Internal relay (memory bit) addressing in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an internal relay at address M100.

PLC Programming

LD [1]

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

AM100

BX0

CY10

DD200

Attempts:

3 left

💡 Hint

Common Mistakes

Using input (X) or output (Y) addresses instead of internal relay (M).

Using data register (D) instead of memory bit (M).

The internal relay (memory bit) is addressed by M followed by the number, so M100 is correct.

2fill in blank

medium

Complete the code to set internal relay M150 ON.

PLC Programming

SET [1]

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

AD150

BM150

CX150

DY150

Attempts:

3 left

💡 Hint

Common Mistakes

Using output or input addresses instead of internal relay.

Confusing data registers with memory bits.

To set an internal relay ON, use SET followed by the internal relay address M150.

3fill in blank

hard

Fix the error in the code to reset internal relay M200.

PLC Programming

RST [1]

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

AX200

BD200

CM200

DY200

Attempts:

3 left

💡 Hint

Common Mistakes

Using input or output addresses instead of internal relay.

Using data register addresses with RST command.

RST command resets the internal relay at address M200, so M200 is correct.

4fill in blank

hard

Fill both blanks to read internal relay M300 and then set internal relay M301.

PLC Programming

LD [1]
SET [2]

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

AM300

BX300

CM301

DY301

Attempts:

3 left

💡 Hint

Common Mistakes

Using input or output addresses instead of internal relays.

Mixing addresses between the two commands.

LD reads internal relay M300, and SET sets internal relay M301.

5fill in blank

hard

Fill all three blanks to load internal relay M400, reset M401, and then set M402.

PLC Programming

LD [1]
RST [2]
SET [3]

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

AM400

BM401

CM402

DX400

Attempts:

3 left

💡 Hint

Common Mistakes

Using input addresses (X) instead of internal relays.

Mixing addresses between commands.

LD loads M400, RST resets M401, and SET sets M402, all internal relays.

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