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Server Overview

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Jens De Ketelaere edited this page Oct 31, 2025 · 9 revisions

Server mode allows you to create and configure virtual Modbus servers for testing, development, and simulation.

What is Server Mode?

In Server mode, Modbux acts as a Modbus server/slave that can:

  • Simulate Modbus devices without physical hardware
  • Create multiple independent server instances
  • Configure registers with various data types
  • Generate dynamic data for realistic testing
  • Save and load server configurations

Supported Server Types

Modbus TCP Server

  • Network-based Modbus server
  • Configurable port (default: 502)
  • Multiple instances on different ports
  • Ideal for Ethernet-based testing

Future support may include Modbus RTU (serial) servers.

Key Features

Multiple Unit IDs Per Server

Each server can simulate up to 248 independent devices (Unit IDs 0-247) on a single port:

  • Each Unit ID has its own complete register configuration

    • Independent coils, discrete inputs, input registers, and holding registers
    • Different data types and values per Unit ID
    • Separate generator settings per Unit ID

  • Use cases:

    • Simulate multi-drop Modbus RTU networks (requires RTU-TCP gateway to convert TCP to RTU)
    • Test client applications that need to communicate with multiple devices
    • Create complex test scenarios with different device types on one server

Example: On port 502, you could have:

  • Unit ID 1: Temperature sensor (10 input registers)
  • Unit ID 2: Pressure sensor (5 input registers)
  • Unit ID 3: Controller (20 holding registers, 10 coils)
  • All accessible through the same IP and port, just different Unit IDs

Multiple Servers

Run up to 10 independent servers simultaneously, each on a different port. Each server can have its own set of Unit IDs.

When to Use Server Mode

Use Server mode when you need to:

  • Test client applications without physical devices
  • Develop Modbus clients with predictable test data
  • Simulate device behavior before hardware is available
  • Create training environments for learning Modbus
  • Debug communication in a controlled environment

Main Interface Components

Top Bar

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All controls are located in a single horizontal bar at the top of the interface (left to right):

HOME BUTTON - Returns to the main Modbux home screen

OPEN - Load a saved server configuration

  • Opens JSON configuration files
  • Loads all 248 Unit IDs and their register configurations
  • Replaces current server configuration entirely

SAVE - Export current server configuration to file

  • Saves the complete server configuration to JSON
  • Includes server settings (port, name)
  • Includes all configured Unit IDs with their registers, values, generator settings, and comments

CLEAR - Reset server configuration

  • Removes all registers from all Unit IDs
  • Resets server to empty state
  • Warning: Cannot be undone

ADD/DELETE Server Buttons - Manage server instances

  • + Icon: Add new server instance (up to 10 total)
  • Trash Icon: Delete current server instance

SERVER SELECTOR - Button group to select which server instance to configure (1-10)

  • Exclusive selection: one server is always active
  • Each button displays the server's port number as text (e.g., "502", "5020")
  • Click a server button to switch to that server's configuration
  • Active server button is visually highlighted
  • Each server runs independently on its own port
  • Default main server starts on port 502

Server Name - Text input for descriptive server name

  • Helps identify servers when running multiple instances
  • Saved in configuration files

Unit ID Dropdown - Select which Unit ID to configure (0-247)

  • Critical: Each Unit ID has completely independent register configurations
  • Each Unit ID acts as a separate virtual device sharing the same IP and port
  • Switch between Unit IDs to configure different "virtual devices"
  • The register grid updates to show only the selected Unit ID's registers
  • Unit IDs with configured registers are visually highlighted in the dropdown

Port - Input field for network port (0-65535)

  • Default: 502 (standard Modbus TCP port)
  • Modbux validates port availability automatically
  • Ports must be unique per server

Important Note: Servers start automatically when configured. There is no manual start/stop control - servers are always running and ready to accept connections.

Register Containers

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Four collapsible containers, one for each Modbus register type. All containers are visible simultaneously - click to expand/collapse:

  • Coils (Function Codes 1, 5, 15)
    • Read/write boolean (1-bit) registers
    • Typical use: Relay control, digital outputs
    • Click on bit values to toggle directly
  • Discrete Inputs (Function Code 2)
    • Read-only boolean registers
    • Typical use: Switch states, digital sensors
    • Click on bit values to toggle (for simulation purposes)
  • Input Registers (Function Code 4)
    • Read-only 16-bit numeric registers
    • Typical use: Sensor measurements, analog inputs
    • Can hold multi-register data types (32-bit, 64-bit)
  • Holding Registers (Function Codes 3, 6, 16)
    • Read/write 16-bit numeric registers
    • Typical use: Configuration, setpoints, general data storage
    • Can hold multi-register data types (32-bit, 64-bit)

Each container expands to show the register grid and an Add Register button. Collapse containers to save screen space while keeping them accessible.

Register Grid (Within Each Container)

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Displays configured registers with the following columns:

  • LE Indicator: ONLY Shows if Little-Endian byte order is used (32/64-bit types only)
    • Default is Big-Endian (Modbus standard)
    • Toggle in register configuration
  • Address: Register address (0-65535)
    • Multi-register types show starting address
  • Data Type: Type of data stored
    • 16-bit: int16, uint16
    • 32-bit: int32, uint32, float (occupies 2 consecutive addresses)
    • 64-bit: int64, uint64, double (occupies 4 consecutive addresses)
  • Value: Current register value
    • Updates in real-time for generators
    • Can be edited directly for static values
  • Comment: User-defined description
    • Document register purpose, units, value ranges
    • Helps when sharing configurations with team members
  • Actions: Per-row action buttons
    • Edit Icon (pencil): Modify register configuration

Register Actions

Add Register Button (within each container):

  • Opens configuration dialog for new register
  • Configure: Address, Data Type, Value (Static or Generator), Comment, Endianness
  • Modbux suggests first available address automatically
  • Validates address conflicts and address space limits

Batch Clear Button (within each container):

  • Removes all registers of that type for the current Unit ID
  • Useful for quickly resetting a register type
  • Warning: Cannot be undone

Typical Workflow

  1. Select Server and Port

    • Choose server instance from dropdown (or use default)
    • Configure port number (default 502)
    • Enter descriptive server name

  2. Select Unit ID

    • Choose Unit ID from dropdown (default 0)
    • Remember: Each Unit ID represents a separate virtual device

  3. Add Registers

    • Expand the desired register type container (Coils, Discrete Inputs, Input Registers, or Holding Registers)
    • Click Add Register button
    • Configure register:
      • Address (automatically suggested)
      • Data type (int16, uint16, int32, uint32, float, int64, uint64, double)
      • Value type: Static (fixed) or Generator (dynamic random within min/max range)
      • Optional comment
    • Repeat for all needed registers

  4. Configure Multiple Unit IDs (optional)

    • Switch to different Unit ID using dropdown
    • Add registers for that Unit ID
    • Repeat for as many Unit IDs as needed

  5. Test Your Server

    • Use Split mode to open built-in client
    • In client: Connect to 127.0.0.1 using the same port
    • Read/write registers to verify configuration
    • For external clients (PLCs, SCADA): Use your PC's network IP address (e.g., 192.168.1.100:502)

  6. Save Configuration (optional)

    • Enter server name
    • Click Save button
    • Choose filename and location
    • All 248 Unit IDs and settings are saved to JSON file

Next Steps

Home

Getting Started

Navigation

Understanding Modbus

Client

Overview

Connection

Register Grid

Advanced Features

Server

Overview

Configuration

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