Siemens S7-1200 PLC Programming: From Basics to Application

PLC in Industrial Automation Systems

PLC (Programmable Logic Controller) is the brain controlling every industrial automation system. From food packaging lines, conveyor systems in warehouses, to HVAC control for buildings — PLCs are everywhere.

Siemens S7-1200 is the most popular PLC series in Vietnam due to reasonable pricing, sufficient performance, and broad ecosystem support. This article will take you from zero to writing your first PLC program.

S7-1200 Hardware Configuration

Popular CPUs

Expansion Modules

Each CPU supports expansion:

• Signal Board (SB): Attach directly to CPU, add compact I/O
• Signal Module (SM): Separate expansion module, add DI/DO/AI/AO
• Communication Module (CM): RS232/RS485, PROFIBUS, AS-i

Example configuration for product sorting conveyor system:

• CPU 1214C DC/DC/DC (14 DI, 10 DO)
• SM 1231 AI (8 analog inputs for weight sensors)
• CM 1241 RS485 (Mitsubishi frequency drive connection)
• SB 1232 AQ (1 analog output for speed control)

Programming with TIA Portal

TIA Portal (Totally Integrated Automation) is Siemens' integrated development environment. Version V17 and above are recommended for full OPC UA support and improvements.

Program Structure

A TIA Portal project is organized as:

Project
├── PLC_1 [CPU 1214C]
│   ├── Program blocks
│   │   ├── OB1 (Main) — Main program, runs each cycle
│   │   ├── FB1 (Motor_Control) — Function Block for motor control
│   │   ├── FC1 (Calculate_Speed) — Function for calculation
│   │   └── DB1 (Motor_Data) — Data Block storing parameters
│   ├── Technology objects
│   │   └── PID_Compact — PID controller
│   └── PLC tags
│       └── Tag table — Physical I/O mapping

Ladder Diagram (LAD)

Most intuitive language, suitable for on/off logic control. Easy for electricians to read and maintain:

// Motor self-holding circuit
|---[ I0.0 ]---[/I0.1 ]---( Q0.0 )---|   I0.0: START button
|                                       |   I0.1: STOP button (NC)
|---[ Q0.0 ]--------------------------|   Q0.0: Motor contactor

// Sequential control of 3 conveyor belts
|---[ I0.0 ]---[TON T1, 2s]---( Q0.0 )---|   Belt 1: start immediately
|---[ Q0.0 ]---[TON T2, 3s]---( Q0.1 )---|   Belt 2: delay 2s
|---[ Q0.1 ]---[TON T3, 3s]---( Q0.2 )---|   Belt 3: delay additional 3s

Function Block Diagram (FBD)

Suitable for analog processing and PID algorithms. Example: oven temperature control:

┌─────────────┐
│ PID_Compact │
│             │
│  Setpoint ──┤── 180.0 (°C)
│  Input ─────┤── IW64 (Thermocouple)
│  Output ────┤── QW80 (Heater SSR)
│  Kp ────────┤── 2.5
│  Ti ────────┤── 10.0s
│  Td ────────┤── 1.0s
└─────────────┘

Structured Text (SCL)

High-level language similar to Pascal, for complex algorithms. Recommended for complex logic:

// Temperature control with hysteresis and alarm
FUNCTION_BLOCK "FB_TempControl"
VAR_INPUT
    Temperature : REAL;    // Measured temperature
    SetPoint    : REAL;    // Target temperature
    Hysteresis  : REAL;    // Deadband (typically 2-5°C)
    AlarmHigh   : REAL;    // High alarm threshold
    AlarmLow    : REAL;    // Low alarm threshold
END_VAR
VAR_OUTPUT
    HeaterOutput : BOOL;
    CoolerOutput : BOOL;
    AlarmActive  : BOOL;
END_VAR

// Control logic
IF Temperature > SetPoint + Hysteresis THEN
    HeaterOutput := FALSE;
    CoolerOutput := TRUE;
ELSIF Temperature < SetPoint - Hysteresis THEN
    HeaterOutput := TRUE;
    CoolerOutput := FALSE;
END_IF;

// Check alarm
AlarmActive := (Temperature > AlarmHigh) OR (Temperature < AlarmLow);

SCADA Connection and Monitoring

S7-1200 supports multiple connection protocols from firmware V4.4:

OPC UA (Recommended)

OPC UA is the industrial standard for M2M communication. S7-1200 V4.4+ has integrated OPC UA Server:

# Python client reading data from PLC via OPC UA
from opcua import Client

client = Client("opc.tcp://192.168.0.1:4840")
client.connect()

# Read temperature from PLC
temp_node = client.get_node("ns=3;s=\"DB_Process\".\"Temperature\"")
temperature = temp_node.get_value()
print(f"Current temperature: {temperature}°C")

# Write setpoint
sp_node = client.get_node("ns=3;s=\"DB_Process\".\"SetPoint\"")
sp_node.set_value(180.0)

client.disconnect()

S7 Protocol + Node-RED

For quick and flexible solutions:

{
    "nodes": [
        {
            "type": "s7 in",
            "address": "DB1,REAL0",
            "name": "Read Temperature",
            "interval": 1000
        }
    ]
}

Integrated Web Server

S7-1200 has an integrated web server allowing simple HTML dashboards. Suitable for basic monitoring without separate SCADA.

Integrating PLC with Robot (ROS 2)

The most interesting part — connecting PLC with modern robot systems. Hybrid PLC + ROS 2 architecture leverages:

• PLC: Stable, deterministic, handles safety I/O per robot safety standards
• ROS 2: Flexible, AI/ML, path planning, computer vision

┌─────────────────────────────────────────┐
│              ROS 2 System               │
│  ┌──────────┐  ┌──────────┐  ┌───────┐ │
│  │ Nav2     │  │ MoveIt2  │  │ Vision│ │
│  │ (AMR)    │  │ (Arm)    │  │ (CV)  │ │
│  └────┬─────┘  └────┬─────┘  └───┬───┘ │
│       └──────────────┼────────────┘     │
│              ┌───────┴───────┐          │
│              │ OPC UA Bridge │          │
│              └───────┬───────┘          │
└──────────────────────┼──────────────────┘
                       │ OPC UA
┌──────────────────────┼──────────────────┐
│              ┌───────┴───────┐          │
│              │  S7-1200 PLC  │          │
│              └───────┬───────┘          │
│       ┌──────────────┼────────────┐     │
│  ┌────┴─────┐  ┌─────┴────┐  ┌───┴───┐ │
│  │ Sensors  │  │ Actuators│  │ Safety│ │
│  │ (I/O)    │  │ (Motor)  │  │ (E-Stop)│ │
│  └──────────┘  └──────────┘  └───────┘ │
│              PLC Layer                  │
└─────────────────────────────────────────┘

Debugging and Best Practices

Diagnostic Buffer

S7-1200 logs the last 50 events. Always check diagnostic buffer when errors occur:

• TIA Portal → Online & Diagnostics → Diagnostic buffer

Tips from Real Experience

1. Always use Symbolic names — Set meaningful tag names (Motor_Conveyor_1_Run instead of Q0.0)
2. Backup project to Git — TIA Portal projects are directories, version-controllable
3. Comment every network — You'll forget logic after 6 months without comments
4. Use FB instead of FC when state needed — FB has instance DB, FC doesn't
5. Test on PLCSIM first — TIA Portal has simulator, test logic before downloading to real PLC
6. Monitor cycle time — Keep cycle time < 10ms for motion, < 100ms for HVAC

Common Errors

Advice for Beginners

• Hardware: Start with CPU 1214C DC/DC/DC — reasonable price (~5.5M VND), sufficient features for learning and real projects
• Software: Use TIA Portal V17+ for full OPC UA and simulator support
• Learning path: LAD → SCL → FB/DB → OPC UA → Robot integration
• Community: Join Siemens Vietnam Facebook group, Siemens Support forums
• Certification: Siemens SITRAIN program — international certification valuable for career

Related Articles

• Digital Twin in Manufacturing: Smart Factory Simulation
• MQTT Protocol: Communication Between Robot and Cloud
• Industrial Robot Safety Standards: ISO 10218 and ISO/TS 15066