Latching circuit (seal-in logic) in PLC Programming - Step-by-Step Execution

Concept Flow - Latching circuit (seal-in logic)

Start

↓

Input: Start Button

↓

Check: Is Start Pressed?

No→Check: Is Output Latched?

↓

Set Output ON

↓

Output ON

↓

Check: Stop Button

↓

Is Stop Pressed?

Yes→Reset Output OFF

No↓

Loop back to Start

The latching circuit uses a start button to turn on an output and keep it on (latched) until a stop button resets it.

Execution Sample

PLC Programming

IF NOT StopButton AND (StartButton OR Output) THEN
  Output := TRUE;
ELSE
  Output := FALSE;
END_IF;

This PLC code latches an output ON when the start button is pressed and keeps it ON until the stop button is pressed.

Execution Table

Step	StartButton	StopButton	Output Before	Condition Evaluated	Output After	Action
1	FALSE	FALSE	FALSE	StartButton=FALSE, Output=FALSE	FALSE	No start, output stays OFF
2	TRUE	FALSE	FALSE	StartButton=TRUE	TRUE	Start pressed, output set ON
3	FALSE	FALSE	TRUE	StartButton=FALSE, StopButton=FALSE	TRUE	Output latched ON
4	FALSE	TRUE	TRUE	StopButton=TRUE	FALSE	Stop pressed, output reset OFF
5	FALSE	FALSE	FALSE	StartButton=FALSE, Output=FALSE	FALSE	No start, output stays OFF

💡 Execution stops after step 5 as no buttons are pressed and output remains OFF.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
StartButton	FALSE	FALSE	TRUE	FALSE	FALSE	FALSE
StopButton	FALSE	FALSE	FALSE	FALSE	TRUE	FALSE
Output	FALSE	FALSE	TRUE	TRUE	FALSE	FALSE

Key Moments - 2 Insights

Why does the output stay ON even when the start button is released at step 3?

Because the output is latched (sealed-in) by its own contact in the logic, as shown in step 3 where StartButton is FALSE but Output remains TRUE.

What causes the output to turn OFF at step 4?

Pressing the stop button resets the output to FALSE, breaking the latch, as seen in step 4 where StopButton=TRUE causes Output to reset.

Visual Quiz - 3 Questions

Look at the execution table, what is the Output value at step 2 when StartButton is TRUE?

A TRUE

B FALSE

C Undefined

D Remains previous value

💡 Hint

Check the 'Output After' column at step 2 in the execution_table.

At which step does the output reset to FALSE due to the StopButton being pressed?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the step where StopButton=TRUE and Output changes to FALSE.

If the StopButton is never pressed, what will happen to the Output after step 3?

A Output will turn OFF immediately

B Output will stay ON (latched)

C Output will toggle ON and OFF

D Output will be undefined

💡 Hint

Refer to the variable_tracker and execution_table rows where StopButton=FALSE and Output remains TRUE.

Concept Snapshot

Latching circuit (seal-in logic):
- Start button sets Output ON.
- Output stays ON (latched) even if Start is released.
- Stop button resets Output OFF.
- Output uses its own contact to keep latched.
- Common in PLCs for motor start/stop control.

Full Transcript

A latching circuit in PLC programming uses a start button to turn an output ON and keep it ON even after releasing the start button. This is done by sealing in the output with its own contact in the logic. The output stays latched until a stop button is pressed, which resets the output OFF. The execution table shows step-by-step how the output changes based on the start and stop buttons. The variable tracker follows the values of StartButton, StopButton, and Output through each step. Key moments include understanding why the output stays ON after releasing start and how pressing stop resets it. The visual quiz tests understanding of output states at different steps and the effect of buttons. This logic is widely used in automation to control devices like motors safely and reliably.

Practice

1. What is the main purpose of a latching circuit (seal-in logic) in PLC programming?

easy

A. To keep a device ON after pressing start until stop is pressed

B. To immediately turn off a device when start is pressed

C. To reset all outputs when the PLC powers on

D. To count the number of times a button is pressed

Solution

1. Step 1: Understand the function of latching circuits

   Latching circuits keep an output ON even after the start input is released.

2. Step 2: Identify the stop input role

   The stop input breaks the latch, turning the output OFF safely.

3. Final Answer:

   To keep a device ON after pressing start until stop is pressed -> Option A

4. Quick Check:

   Latching circuit = Keeps device ON until stopped [OK]

Hint: Remember: latch holds output ON until stop breaks it [OK]

Common Mistakes:

• Thinking latch turns device OFF immediately after start
• Confusing latch with reset logic
• Ignoring the stop input function

2. Which of the following is the correct ladder logic rung for a latching circuit using a start pushbutton Start, stop pushbutton Stop, and output coil Motor?

easy

A. |---[Stop]---[Start]---(Motor)---|
| +----(Motor)---------|

B. |---[Start]---[Stop]---(Motor)---|
| | |
| +----(Motor)---------|

C. |---[Stop]---[Start]---(Motor)---|
| | |
| +----(Motor)---------|

D. |---[Start]---[Stop]---(Motor)---|
| +----(Motor)---------|

Solution

1. Step 1: Check the order of stop and start contacts

   Stop is normally closed and should be first to allow breaking the circuit; start is normally open and second.

2. Step 2: Confirm the seal-in contact

   The output coil Motor is used as a contact in parallel with Start to latch the circuit.

3. Final Answer:

   Stop first, then Start, with Motor contact in parallel to Start -> Option C

4. Quick Check:

   Stop before Start, Motor seals in [OK]

Hint: Stop contact always first; Motor seals in after Start [OK]

Common Mistakes:

• Placing Start before Stop contact
• Missing Motor contact in parallel
• Confusing normally open and normally closed contacts

3. Given this ladder logic snippet:

|---[Stop]---[Start]---(Motor)---|
|          |                   |
|          +----(Motor)---------|

If the Start button is pressed and released, then the Stop button is pressed, what is the state of Motor output?

medium

A. Motor toggles ON and OFF with each Start press

B. Motor remains ON after Start is released and turns OFF when Stop is pressed

C. Motor never turns ON because Stop is first

D. Motor turns ON only while Start is pressed and OFF immediately after release

Solution

1. Step 1: Analyze pressing Start

   Pressing Start energizes Motor coil, which closes Motor contact in parallel, latching the circuit ON.

2. Step 2: Analyze pressing Stop

   Pressing Stop opens the circuit, breaking the latch and turning Motor OFF.

3. Final Answer:

   Motor stays ON after Start release, OFF when Stop pressed -> Option B

4. Quick Check:

   Latching holds Motor ON until Stop breaks it [OK]

Hint: Start latches ON, Stop breaks latch OFF [OK]

Common Mistakes:

• Thinking Motor turns OFF immediately after Start release
• Ignoring Stop button effect
• Confusing latch with toggle

4. Identify the error in this ladder logic for a latching circuit:

|---[Start]---[Stop]---(Motor)---|
|          |                   |
|          +----(Motor)---------|

medium

A. Start contact should be normally closed

B. Motor coil is missing from the rung

C. Motor contact is not in parallel with Stop

D. Start and Stop contacts are in wrong order; Stop should be first

Solution

1. Step 1: Check contact order

   Stop contact should be first and normally closed to allow breaking the circuit safely.

2. Step 2: Confirm Start contact position

   Start contact should be after Stop and normally open to energize Motor.

3. Final Answer:

   Start and Stop contacts are in wrong order; Stop should be first -> Option D

4. Quick Check:

   Stop first, then Start for correct latch [OK]

Hint: Stop contact always first to break circuit safely [OK]

Common Mistakes:

• Placing Start before Stop
• Confusing normally open/closed contacts
• Missing Motor contact in parallel

5. You want to control a motor with a latching circuit but add a safety feature: the motor should only latch ON if a safety switch Safety is closed. Which ladder logic modification correctly adds this condition?

hard

A. Add Safety contact in series before Start and Motor contacts

B. Add Safety contact in parallel with Start

C. Add Safety contact in parallel with Stop

D. Add Safety contact in series after Motor coil

Solution

1. Step 1: Understand safety switch role

   Safety must be closed to allow motor start; it should be in series to block power if open.

2. Step 2: Place Safety contact correctly

   Putting Safety in series before Start and Motor ensures motor cannot latch if Safety is open.

3. Final Answer:

   Add Safety contact in series before Start and Motor contacts -> Option A

4. Quick Check:

   Safety in series blocks motor if open [OK]

Hint: Series Safety contact blocks motor if open [OK]

Common Mistakes:

• Placing Safety in parallel, bypassing it
• Putting Safety after Motor coil, ineffective
• Confusing series and parallel placement