Internal relay (memory bit) addressing in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an internal relay (memory bit) in PLC programming?

An internal relay, also called a memory bit, is a virtual switch inside the PLC. It stores ON or OFF states but does not control physical devices directly. It helps in controlling logic inside the program.

beginner

How do you address an internal relay in a PLC program?

You address an internal relay by its memory bit name or number, like M0, M1, or %M0.0 depending on the PLC brand. This tells the PLC which internal bit to read or write.

intermediate

Why use internal relays instead of physical inputs or outputs?

Internal relays let you store and reuse logic states without needing physical devices. They make programs flexible and help manage complex control sequences inside the PLC.

beginner

Example: What does M10 = 1 mean in a PLC program?

It means the internal relay or memory bit number 10 is set to ON (1). This can be used to remember a condition or control other parts of the program.

intermediate

Can internal relays be used for timers and counters?

Yes, internal relays often store the results of timers and counters. They hold temporary states that help the PLC decide what to do next.

What does an internal relay in a PLC do?

A Stores ON/OFF states inside the PLC

B Controls physical motors directly

C Measures temperature

D Connects to the internet

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

How is an internal relay usually addressed in a PLC program?

A By the motor speed

B By its IP address

C By its memory bit name or number like M0 or %M0.0

D By the physical input number

Internal relays are addressed by memory bit names or numbers such as M0 or %M0.0.

Why use internal relays instead of physical outputs?

A To increase motor speed

B To reduce electricity consumption

C To connect to sensors

D To store logic states without physical devices

Internal relays store logic states inside the PLC without needing physical devices.

What does setting M10 = 1 mean?

A Motor 10 is running

B Memory bit 10 is ON

C Input 10 is active

D Timer 10 is reset

Setting M10 = 1 means memory bit 10 is ON.

Can internal relays be used with timers and counters?

A Yes, to store temporary states

B No, they only control motors

C No, they measure temperature

D Yes, but only for physical outputs

Internal relays store temporary states from timers and counters.

Explain what an internal relay (memory bit) is and how it is used in PLC programming.

Think of it as a light switch inside the PLC that controls logic, not real lights.

Describe how you would address and use an internal relay in a simple PLC program.

Use names like M0 to turn ON or OFF internal bits to remember conditions.

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