Internal relay (memory bit) addressing in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Internal Relay Addressing in Ladder Logic

Given the following ladder logic snippet using internal relay M bits, what is the state of M10 after the rung executes?

Inputs:
- X0 = ON
- X1 = OFF

Rung:
--| X0 |----|/ X1 |----( M10 )

Assuming M10 was OFF before execution, what will be its state after?

A M10 will be ON (true)

B M10 will be OFF (false)

C M10 will toggle its previous state

D M10 state remains unchanged

💡 Hint

Remember that |/ X1| means NOT X1, so the contact is closed when X1 is OFF.

Since X0 is ON and X1 is OFF, the normally open contact X0 is closed and the normally closed contact /X1 is also closed. Both conditions are true, so the coil M10 is energized and turns ON.

🧠 Conceptual

intermediate

Purpose of Internal Relay (Memory Bit) in PLC Programming

What is the main purpose of using internal relay (memory bit) addressing in PLC programs?

A To directly control physical outputs like motors and lights

B To replace all physical inputs with virtual ones

C To store intermediate logic states without physical input/output devices

D To reset the PLC program automatically

💡 Hint

Think about how PLCs handle logic internally before activating outputs.

Internal relays (memory bits) are used to hold intermediate states or flags inside the PLC program. They do not correspond to physical devices but help in complex logic.

🔧 Debug

advanced

Identify the Error in Internal Relay Addressing

Consider this ladder logic snippet:

--| M5 |----| M10 |----( M5 )

What issue will this cause when the PLC runs?

A It causes a latch loop, potentially making M5 always ON

B It causes a syntax error in the PLC program

C It will never turn M5 ON because M10 is not set

D It causes a hardware fault in the PLC

💡 Hint

Look at how M5 is used both as input and output in the same rung.

Using M5 as both input and output in the same rung creates a latch or self-holding circuit. This can cause M5 to stay ON indefinitely once energized.

🚀 Application

advanced

Using Internal Relays for Sequential Control

You want to create a simple sequence where:
1. M0 turns ON when X0 is pressed.
2. M1 turns ON only after M0 is ON.
3. M2 turns ON only after M1 is ON.

Which of the following ladder logic snippets correctly implements this sequence?

A --| X0 |----( M2 )<br>--| M2 |----( M1 )<br>--| M1 |----( M0 )

B --| X0 |----( M0 )<br>--| M0 |----( M1 )<br>--| M1 |----( M2 )

C --| X0 |----( M0 )<br>--| M1 |----( M0 )<br>--| M2 |----( M1 )

D --| M0 |----( X0 )<br>--| M1 |----( M0 )<br>--| M2 |----( M1 )

💡 Hint

Think about the order of activation and how each memory bit depends on the previous one.

Option B correctly sets M0 when X0 is pressed, then M1 depends on M0, and M2 depends on M1, creating a proper sequence.

🧠 Conceptual

expert

Memory Bit Addressing Range and Usage

In a typical PLC, what is the common range of internal relay (memory bit) addresses available, and why is this range important?

A M0 to M31; small range to reduce programming complexity

B M0 to M255; limited range to prevent memory overflow

C M1000 to M1999; reserved only for special functions

D M0 to M9999; it provides enough bits for complex logic without physical I/O

💡 Hint

Think about how many internal bits are typically needed for industrial automation.

Many PLCs provide thousands of internal relay bits (e.g., M0 to M9999) to allow programmers to create complex internal logic without using physical inputs or outputs.

Practice

1. What is the main purpose of an internal relay (memory bit) in PLC programming?

easy

A. To store ON/OFF states inside the PLC without controlling physical devices

B. To directly control motors and actuators

C. To provide power to sensors

D. To display output on the PLC screen

Solution

1. Step 1: Understand internal relay function

   Internal relays store ON/OFF states inside the PLC memory but do not control physical devices.

2. Step 2: Compare options

   Only To store ON/OFF states inside the PLC without controlling physical devices correctly describes this purpose; others mention controlling devices or power, which is incorrect.

3. Final Answer:

   To store ON/OFF states inside the PLC without controlling physical devices -> Option A

4. Quick Check:

   Internal relay = memory bit for logic [OK]

Hint: Internal relays remember states, not control devices [OK]

Common Mistakes:

• Confusing internal relays with output devices
• Thinking internal relays supply power
• Assuming internal relays display information

2. Which of the following is the correct syntax to address an internal relay (memory bit) named M10 in a PLC program?

easy

A. D10

B. X10

C. Y10

D. M10

Solution

1. Step 1: Identify internal relay address format

   Internal relays are commonly addressed with the letter 'M' followed by a number, e.g., M10.

2. Step 2: Match options to address format

   M10 uses 'M10', which is correct. Other options use letters for inputs (X), outputs (Y), or data registers (D).

3. Final Answer:

   M10 -> Option D

4. Quick Check:

   Internal relay = M + number [OK]

Hint: Internal relays start with 'M' in address [OK]

Common Mistakes:

• Using input (X) or output (Y) addresses for internal relays
• Confusing data registers (D) with memory bits
• Omitting the letter prefix

3. Given the following ladder logic snippet, what is the state of internal relay M5 after the rung executes?

--[ ]--[ ]--(M5)\n   I1   I2

Inputs: I1 = ON, I2 = OFF

medium

A. ON

B. OFF

C. Toggle

D. Undefined

Solution

1. Step 1: Analyze input conditions

   The rung has two contacts in series: I1 and I2. I1 is ON, I2 is OFF.

2. Step 2: Determine relay output state

   Both inputs must be ON for M5 to energize. Since I2 is OFF, M5 remains OFF.

3. Final Answer:

   OFF -> Option B

4. Quick Check:

   Series contacts require all ON for output ON [OK]

Hint: Series contacts need all ON to energize relay [OK]

Common Mistakes:

• Assuming relay toggles automatically
• Ignoring OFF input in series
• Confusing parallel and series logic

4. In the following PLC code snippet, there is an error in addressing the internal relay. Identify the error:

LD X0\nAND M1\nOUT M2

What is the error?

medium

A. Using M1 as input in AND instruction is invalid

B. Incorrect internal relay address M2 in OUT instruction

C. No error, code is correct

D. Using input address X0 instead of internal relay

Solution

1. Step 1: Review each instruction

   LD X0 loads input X0 state, AND M1 uses internal relay M1 as condition, OUT M2 sets internal relay M2.

2. Step 2: Check addressing validity

   Inputs (X) and internal relays (M) are correctly used. M1 and M2 are valid internal relay addresses.

3. Final Answer:

   No error, code is correct -> Option C

4. Quick Check:

   Input and internal relay addresses used properly [OK]

Hint: Inputs use X, internal relays use M; both valid here [OK]

Common Mistakes:

• Thinking internal relays can't be inputs
• Assuming OUT can't target internal relays
• Confusing input and output addresses

5. You want to use internal relays M10 and M11 to remember if two sensors S1 and S2 were ON at any time during a process. Which ladder logic sequence correctly sets M10 and M11 to ON when their respective sensors turn ON, and keeps them ON until reset?

hard

A. --[S1]--(M10)\n--[M10]--(M10)\n--[S2]--(M11)\n--[M11]--(M11)\n--[Reset]--(/M10)\n--[Reset]--(/M11)

B. --[S1]--(M10)\n--[S2]--(M11)\n--[Reset]--(/M10)\n--[Reset]--(/M11)

C. --[S1]--(M10)\n--[S2]--(M11)\n--[Reset]--(M10)\n--[Reset]--(M11)

D. --[S1]--(M10)\n--[S2]--(M11)\n--[Reset]--(M10)\n--[Reset]--(/M11)

Solution

1. Step 1: Understand latch logic for memory bits

   To keep M10 and M11 ON after sensors turn ON, latch logic uses the internal relay itself in parallel with the sensor input.

2. Step 2: Analyze options for latch and reset

   --[S1]--(M10)\n--[M10]--(M10)\n--[S2]--(M11)\n--[M11]--(M11)\n--[Reset]--(/M10)\n--[Reset]--(/M11) shows M10 and M11 set by sensors and latched by themselves, with reset contacts to turn them OFF. Others lack latching or proper reset.

3. Final Answer:

   --[S1]--(M10)\n--[M10]--(M10)\n--[S2]--(M11)\n--[M11]--(M11)\n--[Reset]--(/M10)\n--[Reset]--(/M11) -> Option A

4. Quick Check:

   Latching uses self-holding contact + reset to clear [OK]

Hint: Latch with self-contact to hold ON until reset [OK]

Common Mistakes:

• Not using latch (self-holding) contacts
• Resetting only one relay
• Using reset coil instead of reset contact