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Modbus Slave

The Modbus Slave feature enables your device to act as a Modbus slave (server), allowing other Modbus masters to read your device's data and control its functions. This is essential for integrating your device into existing industrial automation systems.

Overview​

As a Modbus slave, your device can:

  • Expose sensor data to SCADA systems
  • Accept control commands from PLCs
  • Integrate with industrial monitoring systems
  • Provide standard Modbus interface to custom hardware
  • Enable remote device control and monitoring

Supported Protocols​

AppBlocks supports both Modbus slave variants:

Modbus RTU Slave​

  • Serial communication (RS-485, RS-232)
  • Binary protocol for efficiency
  • Multi-drop bus topology
  • Unique slave address (1-247)

Modbus TCP Slave​

  • Ethernet-based communication
  • Port 502 (standard Modbus TCP port)
  • Multiple simultaneous master connections
  • Higher bandwidth and speed

Configuration​

Configure Modbus Slave with these properties:

For Modbus RTU:​

  • Slave Address: Device address on Modbus bus (1-247)
  • Serial Port: COM port for communication
  • Baud Rate: Communication speed (9600, 19200, 38400, etc.)
  • Parity: None, Even, or Odd
  • Stop Bits: 1 or 2
  • Data Bits: 7 or 8

For Modbus TCP:​

  • Port: TCP port to listen on (default: 502)
  • Maximum Connections: Concurrent master limit

Register Mapping​

Map your application variables to Modbus registers:

Register Types:​

  • Coils (0x): Digital outputs (read/write)
  • Discrete Inputs (1x): Digital inputs (read-only)
  • Input Registers (3x): Analog inputs (read-only)
  • Holding Registers (4x): Analog data (read/write)

Address Ranges:​

  • Addresses 0-65535 available
  • Map to AppBlocks variables

Use these blocks for Modbus slave functionality:

Register Definition​

Define Modbus registers for your application:

  1. Choose register type (coil, discrete input, input register, holding register)
  2. Assign register address
  3. Register is automatically linked to a variable
  4. Configure data format (int16, uint16, float, etc.)

Automatic Variable Sync​

Register values automatically synchronize with linked variables:

  • Master reads get current variable value
  • Master writes update variable value
  • Variable changes update register values
  • Bidirectional data flow

Function Code Support​

Modbus slave responds to:

  • FC01: Read Coils
  • FC02: Read Discrete Inputs
  • FC03: Read Holding Registers
  • FC04: Read Input Registers
  • FC05: Write Single Coil
  • FC06: Write Single Register
  • FC15: Write Multiple Coils
  • FC16: Write Multiple Registers

Exception Handling​

The slave automatically handles:

  • Invalid function codes
  • Invalid register addresses
  • Invalid data values
  • Communication errors

Masters receive appropriate Modbus exception codes.

Security Considerations​

Modbus protocol has limited built-in security:

  • Use Modbus TCP with VPN for secure remote access
  • Implement firewall rules to restrict access
  • Use Modbus RTU on isolated networks
  • Consider additional authentication layers

Performance​

Optimize slave performance:

  • Minimize register mapping overhead
  • Use continuous register blocks when possible
  • Avoid complex calculations in read handlers
  • Batch variable updates

Linking to Settings​

Modbus slave properties can be linked to settings for:

  • Runtime slave address configuration
  • Dynamic register mapping
  • Field-configurable communication parameters

Use Cases​

Common Modbus slave applications:

  • Exposing sensor data to SCADA systems
  • Integrating custom devices with PLCs
  • Building management system integration
  • Industrial data acquisition
  • Remote monitoring endpoints

Integration Examples​

Connect your AppBlocks device to:

  • Allen-Bradley PLCs
  • Siemens S7 controllers
  • Schneider Electric systems
  • LabVIEW applications
  • Third-party SCADA platforms

Troubleshooting​

Common Modbus slave issues:

  • No response: Check slave address and communication settings
  • CRC errors: Verify baud rate and parity settings
  • Invalid data: Check register mapping and data types
  • Timeout: Verify network connectivity and firewall rules

See Also​