Stop.
Without a seal-in, where the pump remembers it was started, it stops as soon as Start is let go. If that memory ignores Stop, water keeps pumping.
In a block of flats, a basement tank holds water from the mains. Two pumps push it up to the taps above. One runs while the other waits as a spare. A low-water switch stops the pumps before the tank runs dry.
- Street mainsWater enters the building
- Storage tankHolds a supply for the pumps
- Water pumpsPush water to the floors aboveYou are here
- Taps upstairsResidents use the water they need
On a real site: places, parts and safety
- Block of flats — pushing water up to the top floors
- Brewery — sending hot water to a brewing tank
- Water-treatment works — moving water between filter tanks
- Office building — pumping water round the heating pipes
Recognise it on site
- The reservoir stores the water. On this practice rig the water flows back into it, so normal pumping never empties it.
- An electric motor spins the pump to push water through the pipe. A guard covers its moving shaft.
- The standby pump is a spare on the same pipes. It takes over when the main pump cannot run.
- The flow transmitter, a meter in the pipe, measures how much water passes each second.
- The pressure gauge shows how hard the water pushes against the pipe walls.
- The controller cabinet holds the PLC, the industrial computer that runs your code. It also holds the switches that power the motors.
A low-water switch can take away permission to run. The pump must stop before it runs dry and damages its seal.
The PLC can say ON even when a pump has failed. Check the measured flow as well as the command.
The Stop input in these exercises is not an emergency stop. Real machines also need guards, emergency stops and a safe way to cut the power.
The sizes, timings and cutaways are teaching choices, not a drawing of any one real installation.
6. Try it on the rig
Build a Start path and, in parallel, a PumpOn hold path. Then check that Stop breaks both.
Open the mission, make a prediction, run the equipment, then test your programme. Passing the tests completes practice; reading this page alone does not.
04. Make it stay running05. Stop doesn't stop it06. Review the AI: Stop priority7. Check your understanding
Predict a new case. A wrong answer gives you an explanation and another try.
1. Someone is pressing Start, so Start is TRUE. Nobody is pressing Stop, so Stop is FALSE. Does this rung switch PumpOn on?
Scroll sideways to see the full rung.
2. PumpOn is already TRUE. Can Stop turn it off in this diagram?
Scroll sideways to see the full rung.