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modqtt-gw is a unidirectional gateway for passing and publishing Modbus TCP data to MQTT implemented in Python based onuModbus for the Modbus TCP client side and onpaho-mqtt for the MQTT client side.

• modqtt-gw is currently a functional proof of concept, suitable for small scale applications
• My main objectives were to learn about MQTT and hide the complexity of these two protocols behind straightforward configuration/template files
• The Modbus and MQTT elements are currentlytightly coupled (i.e. if the Modbus TCP connection is interrupted the whole script stops and must be restarted). I am aware of this design limitation and plan to work on decoupling these two elements to make the script more robust and more usable.

There are two (2) important configuration files to setup for modqtt-gw to work properly:
 (1) a Modbus and MQTTconfiguration file in .json format (-c CONFIG_FILE, --config CONFIG_FILE); this is where you define the Modbus TCP Server (IP/hostname:Port:ID) and MQTT Broker to connect to (IP/URL:Port), the Modbus scan rate, MQTT version, MQTT TLS settings, MQTT authentication option, etc.
 (2) a Modbus and MQTTtemplate file in .csv format (-t TEMPLATE_FILE, --template TEMPLATE_FILE); this is where you define which Modbus register addresses to query, their read type (ex: Holding Registers, Input Registers) and data type (ex: float32, uint16, coil), their mapping into a more user-friendly tag name, apply linear scaling if needed, and define all the MQTT upload attributes
When calling modqtt-gw, pass the path to these two (2) configuration files as arguments.

$ (python3) path/to/modqtt-gw.py    -c <path to Modbus and MQTT configuration file (.json format)> (--config) [REQUIRED]    -t <path to Modbus and MQTT template file (.csv format)> (--template) [REQUIRED]    -e <path to .env file with username/password information used for authentication with the MQTT broker> (--env) [optional]    -C <string path to the Certificate Authority (CA) certificate files that are to be treated as trusted by this client (ex: some/path/ca.crt)> (--ca-certs) [optional]    -F <string pointing to the PEM encoded client certificate file (ex: some/path/client.crt)> (--certfile) [optional]    -K <string pointing to the PEM encoded client private key file (ex: some/path/client.key)> (--keyfile) [optional]    -f <to force the deadband logic on MQTT interval uploads, i.e. if set to True, do not report unless changes exceed the deadband, default False> (--force-deadband) [optional]    -q <to be quiet and to not display the interval Modbus reads, default False> (--quiet) [optional]    -h to show the help message and exit (--help) [optional]'

Format and content:

mqtt_broker_creds_username=<insert-username-here>mqtt_broker_creds_password=<insert-password-here>

This is where the username and password used for basic authentication with the MQTT Broker are set and stored. Pass the path to this .env file with the -e switch (--env) if needed.

Seetls_set():

• -C (--ca-certs) ~ ca_certs; "Certificate Authority certificate files that are to be treated as trusted by this client"
• -F (--certfile) ~ certfile; "PEM encoded client certificate"
• -K (--keyfile) ~ keyfile; "PEM encoded client private key"

 'modbus_server_ip': a correctly formatted string representing the IP address or hostname of the Modbus TCP Server to connect to; ex: "10.0.1.10" or "localhost"

 'modbus_server_port': a strictly positive integer [1;65535] representing the TCP port where the Modbus TCP Server process is running; ex: 502

 'modbus_server_id': a positive integer [0;255] representing the Modbus Server ID in use by the Modbus TCP Server; ex: 10

 'modbus_server_timeout_seconds': a positive floating point representing the number of seconds to use as timeout when connecting to the Modbus TCP Server; ex: 3.0

 'modbus_poll_interval_seconds': a positive floating point representing the time interval in seconds between two (2) consecutive Modbus polls (i.e. scan rate); ex: 1.0

 'mqtt_client_id': a string representing the MQTT Client ID to use, chosen by the MQTT client and used as prefix/root to all MQTT topics for this client

 'mqtt_broker_ip_or_url': a correctly formatted string representing the IP address, hostname or URL of the MQTT Broker to connect to; ex: "mqtt-broker-online.com" or "localhost"

 'mqtt_broker_port': a strictly positive integer [1;65535] representing the TCP port where the MQTT Broker process is running; ex: 8883

 'mqtt_connection_monitoring': boolean, either true or false; if true, the MQTT Client will monitor its connection status with the MQTT Broker and communicate the times of last connections/disconnections to the broker on a dedicated topic "_connection_monitoring"

 'mqtt_broker_tls': boolean, either true or false; whether to use TLS (true) or not (false); seetls_set()

 'mqtt_tls_insecure_set': boolean, either true or false; trueNOT recommended (insecure), seetls_insecure_set(): "Configure verification of the server hostname in the server certificate."

 'mqtt_v5': boolean, either true or false; set to true to use MQTT v5 (highest priority)

 'mqtt_v311': boolean, either true or false; set to true to use MQTT v3.1.1 (second highest priority)

 'mqtt_v31': boolean, either true or false; set to true to use MQTT v3.1.0 (least priority)

 'mqtt_max_inflight_messages_set': a positive integer value; seemax_inflight_messages_set(): "Set the maximum number of messages with QoS>0 that can be part way through their network flow at once.Defaults to 20. Increasing this value will consume more memory but can increase throughput."

  'address': seeaddress

  'read_type': seeread_type

  'data_type': seedata_type

  'tag_name': seetag_name

  'scaling_coeff': seescaling_coeff

  'scaling_offset': seescaling_offset

  'mqtt_topic': string representing the topic to publish to, this will be prepended to the tag_name (can be empty)

  'mqtt_payload': currently supports "text" (just publish the value) and "json" (publish value with UTC and local timestamps); defaults to "text" if not specified

  'mqtt_qos': the Quality of Service (QoS) level to use; either 0 (at most once), 1 (at least once), or 2 (exactly once); defaults to 0 if not specified

  'mqtt_retain': a string of either "true" or "false"; whether (true) or not (false) to retain the message; defaults to true if not specified

  'mqtt_publish': a string defining the MQTT publish method; either "rbe" (report-by-exception), or a value representing the regular interval upload time in seconds; default to "rbe" if not specified

  'mqtt_deadband': defines a deadband on thescaled value when reporting by exception; ignored by default for interval uploads/publish, use -f (--force-deadband) to force and apply the deadband logic with interval uploads/publish; defaults to 0 if none specified or unsupported, if data_type is 'di', 'coil' or 'packedbool', also if mqqt_publish is not rbe

  'mqtt_alarm_low': defines a low alarm threshold on thescaled value; the scaled value will be published at each scan interval (regardless of mqtt_publish) as long as it remains below this threshold; defaults to None if not specified

  'mqtt_alarm_high': defines a high alarm threshold on thescaled value; the scaled value will be published at each scan interval (regardless of mqtt_publish) as long as it remains above this threshold; defaults to None if not specified

  'mqtt_ignore_low': completly ignore reporting and alarming while thescaled value remains below this threshold; defaults to None if not specified

  'mqtt_ignore_high': completly ignore reporting and alarming while thescaled value remains above this threshold; defaults to None if not specified

./modqtt-gw.py -c config/modqtt_config_local.json -t template/modqtt_template_10.csv \-e config/.env-local-broker

./modqtt-gw.py -c config/modqtt_config_local.json -t template/modqtt_template_10.csv \-C /path/to/ca.crt \-F /path/to/client.crt \-K /path/to/client.key