Modbus devices#

According to gateway specification, all modbus device event types have prefix:

'gateway', <gateway_name>, 'modbus_master', <device_name>, <source>, ...

Together with modbus specific events, generic enable and running events are also supported.

Modbus specific events don’t contain source_timestamp.

Data value#

All modbus native data types are encoded as 1bit or 16bit addressable values. Because of different data encoding schemas, single user defined data values can occupy only parts of modbus registers or span across multiple registers. To accommodate these different encoding rules, modbus gateway devices represent single data point values as list of bits, encoded as single unsigned integer, with legth specified by configuration parameters.

Among other parameters, each data specifies:

  • data type (1bit or 16bit register size)

  • bit count

  • starting register address

  • bit offset

List of bit values is created by iterative reading of register values starting with starting register address and incrementing register address by 1. Bits in each register are read starting with most significant and advancing to the least significant bit. Starting register bit defines bit offset - number of bits which are skipped during start of this iterative procedure. These skipped bits are not included in resulting value and are not taken into account in bit count calculation.

Encoding/decoding example#

In case of 16bit registers with content:

Address

Value

1000

0x1234

1001

0x5678

1002

0x9abc

When data item is configured with properties:

  • bit count: 32

  • starting register address: 1000

  • bit offset: 8

Data value is unlimited unsigned integer 0x3456789a.

Modbus master#

Modbus master device configuration is specified by hat-gateway://modbus.yaml#/definitions/master.

Once enabled, modbus master device will try to open communication connection defined by configuration parameters. While connection is established, this device repeatedly reads modbus registers (in accordance to configured polling interval defined for each data in READ direction) and reports any changes as read responses. Also, received write requests are propagated to remote devices which result in write responses. If connection is lost or could not be established, device repeatedly tries to establish new connection after connect_delay seconds defined in configuration.

Together with read and write processing, device continuously reports current status of communication connection and each modbus device identified by device_id data parameter. Status of modbus device identified by device_id is calculated according to availability of data associated with that device_id. Available remote device statuses:

  • DISABLED

    Data polling is not enabled

  • CONNECTING

    Data polling is enabled and some data read requests have resulted in timeout or haven’t completed.

  • CONNECTED

    Data polling is enabled or and all data read responses have been received.

Polling of data values can be enabled/disabled based on device_id. Initially, polling for all remote devices is disabled and has to be explicitly enabled.

Gateway events#

Events registered by gateway have event type starting with:

'gateway', <gateway_name>, 'modbus_master', <device_name>, 'gateway', ...

Available gateway events are:

  • …, ‘status’

    Connection status of a modbus master device.

    Payload is defined by hat-gateway://modbus.yaml#/definitions/events/master/gateway/status.

  • …, ‘remote_device’, <device_id>, ‘status’

    Status of a remote modbus device, where <device_id> is remote modbus device identifier device_id defined in configuration for each data.

    Payload is defined by hat-gateway://modbus.yaml#/definitions/events/master/gateway/remote_device_status.

  • …, ‘remote_device’, <device_id>, ‘read’, <data_name>

    Read response reporting current data value. Data value and cause of event reporting are available only in case of successful read. <data_name> is configuration defined name for each data.

    First successful read of data value will be reported with INTERROGATE cause and result SUCCESS, while all the following successful reads, where value is different from previous are reported with CHANGE cause. If read is not successful, result equals to any other string that indicate reason of failure (value and cause properties are not included). After value is invalidated, next successful read will be reported with INTERROGATED cause.

    Payload is defined by hat-gateway://modbus.yaml#/definitions/events/master/gateway/read.

    Note

    if data is not received within request_timeout after polling, read event with result TIMEOUT is registered.

  • …, ‘remote_device’, <device_id>, ‘write’, <data_name>

    Write response reporting resulting success value. Together with result that contains success value, this event contains the same request_id as provided in associated write request event.

    Payload is defined by hat-gateway://modbus.yaml#/definitions/events/master/gateway/write.

    Note

    in case response did not arrive within configuration defined request_timeout, response with result TIMEOUT is registered.

System events#

Events registered by other Hat components, which are consumed by gateway, have event type starting with:

'gateway', <gateway_name>, 'modbus_master', <device_name>, 'system', ...

Available system events are:

  • …, ‘remote_device’, <device_id>, ‘enable’

    Enable polling of data associated with remote modbus device with <device_id> identifier.

    Payload is defined by hat-gateway://modbus.yaml#/definitions/events/master/system/enable.

  • …, ‘remote_device’, <device_id>, ‘write’, <data_name>

    Write request containing data value and session identifier used for pairing of request/response messages.

    Payload is defined by hat-gateway://modbus.yaml#/definitions/events/master/system/write.

Configurations and event payloads#

---
"$schema": "http://json-schema.org/schema#"
id: "hat-gateway://modbus.yaml#"
title: Modbus devices
definitions:
    master:
        type: object
        title: Modbus master
        required:
            - connection
            - remote_devices
        properties:
            connection:
                type: object
                required:
                    - modbus_type
                    - transport
                    - connect_timeout
                    - connect_delay
                    - request_timeout
                    - request_retry_count
                    - request_retry_delay
                properties:
                    modbus_type:
                        description: |
                            Modbus message encoding type
                        enum:
                            - TCP
                            - RTU
                            - ASCII
                    transport:
                        oneOf:
                          - type: object
                            required:
                                - type
                                - host
                                - port
                            properties:
                                type:
                                    const: TCP
                                host:
                                    type: string
                                    description: |
                                        Remote host name
                                port:
                                    type: integer
                                    description: |
                                        Remote host TCP port
                                    default: 502
                          - type: object
                            required:
                                - type
                                - port
                                - baudrate
                                - bytesize
                                - parity
                                - stopbits
                                - flow_control
                                - silent_interval
                            properties:
                                type:
                                    const: SERIAL
                                port:
                                    type: string
                                    description: |
                                        Serial port name (e.g. /dev/ttyS0)
                                baudrate:
                                    type: integer
                                    description: |
                                        Baud rate (e.g. 9600)
                                bytesize:
                                    description: |
                                        Number of data bits
                                    enum:
                                        - FIVEBITS
                                        - SIXBITS
                                        - SEVENBITS
                                        - EIGHTBITS
                                parity:
                                    description: |
                                        Parity checking
                                    enum:
                                        - NONE
                                        - EVEN
                                        - ODD
                                        - MARK
                                        - SPACE
                                stopbits:
                                    description: |
                                        Number of stop bits
                                    enum:
                                        - ONE
                                        - ONE_POINT_FIVE
                                        - TWO
                                flow_control:
                                    type: object
                                    required:
                                        - xonxoff
                                        - rtscts
                                        - dsrdtr
                                    properties:
                                        xonxoff:
                                            type: boolean
                                            description: |
                                                Enable software flow control
                                        rtscts:
                                            type: boolean
                                            description: |
                                                Enable hardware (RTS/CTS) flow control
                                        dsrdtr:
                                            type: boolean
                                            description: |
                                                Enable hardware (DSR/DTR) flow control
                                silent_interval:
                                    type: number
                                    description: |
                                        Serial communication silet interval
                    connect_timeout:
                        type: number
                        description: |
                            Maximum number of seconds available to single connection
                            attempt
                    connect_delay:
                        type: number
                        description: |
                            Delay (in seconds) between two consecutive connection
                            establishment attempts
                    request_timeout:
                        type: number
                        description: |
                            Maximum duration (in seconds) of read or write
                            request/response exchange.
                    request_retry_count:
                        type: integer
                        description: |
                            Number of request retries before remote data is considered
                            unavailable
                    request_retry_delay:
                        type: number
                        description: |
                            Delay (in seconds) between two consecutive request retries
            remote_devices:
                type: array
                items:
                    type: object
                    required:
                        - device_id
                        - data
                    properties:
                        device_id:
                            type: integer
                            description: |
                                Modbus device identifier
                        data:
                            type: array
                            items:
                                type: object
                                required:
                                    - name
                                    - interval
                                    - data_type
                                    - start_address
                                    - bit_offset
                                    - bit_count
                                properties:
                                    name:
                                        type: string
                                        description: |
                                            Data point name
                                    interval:
                                        type:
                                            - number
                                            - "null"
                                        description: |
                                            Polling interval in seconds or
                                            null if polling is disabled
                                    data_type:
                                        description: |
                                            Modbus register type
                                        enum:
                                            - COIL
                                            - DISCRETE_INPUT
                                            - HOLDING_REGISTER
                                            - INPUT_REGISTER
                                            - QUEUE
                                    start_address:
                                        type: integer
                                        description: |
                                            Starting address of modbus register
                                    bit_offset:
                                        type: integer
                                        description: |
                                            Bit offset (number of bits skipped)
                                    bit_count:
                                        type: integer
                                        description: |
                                            Number of bits used for
                                            encoding/decoding value (not
                                            including offset bits)
    events:
        master:
            gateway:
                status:
                    enum:
                        - DISCONNECTED
                        - CONNECTING
                        - CONNECTED
                remote_device_status:
                    enum:
                        - DISABLED
                        - CONNECTING
                        - CONNECTED
                read:
                    type: object
                    required:
                        - result
                    properties:
                        result:
                            enum:
                                - SUCCESS
                                - INVALID_FUNCTION_CODE
                                - INVALID_DATA_ADDRESS
                                - INVALID_DATA_VALUE
                                - FUNCTION_ERROR
                                - TIMEOUT
                        value:
                            type: integer
                        cause:
                            enum:
                                - INTERROGATE
                                - CHANGE
                write:
                    type: object
                    required:
                        - request_id
                        - result
                    properties:
                        request_id:
                            type: string
                        result:
                            enum:
                                - SUCCESS
                                - INVALID_FUNCTION_CODE
                                - INVALID_DATA_ADDRESS
                                - INVALID_DATA_VALUE
                                - FUNCTION_ERROR
                                - TIMEOUT
            system:
                enable:
                    type: boolean
                write:
                    type: object
                    required:
                        - request_id
                        - value
                    properties:
                        request_id:
                            type: string
                        value:
                            type: integer
...