Skip to content
Meddle Docs

Siemens S7 Connector

Overview

Section titled “Overview”

The Siemens S7 connector enables direct communication with Siemens S7-series PLCs without requiring OPC UA or other middleware.

Connector Types:

  • S7Reader - Read data from S7 PLCs
  • S7Writer - Write data to S7 PLCs

Supported PLCs:

  • S7-300
  • S7-400
  • S7-1200
  • S7-1500

Basic Configuration

Section titled “Basic Configuration”

S7 Reader

Section titled “S7 Reader”
{
  "type": "S7Reader",
  "config": {
    "endpoint": "192.168.0.1:102",
    "pollingRate": 1000,
    "rack": 0,
    "slot": 2
  },
  "variables": [
    {
      "key": "temperature",
      "dataType": "Real",
      "memoryArea": "DB",
      "blockNumber": 1,
      "startIdx": 0,
      "size": 4,
      "bitOrder": "BigEndian"
    },
    {
      "key": "counter",
      "dataType": "Dint",
      "memoryArea": "DB",
      "blockNumber": 1,
      "startIdx": 4,
      "size": 4,
      "bitOrder": "BigEndian"
    }
  ]
}

S7 Writer

Section titled “S7 Writer”
{
  "type": "S7Writer",
  "config": {
    "endpoint": "192.168.0.1:102",
    "rack": 0,
    "slot": 2
  },
  "variables": [
    {
      "key": "setpoint",
      "dataType": "Real",
      "memoryArea": "DB",
      "blockNumber": 1,
      "startIdx": 100,
      "size": 4,
      "bitOrder": "BigEndian"
    }
  ]
}

Configuration Parameters

Section titled “Configuration Parameters”

Endpoint

Section titled “Endpoint”

PLC IP address and port (default port is 102):

{
  "endpoint": "192.168.0.1:102"
}

Rack and Slot

Section titled “Rack and Slot”

Physical location of the CPU module:

{
  "rack": 0,
  "slot": 2
}

Common configurations:

  • S7-300: rack=0, slot=2
  • S7-400: rack=0, slot=2 or 3
  • S7-1200: rack=0, slot=1
  • S7-1500: rack=0, slot=1

Polling Rate

Section titled “Polling Rate”

For readers, interval in milliseconds:

{
  "pollingRate": 1000
}

Memory Areas

Section titled “Memory Areas”

DB (Data Blocks)

Section titled “DB (Data Blocks)”

Most common area for structured data:

{
  "key": "temperature",
  "dataType": "Real",
  "memoryArea": "DB",
  "blockNumber": 1,
  "startIdx": 0,
  "size": 4
}

Requires blockNumber to specify which DB.

M (Merkers/Flags)

Section titled “M (Merkers/Flags)”

General-purpose memory:

{
  "key": "flag_value",
  "dataType": "Word",
  "memoryArea": "M",
  "startIdx": 0,
  "size": 2
}

I (Inputs)

Section titled “I (Inputs)”

Physical input signals (read-only):

{
  "key": "sensor_input",
  "dataType": "Bool",
  "memoryArea": "I",
  "startIdx": 0,
  "size": 1
}

Q (Outputs)

Section titled “Q (Outputs)”

Physical output signals (read/write):

{
  "key": "motor_output",
  "dataType": "Bool",
  "memoryArea": "Q",
  "startIdx": 0,
  "size": 1
}

C (Counters)

Section titled “C (Counters)”

Counter values:

{
  "key": "production_count",
  "dataType": "Word",
  "memoryArea": "C",
  "startIdx": 0,
  "size": 2
}

Data Types

Section titled “Data Types”

Bool (1 bit)

Section titled “Bool (1 bit)”

Boolean value:

{
  "dataType": "Bool",
  "size": 1
}

Byte (8 bit)

Section titled “Byte (8 bit)”

Unsigned 8-bit integer (0-255):

{
  "dataType": "Byte",
  "size": 1
}

Word (16 bit)

Section titled “Word (16 bit)”

Unsigned 16-bit integer (0-65535):

{
  "dataType": "Word",
  "size": 2
}

Dword (32 bit)

Section titled “Dword (32 bit)”

Unsigned 32-bit integer:

{
  "dataType": "Dword",
  "size": 4
}

Int (16 bit signed)

Section titled “Int (16 bit signed)”

Signed 16-bit integer (-32768 to 32767):

{
  "dataType": "Int",
  "size": 2
}

Dint (32 bit signed)

Section titled “Dint (32 bit signed)”

Signed 32-bit integer:

{
  "dataType": "Dint",
  "size": 4
}

Real (32 bit float)

Section titled “Real (32 bit float)”

32-bit floating point:

{
  "dataType": "Real",
  "size": 4
}

Char (8 bit ASCII)

Section titled “Char (8 bit ASCII)”

ASCII character:

{
  "dataType": "Char",
  "size": 1
}

Byte Order

Section titled “Byte Order”

S7 PLCs use Big Endian byte order by default:

{
  "bitOrder": "BigEndian"
}

Variable Structure

Section titled “Variable Structure”

Complete variable definition:

{
  "key": "temperature",        // Meddle payload key
  "dataType": "Real",          // S7 data type
  "memoryArea": "DB",          // Memory area
  "blockNumber": 1,            // DB number (for DB area)
  "startIdx": 0,               // Start byte offset
  "size": 4,                   // Size in bytes
  "bitOrder": "BigEndian"      // Byte order
}

Common Use Cases

Section titled “Common Use Cases”

1. Production Monitoring

Section titled “1. Production Monitoring”
{
  "type": "S7Reader",
  "config": {
    "endpoint": "192.168.1.10:102",
    "pollingRate": 1000,
    "rack": 0,
    "slot": 2
  },
  "variables": [
    {
      "key": "machine_status",
      "dataType": "Word",
      "memoryArea": "DB",
      "blockNumber": 10,
      "startIdx": 0,
      "size": 2,
      "bitOrder": "BigEndian"
    },
    {
      "key": "production_count",
      "dataType": "Dint",
      "memoryArea": "DB",
      "blockNumber": 10,
      "startIdx": 2,
      "size": 4,
      "bitOrder": "BigEndian"
    },
    {
      "key": "cycle_time",
      "dataType": "Real",
      "memoryArea": "DB",
      "blockNumber": 10,
      "startIdx": 6,
      "size": 4,
      "bitOrder": "BigEndian"
    }
  ]
}

2. Process Control

Section titled “2. Process Control”
{
  "type": "S7Writer",
  "config": {
    "endpoint": "192.168.1.10:102",
    "rack": 0,
    "slot": 2
  },
  "variables": [
    {
      "key": "temperature_setpoint",
      "dataType": "Real",
      "memoryArea": "DB",
      "blockNumber": 20,
      "startIdx": 0,
      "size": 4,
      "bitOrder": "BigEndian"
    },
    {
      "key": "enable_process",
      "dataType": "Bool",
      "memoryArea": "M",
      "startIdx": 0,
      "size": 1,
      "bitOrder": "BigEndian"
    }
  ]
}

3. Digital I/O Monitoring

Section titled “3. Digital I/O Monitoring”
{
  "type": "S7Reader",
  "config": {
    "endpoint": "192.168.1.10:102",
    "pollingRate": 500,
    "rack": 0,
    "slot": 2
  },
  "variables": [
    {
      "key": "emergency_stop",
      "dataType": "Bool",
      "memoryArea": "I",
      "startIdx": 0,
      "size": 1,
      "bitOrder": "BigEndian"
    },
    {
      "key": "motor_running",
      "dataType": "Bool",
      "memoryArea": "Q",
      "startIdx": 0,
      "size": 1,
      "bitOrder": "BigEndian"
    }
  ]
}

Troubleshooting

Section titled “Troubleshooting”

Connection Failed

Section titled “Connection Failed”

Solutions:

  • Verify PLC IP address and port
  • Check network connectivity: ping [plc-ip]
  • Ensure PLC allows external connections (check PLC configuration)
  • Verify rack and slot numbers match your PLC configuration
  • Check firewall rules (port 102)

Access Denied

Section titled “Access Denied”

Solutions:

  • Check PLC protection settings
  • Verify PUT/GET communication is enabled in PLC
  • Ensure no password protection on the PLC
  • Check if PLC is in RUN mode (some operations require STOP mode)

Wrong Data Values

Section titled “Wrong Data Values”

Solutions:

  • Verify startIdx (byte offset) is correct
  • Check size matches the data type
  • Confirm blockNumber for DB areas
  • Verify bitOrder (should be BigEndian for S7)
  • Use TIA Portal or STEP 7 to verify memory layout

Slow Performance

Section titled “Slow Performance”

Solutions:

  • Increase pollingRate to reduce frequency
  • Group consecutive memory addresses
  • Reduce number of variables
  • Check network latency

Best Practices

Section titled “Best Practices”
  1. Use Data Blocks: Organize data in DBs for better structure
  2. Document Memory Layout: Keep a map of your DB structure
  3. Group Consecutive Addresses: More efficient than scattered reads
  4. Test in STOP Mode First: Safer for initial testing
  5. Use Appropriate Data Types: Match PLC program data types exactly

Limitations

Section titled “Limitations”
Section titled “Related Connectors”

Additional Resources

Section titled “Additional Resources”