Why ladder logic mimics relay circuits in PLC Programming - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to represent a normally open contact in ladder logic.

PLC Programming

if input [1] True:
    output = True

Drag options to blanks, or click blank then click option'

A>

B==

C<

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' causes the logic to invert.

In ladder logic, a normally open contact passes power when the input is True, so we check if input == True.

2fill in blank

medium

Complete the code to represent a coil output energized when the rung is true.

PLC Programming

output = [1]

Drag options to blanks, or click blank then click option'

Arung_condition

BFalse

Cinput

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning output directly to input instead of rung condition.

The coil output energizes when the rung condition is True, so output equals rung_condition.

3fill in blank

hard

Fix the error in the rung logic that combines two contacts in series.

PLC Programming

rung_condition = contact1 [1] contact2

Drag options to blanks, or click blank then click option'

Axor

Bnot

Cand

Dor

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'or' causes the rung to energize if either contact is True.

Contacts in series require both to be True, so we use 'and' to combine them.

4fill in blank

hard

Fill both blanks to create a rung with parallel contacts.

PLC Programming

rung_condition = contact1 [1] contact2 [2] contact3

Drag options to blanks, or click blank then click option'

Aor

Band

Cnot

Dxor

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'and' causes the rung to energize only if all contacts are True.

Parallel contacts in ladder logic use 'or' to energize the rung if any contact is True.

5fill in blank

hard

Fill all three blanks to build a rung with series and parallel contacts.

PLC Programming

rung_condition = ((contact1 [1] contact2) [2] contact3) [3] contact4

Drag options to blanks, or click blank then click option'

Aand

Bor

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing 'and' and 'or' incorrectly changes the logic flow.

The rung has contact1 and contact2 in series, then in parallel with contact3, and finally in series with contact4.

Practice

1. Why does ladder logic mimic relay circuits in PLC programming?

easy

A. To allow programming in multiple languages

B. To increase the speed of the PLC processor

C. To reduce the cost of hardware components

D. To make it easy to understand and troubleshoot

Solution

1. Step 1: Understand the design goal of ladder logic

   Ladder logic was designed to look like relay circuits to help technicians easily understand and troubleshoot programs.

2. Step 2: Compare options with this goal

   Only To make it easy to understand and troubleshoot matches this purpose; others relate to hardware or languages, which are unrelated.

3. Final Answer:

   To make it easy to understand and troubleshoot -> Option D

4. Quick Check:

   Ease of understanding = To mimic relay circuits [OK]

Hint: Think about why technicians prefer relay diagrams [OK]

Common Mistakes:

• Choosing hardware cost reduction as the reason
• Confusing programming language support with design
• Assuming processor speed is the main goal

2. Which of the following is the correct symbol in ladder logic that mimics a relay contact?

easy

A. -->-- (Wire Connection)

B. --| |-- (Normally Open Contact)

C. --[ ]-- (Timer)

D. --( )-- (Coil)

Solution

1. Step 1: Identify relay contact symbol in ladder logic

   The normally open contact is represented by --| |--, mimicking a relay contact.

2. Step 2: Verify other symbols

   --( )-- is a coil, --[ ]-- is a timer, and -->-- is not a standard symbol.

3. Final Answer:

   --| |-- (Normally Open Contact) -> Option B

4. Quick Check:

   Relay contact symbol = --| |-- [OK]

Hint: Remember contacts look like switches: two vertical lines [OK]

Common Mistakes:

• Confusing coil symbol with contact
• Selecting timer symbol as contact
• Choosing non-standard symbols

3. Given this ladder logic snippet:

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

What happens when the Start button is pressed and Stop button is not pressed?

medium

A. Motor coil energizes and motor runs

B. Motor coil de-energizes and motor stops

C. Motor runs only if Stop is pressed

D. Nothing happens because Start is normally closed

Solution

1. Step 1: Analyze the contacts and coil

   Start is a normally open contact; pressing it closes the contact. Stop is normally closed contact |/|, so not pressing Stop keeps it closed.

2. Step 2: Determine coil state

   With Start pressed (closed) and Stop not pressed (closed), current flows to the Motor coil, energizing it and running the motor.

3. Final Answer:

   Motor coil energizes and motor runs -> Option A

4. Quick Check:

   Start pressed + Stop not pressed = Motor ON [OK]

Hint: Normally closed Stop contact opens when pressed [OK]

Common Mistakes:

• Thinking motor runs only if Stop is pressed
• Confusing normally open and normally closed contacts
• Assuming Start is normally closed

4. This ladder logic rung is intended to energize a light when a sensor is active:

--|/|--[Sensor]----( )--[Light]

But the light never turns on. What is the likely error?

medium

A. Using normally closed contact instead of normally open

B. Using coil symbol instead of contact symbol

C. Sensor input is wired incorrectly

D. Light coil is missing from the rung

Solution

1. Step 1: Understand contact types and sensor behavior

   The rung uses a normally closed contact |/| for Sensor, so when sensor is active (ON), contact opens, breaking the circuit.

2. Step 2: Identify correct contact type

   To energize light when sensor is active, a normally open contact | | should be used instead.

3. Final Answer:

   Using normally closed contact instead of normally open -> Option A

4. Quick Check:

   Wrong contact type = Light never turns on [OK]

Hint: Active sensor needs normally open contact to energize output [OK]

Common Mistakes:

• Assuming coil symbol error instead of contact
• Ignoring wiring issues
• Thinking light coil is missing

5. In a complex control system, why is it beneficial that ladder logic mimics relay circuits when troubleshooting?

hard

A. Because ladder logic runs faster than other languages

B. Because it requires fewer PLC inputs and outputs

C. Because technicians can use their existing knowledge of relay wiring

D. Because it automatically documents the control system

Solution

1. Step 1: Consider technician familiarity

   Technicians trained on relay wiring can read ladder logic easily since it looks like relay circuits.

2. Step 2: Evaluate other options

   Speed, I/O count, and automatic documentation are unrelated to why ladder logic mimics relay circuits for troubleshooting.

3. Final Answer:

   Because technicians can use their existing knowledge of relay wiring -> Option C

4. Quick Check:

   Familiarity aids troubleshooting = Mimics relay circuits [OK]

Hint: Think about who fixes the system and their skills [OK]

Common Mistakes:

• Choosing speed or hardware reasons
• Assuming automatic documentation is a feature
• Confusing I/O count with logic style