Source code: GitHub

Container image: DockerHub

• latest, 1.1.5

The Modbus TCP Server is a simple, in python written, Modbus TCP server. The Modbus registers can be also predefined with values.

The Modbus server was initially created to act as a Modbus slave mock system for enhanced tests with modbus masters and to test collecting values from different registers.

The Modbus specification can be found here: PDF

Step - 1 : Pull the Modbus TCP Server

docker pull jforge/modbus-server

Step - 2 : Run the Modbus TCP Server

docker run --rm -p 5020:5020 jforge/modbus-server:latest

Step - 3 : Predefine registers

The default configuration file is configured to initialize every register with a 0x0000. To set register values, you need to create your own configuration file.

docker run --rm -p 5020:5020 -v ./server_config.json:/server_config.json jforge/modbus-server:latest -f /server_config.json

or you mount the config file over the default file, then you can skip the file parameter:

docker run --rm -p 5020:5020 -v ./server_config.json:/app/modbus_server.json jforge/modbus-server:latest

The path and filename to the general configuration file can be set via command option -f <file> after mounting it inside of the container. By default, the script will use /app/modbus_server.json.

The /app/modbus_server.json file comes with following content:

{
"server": {
  "listenerAddress": "0.0.0.0",
  "listenerPort": 5020,
  "tlsParams": {
    "description": "path to certificate and private key to enable tls",
    "privateKey": null,
    "certificate": null
    },
  "logging": {
    "format": "%(asctime)-15s %(threadName)-15s  %(levelname)-8s %(module)-15s:%(lineno)-8s %(message)s",
    "logLevel": "INFO"
    }
  },
"registers": {
  "description": "initial values for the register types",
  "zeroMode": false,
  "initializeUndefinedRegisters": true,
  "discreteInput": {},
  "coils": {},
  "holdingRegister": {},
  "inputRegister": {}
  }
}

Pre-define registers always starts with the register number. We use a json format as configuration file, so the "key" needs to be a string. So, the register number needs also to be a string. During server initialization, the json key that represents the register number will be converted to an integer.

As by the modbus spec, the "Discrete Input" and "Coils" registers contains a single bit. In the json configuration file, we use true or false as register values.

Example configuration of pre-defined registers from type "Discrete Input" or "Coils":

[..]
  "<register_type>": {
    "0": true,
    "1": true,
    "42": true,
    "166": false
  }
[..]

As by the modbus spec, the "Holding Registers" and "Input Registers" tables contains a 16-bit word. In the json configuration file, we use a hexadecimal notation, starting with 0x, as register values.

Example configuration of pre-defined registers from type "Holding Registers" or "Input Registers":

[..]
  "<register_type>": {
    "9": "0xAA00",
    "23": "0xBB11",
    "142": "0x1FC3",
    "2346": "0x00FF"
  }
[..]

• src/app/modbus_server.json
• examples/abb_coretec_example.json
• examples/test.json

version: '3.8'

services:
  modbus-server:
    container_name: modbus-server
    image: jforge/modbus-server:latest
    restart: always
    command: -f /server_config.json
    ports:
      - 5020:5020
    volumes:
      - ./server.json:/server_config.json:ro