Special Rules to design ladder diagram logic

-Signals selection, Decisions section and Actions Section

-The sequence of the logic should be left to right

NORM_X AND SCALE_X value scaling in the S7-1200 PLC

Design and implement a PLC program in Siemens S7‑1200 that processes the analog signal from the pressure transmitter, normalizes it, and scales it into engineering units (bar), ensuring accurate and reliable pressure monitoring for display and control using NORMA_X and SCALE_X

PLCSCADACADEMY

Explanation

1. Hardware Integration (S7-1200)

The S7-1200 typically features onboard analog inputs (0-10V) or uses signal boards/modules for current signals (4-20mA).

Wiring the Pressure Transmitter

• 2-Wire Transmitter: The transmitter is powered by the loop. The PLC provides 24V DC.

             Path: L+ → Transmitter (+) → Transmitter (-) → PLC Input (+) → PLC Input (-) → M.

• 4-Wire Transmitter: The transmitter has separate power and signal wires.

             Path: Signal (+) → PLC Input (+) | Signal (-) → PLC Input (-).

S7-1200 Addressing & Range

• Onboard AI: Usually %IW64 and %IW66.
• Expansion Modules: Addressing starts at %IW96 or higher.
• Digital Range: 0 to 27648 (Normal range).
• Deadband/Wire Break: For 4-20mA, if the raw value drops to -4864, it indicates 0 mA (wire break).

2. Technical Case Study: Pressure Measurement

Application: Monitoring a hydraulic line pressure.

• Sensor: 4-20 mA Pressure Transmitter.
• Physical Range: 0.0 to 100.0 Bar.
• PLC: S7-1200 with an AI 4x13 bit SM 1231 module.

Signal Mapping Table

Pressure (Bar)	Signal (mA)	S7 Raw Value (%IW)	S7 Normalized (0.0 - 1.0)
0.0 Bar	4 mA	0	0.00
25.0 Bar	8 mA	6,912	0.25
50.0 Bar	12 mA	13,824	0.50
75.0 Bar	16 mA	20,736	0.75
100.0Bar	20 mA	27,648	1.00

 

PLC program:-

Write the ladder program for the above application using ladder diagram language. Here for ladder logic, we can use any supported PLC system like S7-1200, S7-300, S7-1500, or any other PLC which can support this instruction. For PLC logic we need digital inputs and outputs.

Define the memory register for the program as per the following address,

List of M Memory

MW10:-For analog value (0 to 27648). ( Convert int to real for real value )

MW12:-Out of NORM_X ( Convert int to real for real value )

MW14:-Output value ( Convert int to real for real value )

 

PLCSCADAACADEMY

PLCSCADAACADEMY

PLC program Explanation:-

 Network 1 – Input Value Conversion (NORM_X)

• Instruction used: NORM_X
• Purpose: To normalize the raw analog input value into a floating‑point number between 0.0 and 1.0.
• Why needed:
• Analog input modules return integer values (e.g., 0 to 27648 for 0–20 mA).
• These raw values are not directly meaningful in engineering terms.
• Normalization ensures that no matter the input range, the value is scaled into a standardized ratio (0.0–1.0).

Process:

1. The PLC reads the raw analog input (e.g., 0–27648).
2. NORM_X divides the input by the maximum possible value.

3. Result is always between 0.0 (minimum) and 1.0 (maximum).

Example:

• Raw input = 13,824 (half of 27,648)
• Normalized value = 0.5

So, the sensor signal is now expressed as a percentage of full scale.

Network 2 – Output Value Conversion (SCALE_X)

• Instruction used: SCALE_X
• Purpose: To convert the normalized value (0.0–1.0) into a desired engineering unit range.
• Why needed:
• Engineers want values in real units (temperature, pressure, speed, etc.).
• SCALE_X maps the normalized ratio into the chosen engineering range.

Process:

1. Take the normalized value from Network 1.
2. Multiply it by the engineering unit span (Max – Min).
3. Add the minimum engineering unit value.

Example:

• Normalized value = 0.5
• Engineering unit range = 0–100
• Calculation:

So, the PLC output is 50 units (e.g., 50.0 bar if measuring bar).

Putting It Together

• Network 1 (NORM_X): Converts raw sensor signal → normalized ratio (0.0–1.0).
• Network 2 (SCALE_X): Converts normalized ratio → engineering units (0–100).

This two‑step process ensures:

• Flexibility (any input range can be normalized).
• Accuracy (engineering values are scaled correctly).
• Simplicity (easy to reuse across different sensors and outputs).

 

Explain Leak testing System using example.

Application:-Make a leak test system. Write PLC program for leak test system using ladder diagram language.

Diagram:-

Explanation:-

As per the above application, there is one pipe in the system. We need to check its leakage by applying pressure up to 4- 5 bar. For leakage testing, the first pipe will be put into the fixture and one side pipe will be blocked completely. From the other side, the pressure will be applied.

The transmitter will read the pressure. Once the pressure achieved into the pipe, the system will close the inlet pressure valve and hold pressure into the pipe. If there is no pressure drop for 1 minute, the pipe is ok and if the pressure drop measured during 1 minute, the pipe is leaked.

PLC program:-

Write the ladder program for the above application using ladder diagram language. Here for ladder logic, we can use any supported PLC system like S7-1200, S7-300, S7-1500 or any other PLC which can support this instruction. For PLC logic we need digital inputs and outputs.

Digital inputs:-

Start Button=I0.0

Digital Output:-

Inlet Valve=Q0.0

Ok lamp=Q0.1

M Memory:-

MD24= Set Pressure

MD20=Actual Pressure

Network 1:-Inlet valve for air supply

 

Network 2:-Inlet valve reset when pressure achieved

Network 3:-Indication timer for ok pipe

PLC program Explanation:-

In network 1, the inlet valve (Q0.0) will on when the start button (I0.0) is pressed. Here we used set coil so the output will be set after the start button pressed.

In network 2, the inlet valve will be off when Actual pressure (MD20) is greater or equal to set pressure (MD24).

In network 3, if the pressure will not drop from the pipe for 60 seconds, ok indication lamp will be on.