Modbus Publisher

The Modbus publisher writes Mango data point values to registers and coils on external Modbus slave devices. While the Modbus data source reads data into Mango, the Modbus publisher sends data out -- allowing Mango to act as the central coordinator that pushes calculated values, setpoints, or aggregated data to PLCs, RTUs, HMI panels, and other Modbus-capable equipment.

This publisher supports both Modbus TCP/IP and Modbus Serial (RTU/ASCII) connections, and can write to holding registers, coils, and multiple register types in a single publish cycle.

Overview

The Modbus publisher monitors configured Mango data points and writes their values to specified Modbus register addresses on a target device whenever the point values change, update, or at scheduled snapshot intervals. Each published point maps to one or more Modbus registers, with automatic data type conversion between Mango's internal representation and the Modbus register format.

Key capabilities:

• Write on change or on schedule -- publish values immediately when they change, on every update, or at regular snapshot intervals.
• Multiple data types -- write 16-bit integers, 32-bit integers, 32-bit floats, 64-bit doubles, and boolean coil values.
• Multiple slave support -- write to different slave IDs within a single publisher (useful for gateway devices).
• Batch writes -- multiple registers to the same slave can be written in a single Modbus transaction using function code 16 (Write Multiple Registers).

Prerequisites

Before you begin, make sure you have:

• A Modbus TCP/IP or Serial device accessible from the Mango host.
• The device's register map documentation, specifying register addresses, data types, and byte order.
• Network connectivity (for TCP) or a serial connection (for RTU/ASCII) to the target device.
• Knowledge of the device's slave ID (unit ID).
• The target registers must be writable (holding registers or coils, not input registers).

Step-by-step guide

Step 1: Create the publisher

1. Navigate to Administration > Publishers.
2. Click New and select Modbus Publisher from the publisher type dropdown.
3. Enter a descriptive Name (e.g., "PLC Setpoint Writer").
4. Configure the common publisher settings (update event, cache sizes).

Step 2: Configure the connection

Switch to the Modbus configuration tab and select the transport type.

For Modbus TCP/IP:

Setting	Description
Transport type	Select TCP/IP.
Host	IP address or hostname of the Modbus slave device.
Port	TCP port (default: 502).
Connection timeout	Maximum milliseconds to wait for a TCP connection (default: 3000).

For Modbus Serial (RTU/ASCII):

Setting	Description
Transport type	Select Serial (RTU) or Serial (ASCII).
Comm port	Serial port connected to the Modbus network (e.g., /dev/ttyUSB0, COM1).
Baud rate	Communication speed (common values: 9600, 19200, 38400, 115200).
Data bits	Number of data bits per frame (typically 8).
Stop bits	Number of stop bits (typically 1).
Parity	Parity checking (None, Even, or Odd). Must match the slave device.

Step 3: Configure common publisher settings

Setting	Description
Update event	When to publish values: All (every update), Changes only (only when value changes), Logged only (when logged), or None (snapshots only).
Cache warning size	Raises a warning alarm when the publish queue exceeds this number of entries.
Cache discard size	Discards the oldest entries when the queue reaches this size, preventing unbounded memory growth.
Send regular snapshots	When enabled, publishes all point values at a regular interval regardless of whether they have changed.
Snapshot period	The interval between snapshot publishes (e.g., every 5 minutes).

Step 4: Add published points

For each Mango data point you want to write to the Modbus device:

1. Click Add point in the published points section.
2. Select the Mango data point to publish.
3. Configure the Modbus register mapping:

Setting	Type	Default	Description
Slave ID	Integer	1	The Modbus unit ID (slave address) of the target device. Range: 1-247.
Register type	Enum	Holding register	The register type to write. Options: Holding register (function code 6/16) or Coil (function code 5/15).
Register offset	Integer	0	The starting register address (0-based). Consult the device register map. Some devices use 1-based addressing -- subtract 1 for the Mango configuration.
Data type	Enum	Two byte int (unsigned)	How to encode the Mango value into Modbus registers.
Byte order	Enum	Big-endian	Byte order within multi-byte registers. Most Modbus devices use big-endian, but some (especially Schneider, Wago) use little-endian.
Word order	Enum	Big-endian	Word order for 32-bit and 64-bit values that span multiple registers. Determines whether the high-order or low-order register comes first.
Multiplier	Numeric	1	Scaling factor applied before writing: written = mango_value * multiplier + additive.
Additive	Numeric	0	Offset applied after multiplication.

Step 5: Enable and verify

1. Click Save to save the publisher configuration.
2. Toggle Enabled to start publishing.
3. Monitor the publisher status for successful writes. The queue size should remain near zero during normal operation.
4. Verify on the Modbus slave device (or via a Modbus data source reading the same registers) that values are being received correctly.

Configuration reference

Supported data types

Data Type	Registers	Range / Precision	Description
Two byte int (signed)	1	-32,768 to 32,767	Standard 16-bit signed integer.
Two byte int (unsigned)	1	0 to 65,535	Standard 16-bit unsigned integer.
Four byte int (signed)	2	-2,147,483,648 to 2,147,483,647	32-bit signed integer across two registers.
Four byte int (unsigned)	2	0 to 4,294,967,295	32-bit unsigned integer across two registers.
Four byte float	2	IEEE 754 single precision	32-bit floating point across two registers.
Eight byte float	4	IEEE 754 double precision	64-bit floating point across four registers.
Coil (boolean)	N/A	true/false	Single bit written via coil function codes.

Register addressing

Modbus register addresses in Mango use 0-based offsets. Many device manuals use 1-based addressing or include a register type prefix (e.g., 40001 for the first holding register). To convert:

Manual Address	Mango Offset	Register Type
40001	0	Holding register
40100	99	Holding register
00001	0	Coil
00050	49	Coil

When the manual uses 1-based addressing without a prefix, subtract 1 from the documented address to get the Mango offset.

Write modes

The publisher automatically selects the appropriate Modbus function code:

Scenario	Function Code	Description
Single holding register	FC 06 (Write Single Register)	Writing one 16-bit register.
Multiple holding registers	FC 16 (Write Multiple Registers)	Writing 32-bit or 64-bit values that span multiple registers.
Single coil	FC 05 (Write Single Coil)	Writing a single boolean coil.
Multiple coils	FC 15 (Write Multiple Coils)	Writing multiple coils in a batch.

Troubleshooting

Symptom	Cause	Solution
"Connection refused" on TCP	Wrong IP/port, device offline, firewall	Verify the Modbus slave is reachable with telnet host 502. Check firewall rules.
"Timeout waiting for response"	Slave not responding, wrong slave ID	Verify the slave ID matches the device configuration. Check that the device supports the register type and address being written.
Values written but incorrect on device	Byte order or word order mismatch	Try switching between big-endian and little-endian byte/word order. Consult the device documentation for the expected byte order.
Float values corrupted	Wrong word order for 32-bit floats	32-bit floats span two registers. If the high and low words are swapped, change the word order setting.
"Illegal data address" error (exception code 2)	Register offset out of range or register is read-only	Verify the register address against the device register map. Ensure you are writing to holding registers or coils, not input registers.
"Illegal function" error (exception code 1)	Device does not support the function code	Some devices only support FC 06 (single register), not FC 16 (multiple registers). Ensure the data type fits in a single register, or check device capabilities.
Queue growing indefinitely	Write failures faster than the publish rate	Check the connection to the Modbus device. If the device is intermittently unavailable, increase the cache warning size and investigate the root cause of connectivity issues.
Scaling produces unexpected values	Multiplier/additive applied in wrong direction	Remember: written_value = (mango_value * multiplier) + additive. To write raw register values, set multiplier to 1 and additive to 0.

Related pages

• Publishers Overview -- Common publisher configuration and queue management
• Modbus Data Source -- Reading data from Modbus devices
• HTTP Sender Publisher -- Publishing data via HTTP
• gRPC Publisher -- Publishing data between Mango instances