Modbus Nodes

Modbus is the most widely deployed protocol in industrial automation. MaestroHub supports all three variants: TCP, RTU, and ASCII so you can communicate with PLCs, sensors, meters, and supervisory systems from a single node set.

Configuration Quick Reference

Field	What you choose	Details
Parameters	Connection, Function, Function Parameters, Timeout Override	Select the connection profile, function, configure function parameters with expression support, and optionally override timeout.
Settings	Description, Timeout (seconds), Retry on Timeout, Retry on Fail, On Error	Node description, maximum execution time, retry behavior on timeout or failure, and error handling strategy. All execution settings default to pipeline-level values.

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Modbus Read Node

Modbus Read Node

Read data from PLCs, sensors, meters, and industrial devices.

Supported Function Types:

Function Name	Purpose	Common Use Cases
Read Coils (0x01)	Read digital outputs	Motor status, valve positions
Read Discrete Inputs (0x02)	Read digital inputs	Limit switches, sensors
Read Holding Registers (0x03)	Read holding registers	Setpoints, configuration
Read Input Registers (0x04)	Read input registers	Temperature, pressure, flow
Read FIFO Queue (0x18)	Read FIFO queue	Event logs, alarm history

Node Configuration

Parameter	Type	Required	Description
Connection	Selection	Yes	Modbus connection profile to use
Function	Selection	Yes	Read function from the selected connection
Function Parameters	Dynamic	Varies	Auto-populated from the function schema (e.g., address, quantity, dataType). See your Modbus connection functions for full parameter details.
Timeout Override	Number (seconds)	No	Override the default function timeout

All function parameters support expression syntax ({{ expression }}) for dynamic values from the pipeline context.

Input

The node receives the output of the previous node as input. Input data can be referenced in function parameter expressions using $input.

Output Structure

On success the node produces:

{
  "success": true,
  "functionId": "<function-id>",
  "data": { ... },
  "durationMs": 42,
  "timestamp": "2026-01-15T08:30:00Z"
}

Field	Type	Description
success	boolean	true when the function executed without errors
functionId	string	ID of the executed function
data	any	Function-specific result data (see below)
durationMs	number	Execution time in milliseconds
timestamp	string	ISO 8601 / RFC 3339 UTC timestamp

Function-Specific Output Data

The shape of data depends on the function code:

Function Code	data Contents
FC01 Read Coils	Array of boolean values, one per coil
FC02 Read Discrete Inputs	Array of boolean values, one per input
FC03 Read Holding Registers	Converted values based on the configured dataType (e.g., int16, float32, raw)
FC04 Read Input Registers	Converted values based on the configured dataType
FC24 Read FIFO Queue	FIFO queue contents from the target address

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Modbus Write Node

Modbus Write Node

Write control values, setpoints, and configuration to devices.

Supported Function Types:

Function Name	Purpose	Common Use Cases
Write Single Coil (0x05)	Write single digital output	Start/stop motor
Write Single Register (0x06)	Write single register	Update setpoint
Write Multiple Coils (0x0F)	Write multiple digital outputs	Batch control
Write Multiple Registers (0x10)	Write multiple registers	Configuration update
Write Masked Register (0x16)	Masked register write	Bit manipulation
Read/Write Multiple Registers (0x17)	Atomic read+write	Synchronized operations

Node Configuration

Parameter	Type	Required	Description
Connection	Selection	Yes	Modbus connection profile to use
Function	Selection	Yes	Write function from the selected connection
Function Parameters	Dynamic	Varies	Auto-populated from the function schema (e.g., address, value, values). See your Modbus connection functions for full parameter details.
Timeout Override	Number (seconds)	No	Override the default function timeout

All function parameters support expression syntax ({{ expression }}) for dynamic values.

Input

The node receives the output of the previous node as input. Use expressions like {{ $input.payload.value }} to pass dynamic values to write parameters.

Output Structure

The write node uses the same output envelope as the read node:

{
  "success": true,
  "functionId": "<function-id>",
  "data": { ... },
  "durationMs": 15,
  "timestamp": "2026-01-15T08:30:00Z"
}

Function-Specific Output Data

Function Code	data Contents
FC05 Write Single Coil	Confirmation of the written coil address and value
FC06 Write Single Register	Confirmation of the written register address and value
FC0F Write Multiple Coils	Address and quantity of coils written
FC10 Write Multiple Registers	Address and quantity of registers written
FC16 Write Masked Register	Address, AND mask, and OR mask applied
FC17 Read/Write Multiple Registers	Read results from the combined operation

Safety First

Always implement validation and safety checks before writing to industrial equipment. Consider adding condition nodes to verify values are within safe ranges.

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Modbus Read Group Node

The Modbus Read Group node executes multiple Modbus read operations in a single node. It supports selective function lists, a Read All mode for complete device snapshots, and parallel or sequential execution.

Modbus Read Group Node

Node Configuration

Parameter	Type	Required	Default	Description
Connection	Selection	Yes	—	Modbus connection profile
Read All	Toggle	No	false	When enabled, executes all read functions (FC01–FC04, FC24) from the connection
Functions	Array of {functionId, alias}	When Read All is off	[]	List of specific functions to execute. Each entry requires a functionId; alias is optional.
Execution Mode	Selection	No	parallel	parallel — all functions run concurrently. sequential — functions run one after another.
Continue on Error	Toggle	No	true	When enabled, the node continues executing remaining functions even if one fails
Debug Mode	Toggle	No	false	Enable detailed logging for troubleshooting

Read All Mode

When Read All is enabled the function list is ignored. The node automatically discovers and executes every read function defined on the selected connection.

Supported read function types for Read All:

Function Type	Function Code
modbus.read.coils	FC01
modbus.read.discreteInputs	FC02
modbus.read.holdingRegisters	FC03
modbus.read.inputRegisters	FC04
modbus.read.fifoQueue	FC24

Input

The node receives the output of the previous node as input.

Output Structure

{
  "Temperature Sensor": {
    "value": [22.5, 23.1],
    "success": true,
    "duration": 45,
    "timestamp": "2026-01-15T08:30:00Z"
  },
  "Pressure Gauge": {
    "value": [1013],
    "success": true,
    "duration": 38,
    "timestamp": "2026-01-15T08:30:00Z"
  },
  "_meta": {
    "connectionId": "conn-123",
    "connectionName": "PLC-01",
    "total": 2,
    "successful": 2,
    "failed": 0,
    "totalDuration": 83,
    "executionMode": "parallel",
    "originalRequests": 2
  }
}

Each function result contains:

Field	Type	Description
value	any	The function's return data
success	boolean	Whether this function succeeded
error	string	Error message (present only on failure)
duration	number	Execution time in milliseconds
timestamp	string	ISO 8601 / RFC 3339 UTC timestamp

The _meta object provides execution summary:

Field	Type	Description
connectionId	string	Connection profile ID
connectionName	string	Connection profile display name
total	number	Total functions executed
successful	number	Count of successful executions
failed	number	Count of failed executions
totalDuration	number	Wall-clock time for the entire group (ms)
executionMode	string	parallel or sequential
originalRequests	number	Number of function configs submitted
keyCollisions	string[]	Output keys that appeared more than once (omitted when empty)

Output Key Resolution

Each function result is keyed in the output map using the first available value:

1. Alias — the custom alias set in the function entry
2. Function Name — the name defined on the connection function
3. Function ID — the unique function identifier (fallback)

Use aliases to give results predictable, human-readable keys, especially when the same function type is read from multiple address ranges.

Execution Modes

Parallel (Recommended) (default)

• All functions execute concurrently.
• Fastest total execution time.
• Utilizers Coalescing to efficiently handle overlapping function ranges.
• Best for independent read operations across different address ranges.

Sequential

• Functions execute one after another in list order.
• More predictable timing and lower peak load on the device.
• When Continue on Error is off, execution stops at the first failure.

Error Handling

Continue on Error	Behavior
true (default)	The node completes even if individual functions fail. Failed functions include an error field in their result. The node output remains successful.
false	The node fails as soon as any function fails. In sequential mode, remaining functions are skipped. The node result is marked as failed.

Validation Rules

• connectionId is required.
• executionMode must be parallel or sequential.
• When Read All is off, at least one function entry is required.
• Duplicate functionId values are not allowed.
• Duplicate alias values are not allowed.
• The connection type must be modbus.

Best Practices

• Use Read All for device commissioning, complete state backups, and monitoring dashboards where you need every data point.
• Use specific functions in production pipelines with known data requirements for better performance and clarity.
• Assign aliases when reading multiple functions of the same type to keep output keys readable.
• Prefer parallel mode unless the target device has limited concurrent request handling.
• Enable Continue on Error for monitoring scenarios where partial data is still valuable.

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Settings Tab

All three Modbus node types share the same Settings tab:

Setting	Type	Default	Description
Description	Text	—	Optional description displayed on the node
Timeout (seconds)	Number	Pipeline default	Maximum time the node may run before timing out
Retry on Timeout	Toggle	Pipeline default	Automatically retry the node if it times out
Retry on Fail	Toggle	Pipeline default	Automatically retry the node if it fails
On Error	Selection	Pipeline default	Error strategy: stop the pipeline, continue to the next node, or follow the error output path

When left at their defaults, these settings inherit from the pipeline-level execution configuration.