feat: Add Slave ID support for Modbus TCP master plugin

[thiagoralves]

Summary

• Add Slave ID (Unit ID) support to the Modbus master plugin to enable communication through Modbus TCP gateways that forward messages to RS485 serial networks
• The Slave ID is passed to all pymodbus read/write operations via the slave parameter
• Default value is 1 for backward compatibility with existing configurations

Changes

• Add slave_id field to ModbusDeviceConfig with validation (0-255)
• Parse slave_id from JSON configuration (default 1)
• Store slave_id in ModbusConnectionManager
• Pass slave parameter to all Modbus operations (FC 1, 2, 3, 4, 5, 6, 15, 16)

JSON Configuration Format

{
  "name": "device_name",
  "protocol": "MODBUS",
  "config": {
    "type": "SLAVE",
    "host": "192.168.1.100",
    "port": 502,
    "timeout_ms": 1000,
    "slave_id": 5,
    "io_points": [...]
  }
}

Test plan

• Test with Modbus TCP device directly (slave_id = 1)
• Test with Modbus TCP gateway forwarding to RS485 devices with different slave IDs
• Verify backward compatibility with existing configurations without slave_id
• Verify validation rejects invalid slave_id values (<0 or >255)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds Slave ID (Unit ID) support to the Modbus TCP master plugin, enabling communication through Modbus TCP gateways that forward messages to RS485 serial networks. The implementation maintains backward compatibility by defaulting to slave_id=1.

Changes:

• Added slave_id field to ModbusDeviceConfig with validation for values 0-255
• Updated ModbusConnectionManager to store and use the slave_id parameter
• Modified all Modbus read/write operations (FC 1, 2, 3, 4, 5, 6, 15, 16) to pass the slave parameter

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
modbus_master_config_model.py	Added slave_id field with default value 1, validation logic, and updated repr method
modbus_master_plugin.py	Updated ModbusConnectionManager initialization and all Modbus read/write operations to include slave parameter
modbus_master_connection.py	Added slave_id parameter to init method with default value 1

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