Ladder Diagram Cheat Sheet
Give me a cheat sheet for making or reading a lader diagram
Ladder Diagram Cheat Sheet
How to Read or Make One
A ladder diagram is an electrical control drawing that looks like a ladder.
Left rail = power/hot side
Right rail = neutral/return side
Rungs = control logic paths
Read it left to right and top to bottom
1. Basic Ladder Symbols
Symbol
Name
`—
Meaning
—`
`—
/
—`
( )
Coil / Output
Turns on a device
(L)
Latch Coil
Stays on after being triggered
(U)
Unlatch Coil
Turns latch off
[TON]
Timer On Delay
Delays turning ON
[TOF]
Timer Off Delay
Delays turning OFF
[CTU]
Counter Up
Counts events
(M) or (B3)
Internal Relay / Memory Bit
Logic-only output, not physical
2. The Big Rule
A rung is true when there is a complete path from the left rail to the right rail.
Think:
“Can power flow through this rung?”
If yes, the output turns ON.
3. Normally Open vs Normally Closed
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Normally Open Contact
—| |—
Means:
The signal must be ON for power to pass.
Example:
Start Button
—| |—
The button must be pressed.
Normally Closed Contact
—|/|—
Means:
The signal must be OFF for power to pass.
Example:
Stop Button
—|/|—
The stop button normally allows power through.
When pressed, it breaks the circuit.
4. Series vs Parallel Logic
Series = AND
—| |——| |——( )
A
B
Output
Output turns ON only if:
A AND B are ON
Example:
Motor runs only if Start is pressed AND Guard is closed.
Parallel = OR
|
—---|
|
—| |—
A |
|----( )
B |
—| |—
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Output turns ON if:
A OR B is ON
Example:
Light turns on if Button 1 OR Button 2 is pressed.
5. Common Ladder Diagram Pattern: Start/Stop Motor Circuit
Stop
Start
Motor
—|/|———| |——————( Motor )
|
| Motor
—| |—
What it does:
1. Stop is normally closed, so power can pass.
2. Press Start, and the motor turns on.
3. The Motor contact seals itself in.
4. When you release Start, the motor stays on.
5. Press Stop, and the circuit opens.
This is called a seal-in circuit or latching circuit.
6. How to Read a Ladder Diagram
Use this order:
Step 1: Find the output
Look at the right side of the rung.
Example:
—| |——| |——( Motor )
The output is:
Motor
Ask:
“What turns this output on?”
Step 2: Read left to right
Every contact before the output is a condition.
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Example:
—| |——|/|——( Fan )
A
B
Fan turns ON if:
A is ON
AND
B is OFF
Because B is normally closed.
Step 3: Identify AND/OR logic
Series contacts = AND
Parallel contacts = OR
Step 4: Check for timers, counters, and latches
Timers delay actions.
Counters track repeated events.
Latches hold an output ON until reset.
7. How to Make a Ladder Diagram
Start with the sentence:
“I want the output to turn on when…”
Example:
“I want the motor to turn on when the start button is pressed, unless the stop button is
pressed.”
Turn that into logic:
Motor = Stop NOT pressed AND Start pressed
Then draw:
Stop
Start
Motor
—|/|———| |————( Motor )
8. Common Inputs and Outputs
Inputs
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Device
Usually Drawn As
Start button
Normally open
Stop button
Normally closed
Limit switch
NO or NC depending on use
Sensor
NO or NC contact
Emergency stop
Normally closed
Overload contact
Normally closed
Outputs
Device
Drawn As
Motor starter
Coil
Light
Coil/output
Relay
Coil
Solenoid valve
Coil
Alarm
Coil
Conveyor
Coil/output
9. Important Safety Rule
Stop buttons, emergency stops, and overloads are usually normally closed.
Why?
Because if a wire breaks, the machine shuts off instead of staying on.
That is called fail-safe design.
10. Ladder Diagram Logic Translation
Ladder Logic
Meaning
A NO contact
A is ON
A NC contact
A is OFF
A and B in series
A AND B
A and B in parallel
A OR B
Coil energized
Output ON
Timer done bit
Timer finished
Counter done bit
Count reached
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11. Example Problems
Example 1
A
B
Light
—| |———| |————( Light )
Light turns ON when:
A AND B are ON
Example 2
A
B
Alarm
—| |———|/|————( Alarm )
Alarm turns ON when:
A is ON
AND
B is OFF
Example 3
A
—---| |----|
|----( Pump )
—---| |----|
B
Pump turns ON when:
A OR B is ON
12. Memory Trick
Ladder logic is just electrical “if statements.”
Example:
If Start is pressed
and Stop is not pressed
then Motor turns on.
In ladder form:
Stop
Start
Motor
—|/|———| |————( Motor )
13. Quick Design Formula
When making one, use this structure:
Safety conditions → Permission conditions → Start command → Output
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Example:
E-Stop
Overload
Start
—|/|—————|/|—————| |————( Motor )
Motor
Meaning:
Motor runs if:
E-stop is not pressed
AND overload is not tripped
AND start is pressed
14. Ladder Diagram Checklist
Before finishing, ask:
1. What is the output?
2. What conditions turn it on?
3. What conditions turn it off?
4. Are stop and safety devices normally closed?
5. Do I need a latch/seal-in?
6. Do I need a timer?
7. Do I need a sensor input?
8. Does the logic fail safely?
Super Simple Summary
—| |—
means the input must be ON
—|/|—
means the input must be OFF
( )
means output turns ON
Series = AND
Parallel = OR
Left to right = conditions
Right side = result
Ladder diagrams are basically:
“If these conditions are true, turn this thing on.”
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