Modbus/TCP¶

Overview¶

Modbus/TCP is a communication protocol which is a variant of the Modbus family. It is based on a master/slave architecture and intended for use in industrial automation especially with PLCs or IO modules. Modbus/TCP is defined in IEC 61158.

Modbus uses big-endian representation for addresses and data items.

Protocol structure:

transaction identifier	protocol identifier	length field	unit identifier	function code	data
2 byte	2 byte (always 0x0000)	2 byte (n+2)	1 byte	1 byte	n byte

Connection Properties¶

`host` (string, required)¶

Hostname or IP address of the modbus device

Example: "126.4.10.52"

`port` (integer, required)¶

Port of the Modbus device

Example: "15200"

`unitId` (integer)¶

Default: 1

`socketTimeout` (integer)¶

Default: 30000

Additional restrictions:

Minimum: 2000

`connectionStrategy` (object)¶

If a connection attempt fails, retries will be performed with increasing delay (waiting time) in between. The following parameters control how these delays behave.

Properties of the connectionStrategy object:

`initialDelay` (integer)¶

Delay (waiting time) of the first connection retry (in milliseconds). For subsequent retries, the delay will be increased according to the parameter incrementFactor which has a default value of 2.

Default: 1000

Additional restrictions:

Minimum: 1000

`maxDelay` (integer)¶

Maximum delay (waiting time) to wait until the next retry (in milliseconds). The delay (waiting time) for any subsequent connection retry will not be larger than this value. Must be strictly greater than initialDelay.

Default: 30000

`incrementFactor` (integer)¶

The factor used to increment initialDelay up to maxDelay. For example if initialDelay is set to 1000 and maxDelay to 5000 the values for the delay would be 1000, 2000, 4000, 5000.

Default: 2

Additional restrictions:

Minimum: 2

Endpoint Properties¶

`unitId` (integer) e.g.: `1,2`¶

By setting this optional parameter, the unitId defined in the Connection resource will be overwritten.

Example: [1,2]

Additional restrictions:

Maximum: 255

`fc` (integer, enum, required)¶

The Function Code (fc) defines the operation of the request. This can be any of the common Modbus codes, but the exact implementation depends on the concrete device manufacturer.

This element must be one of the following enum values:

1
2
3
4
5
6
15
16

Example: 2

`address` (integer, required)¶

Example: 0

`length` (integer, required)¶

The length describes how many registers should be read starting at the specified address. The registers of a modbus device have a length of 16 bits.

Example: 4

`dataType` (string, enum)¶

If given, convert the Modbus value to this specified type. Types are available in byte ordering Big Endian (BE), or Little Endian (LE), or with additional word swap (BEWS, LEWS).

This element must be one of the following enum values:

raw
base64
boolean
doubleBE
doubleLE
floatBE
floatBEWS
floatLE
floatLEWS
int16BE
int16LE
int32BE
int32LE
uint16BE
uint16LE
uint32BE
uint32LE

Example: "floatLE"

`interval` (integer)¶

Poll interval (best effort) in milliseconds

Default: 1000

Example: 2000

`cronExpression` (string)¶

The Cron expression used to poll the endpoint. (For examples, see: https://github.com/node-cron/node-cron)

Examples: "1,2,4,5 * * * *", "1-5 * * * *", "*/2 * * * *", "* * * January,September Sunday"

Supported function codes¶

For reading or writing data over Modbus/TCP the protocol provides a set of functions. Which action should be performed on the other end of the connection is defined by the function code (fc).

Supported function codes are:

Function code	Action	Operation
1	Read coils	subscribe
2	Read discrete inputs
3	Read holding registers
4	Read input registers
5	Write single coil	write
6	Write single holding register
15	Write multiple coils
16	Write multiple holding registers

Supported Data-types¶

In Modbus/TCP there are no predefined data-types. The Connectware supports sending the raw payload over MQTT as binary. If the data-type is known and supported by the respective fc, the payload is converted to JSON format.

The desired data type on read or on write is specified with the property dataType. The possible values are as follows:

DataType/FC	1	2	3	4	Size
raw	✓	✓	✓	✓	variable
bool	✓	✓	x	x	1 bit
ascii	x	x	✓	✓	variable
base64	x	x	✓	✓	variable
int16BE	x	x	✓	✓	16 bit
int16LE	x	x	✓	✓	16 bit
int32BE	x	x	✓	✓	32 bit
int32LE	x	x	✓	✓	32 bit
uint16BE	x	x	✓	✓	16 bit
uint16LE	x	x	✓	✓	16 bit
uint32BE	x	x	✓	✓	32 bit
uint32LE	x	x	✓	✓	32 bit
floatBE	x	x	✓	✓	32 bit
floatLE	x	x	✓	✓	32 bit
doubleBE	x	x	✓	✓	64 bit
doubleLE	x	x	✓	✓	64 bit
bigUInt64BE	x	x	✓	✓	64 bit
bigUInt64LE	x	x	✓	✓	64 bit

If the payload is smaller than the required bits for the conversion (payload.length < dataType.length), an error is displayed in the logs and no message is sent over MQTT. If the payload has more bits (payload.length > dataType.length) than needed for the conversion, the “extra” bits are ignored and a message is sent over MQTT.

As a comparison to other documentation, this is a list of other common names for the supported types:

Common type name	Size	Minimum value	Maximum value
char	8 bit	0	255
byte	8 bit	-128	127
short (int16)	16 bit	-2^15	2^15-1
int (int32)	32 bit	-2^31	2^31-1
uint (uint32)	32 bit	0	2^32-1
long64	64 bit	-2^63	2^63-1
float	32 bit	IEEE 754	IEEE 754
double	64 bit	IEEE 754	IEEE 754

Input Format¶

This Modbus implementation supports writing using the standard function calls 5, 6, 15 and 16. In most cases you can directly send a number in the message payload when you are trying to write to a coil or a single register (function call 5 or 6, respectively).

When trying to write several coils at once (function call 15) your data message’s value must consist of an array of boolean values, for example: [true, true, false, true].

Alternatively, if your goal is to write several registers at once, the data type of the endpoint needs to be considered. The property dataType (see Endpoint properties) specifies how to properly serialize the data into an array of bytes suitable to write into the Modbus registers.

For the available integer and float data types (see Supported Data-types above) you can directly send the value in the message payload. A BigInt must be sent as a string value. You can also send ascii, base64, and utf8 encoded strings that will be parsed using Node.js Buffer class.

Raw data in the form of an array of bytes is also supported.

Input Format on Write¶

For writing data to modbus a message needs to be published to the /set topic of the Endpoint with the following properties:

{
  "fc": "<function call value>",
  "dataType": "<one of the modbus data types>",
  "data": "<the data to be written>"
}

Output Format on Write¶

Results are published to the /res topic of the Endpoint. The format depends on the modbus function being called and it is returned as a JSON object.

Output Format¶

If data is read from Modbus and dataType is set to any of the potential values

Output Format on Read¶

When data is read from the endpoint, results are published to the /res topic of the Endpoint. The output message is an object with two properties:

{
  "timestamp": "<unix timestamp in ms>",
  "value": "<value>"
}

The value of the value property will be the JSON representation of the configured dataType, i.e. a number, or a string, or for ‘raw’ the JSON representation of a Javascript buffer object.

Alternatively, the property dataType can be left undefined, in which case the output is a Javascript buffer object directly. This is probably useful for further processing using suitable mappings.

Example Configuration¶

The following example demonstrates how to configure a simple Modbus connection and endpoint that reads holding registers (fc 3) from the device.

Download: modbus-example.yml

# ----------------------------------------------------------------------------#
# Commissioning File
# ----------------------------------------------------------------------------#
# Copyright: Cybus GmbH (2020)
# Contact: support@cybus.io
# ----------------------------------------------------------------------------#
# Source Interface Definition - Modbus TCP
# ----------------------------------------------------------------------------#

description: |
  Sample commissioning file for Modbus TCP protocol connectivity and data mapping

metadata:

  name: ModBus TCP Protocol Connectivity
  icon: https://www.cybus.io/wp-content/uploads/2019/03/Cybus-logo-Claim-lang.svg
  provider: cybus
  homepage: https://www.cybus.io
  version: 0.0.1

parameters:
  IP_Address:
    type: string
    default: 192.168.10.30

  Port_Number:
    type: number
    default: 502

  initialReconnectDelay:
    type: integer
    default: 1000

  maxReconnectDelay:
    type: integer
    default: 30000

  factorReconnectDelay:
    type: integer
    default: 2

resources:

  modbusConnection:
    type: Cybus::Connection
    properties:
      protocol: Modbus
      targetState: connected
      connection:
        host: !ref IP_Address
        port: !ref Port_Number
        connectionStrategy:
          initialDelay: !ref initialReconnectDelay
          maxDelay: !ref maxReconnectDelay
          incrementFactor: !ref factorReconnectDelay

  readCoil:
    type: Cybus::Endpoint
    properties:
      protocol: Modbus
      connection: !ref modbusConnection
      subscribe:
        fc: 1
        length: 2
        interval: 1000
        address: 3
        dataType: boolean

  writeCoil:
    type: Cybus::Endpoint
    properties:
      protocol: Modbus
      connection: !ref modbusConnection
      write:
        fc: 5
        length: 2
        address: 3
        dataType: boolean

  readRegister:
    type: Cybus::Endpoint
    properties:
      protocol: Modbus
      connection: !ref modbusConnection
      subscribe:
        fc: 3
        length: 2
        interval: 1000
        address: 1
        dataType: int16BE

  writeRegister:
    type: Cybus::Endpoint
    properties:
      protocol: Modbus
      connection: !ref modbusConnection
      write:
        fc: 6
        length: 2
        address: 1
        dataType: int16BE

  mapping:
    type: Cybus::Mapping
    properties:
      mappings:
        - subscribe:
            endpoint: !ref readCoil
          publish:
            topic: !sub '${Cybus::MqttRoot}/read/coil'
        - subscribe:
            topic: !sub '${Cybus::MqttRoot}/write/coil'
          publish:
            endpoint: !ref writeCoil
        - subscribe:
            endpoint: !ref readRegister
          publish:
            topic: !sub '${Cybus::MqttRoot}/read/register'
        - subscribe:
            topic: !sub '${Cybus::MqttRoot}/write/register'
          publish:
            endpoint: !ref writeRegister

References¶

https://www.rtaautomation.com/technologies/modbus-tcpip/

Modbus/TCP¶

Overview¶

Connection Properties¶

host (string, required)¶

port (integer, required)¶

unitId (integer)¶

socketTimeout (integer)¶

connectionStrategy (object)¶

initialDelay (integer)¶

maxDelay (integer)¶

incrementFactor (integer)¶

Endpoint Properties¶

unitId (integer) e.g.: 1,2¶

fc (integer, enum, required)¶

address (integer, required)¶

length (integer, required)¶

dataType (string, enum)¶

interval (integer)¶

cronExpression (string)¶

Supported function codes¶

Supported Data-types¶

Input Format¶

Input Format on Write¶

Output Format on Write¶

Output Format¶

Output Format on Read¶

Example Configuration¶

References¶

`host` (string, required)¶

`port` (integer, required)¶

`unitId` (integer)¶

`socketTimeout` (integer)¶

`connectionStrategy` (object)¶

`initialDelay` (integer)¶

`maxDelay` (integer)¶

`incrementFactor` (integer)¶

`unitId` (integer) e.g.: `1,2`¶

`fc` (integer, enum, required)¶

`address` (integer, required)¶

`length` (integer, required)¶

`dataType` (string, enum)¶

`interval` (integer)¶

`cronExpression` (string)¶