Modbus Server

Purpose

This target exposes variables from a forwarding group to an external Modbus master through Modbus TCP or Modbus RTU. The current Modbus type list has no ASCII option.

Configure the forwarding group and target through Data Forwarding. This page covers Modbus server properties, address mapping, client permissions, and protocol debug.

Open the plugin

Open Development Configuration → Data Forwarding and:

1. Save the forwarding-group variable range.
2. Add a target, choose Modbus Server, and fill in its basic target information.
3. Open Target properties, choose the Modbus type and channel, then configure protocol, permissions, serial, DTU, or TLS settings.
4. Save the target and configure a required Slave variable address for each exposed variable.
5. Enable the target and verify with an external Modbus master or the debug page.

Basic target information

Setting	Default	Configuration
Group	-	Select a saved forwarding group.
Target name	-	Required and unique within the group.
Enable	Enabled	Disable to stop listening.
Log level	Info	Temporarily use Debug for protocol diagnostics.
Start timeout	60 seconds	Valid range is 1 to 3600 seconds.

Target properties

Protocol

Setting	Default	Configuration
Modbus type	ModbusTcp	Current choices are ModbusTcp (MBAP) and ModbusRtu (binary RTU + CRC16). TCP normally uses TcpService; RTU normally uses SerialPort.
Default station	1	Used when an address has no S= prefix.
Multiple-station mode	Enabled	Allows multiple stations in one target.
Channel type	TcpService	Select TcpService, SerialPort, or the channel required by the deployment.

Data format

Setting	Default	Configuration
Data format	ABCD	Select ABCD, BADC, CDAB, or DCBA to match the external master.
Reverse string bytes	Disabled	Reverse string bytes by word only when required by the external layout.

Permissions

Setting	Default	Configuration
Allow RPC write	Enabled	Target-level write switch. Disable for read-only monitoring.
Client permission list	Empty	Empty allows all TCP client IPs. When non-empty, only matching AllowedClientIp entries connect; AllowWrite must also be true for writes.

Runtime

Setting	Default	Configuration
Write to memory immediately	Enabled	Applies an accepted write to the gateway variable immediately.
Send delay	0 ms	Delay before a reply frame when required by the master.
Connection timeout	3000 ms	Connection-operation timeout; minimum 100 ms.
Heartbeat content	Empty	Optional application heartbeat content.
Heartbeat is Hex	Disabled	Treats heartbeat content as hexadecimal when enabled.
Heartbeat interval	60000 ms	Server heartbeat interval.

Connection

Setting	Default	Configuration
Remote address	127.0.0.1:502	Used only when the selected channel requires a remote endpoint.
Local bind address	0.0.0.0:502	TCP service listen address. Restrict it to a required interface in production.
Serial port	COM1	Used for serial channels.
DTU ID	Empty	Identifier used to match a DTU service connection.
DTU ID is Hex	Disabled	Parses the DTU ID as hexadecimal bytes when enabled.
DTU service type	Default	Select the DTU registration mode required by the terminal.

TLS

Setting	Default	Configuration
Enable SSL	Disabled	Enables TLS for the channel.
SSL target host	Empty	Host name used for client certificate validation.
SSL client certificate	Empty	Client certificate for mutual TLS.
SSL server certificate	Empty	Certificate presented by a server channel.
SSL CA certificate	Empty	CA used to validate peer certificates.
Allow untrusted certificate	Enabled	Temporary self-signed testing only; disable in production.
SSL protocol	None	Keep the system default unless the peer requires a specific TLS version.
Check certificate revocation	Disabled	Enable according to the security policy.

Serial port

Setting	Default	Configuration
Baud rate	9600	Must match the external master and serial device.
Data bits	8	Must match the external master.
Parity	None	Must match the external master.
Stop bits	One	Must match the external master.
DTR / RTS	Enabled / Enabled	Hardware flow-control switches.
Async stream	Enabled	Enables asynchronous serial I/O.
Handshake	Default	Serial handshake mode.
Input / output buffer	4096 / 2048 bytes	Increase only for long or continuous frames.

Capacity and reliability

Setting	Default	Configuration
Maximum concurrency	1	Maximum concurrent protocol requests; minimum 1.
Maximum connections	10000	Maximum client connections.
Frame cache time	500 ms	Frame assembly cache window; minimum 100 ms.
Client cleanup time	120000 ms	Idle-client cleanup interval.

Target variable properties

Setting	Description
Slave variable address	Required. Modbus address exposed to the external master.
Data type	Default Int16. Data type used for this target mapping.
Allow RPC write	Allows external clients to write this variable when target-level and channel-level write permissions also allow it.

The variable must already be inside the forwarding group's range. Adding a target variable property does not add a member to the group.

Address rules

Address	Data area	Common read function
00001	Coils	FC1
10001	Discrete inputs	FC2
30001	Input registers	FC4
40001	Holding registers	FC3

Addresses use one-based notation. S=2;40001 exposes holding register 40001 at station 2; F=3;0 explicitly uses FC3 with zero-based offset 0; 40001.3 exposes bit 3 of the register.

Within one target, station, function, and start-address combinations must not overlap. Match the data type and register size to the external master's point list.

Functions

Function	Purpose
Read and write	Read a confirmed point and write only to an approved test point.
Forwarding pipeline	Use Forwarding pipeline and review the returned result.
Protocol service	Use Protocol service to send or inspect protocol frames and view the device or target response.

Target debug

Open Development Configuration → Data Forwarding, select the forwarding group and target, and open Debug.

Read and write

Read a mapped address first. Before writing, confirm target, channel, client, and variable write permissions.

Forwarding pipeline

Use the pipeline view to confirm how a group variable becomes an exposed Modbus value.

Protocol service

Check the listening endpoint, connected clients, and protocol-service state.

Verify the target

1. Start the target and confirm the server is listening.
2. Connect an external Modbus master.
3. Read one mapped variable in each configured data area.
4. If writing is enabled, write one safe value and confirm the gateway variable changes as expected.

Troubleshooting

Symptom	Check
Master cannot read a value	Listening channel, bind address, station, function code, target variable property, and data type.
Client cannot connect	Channel type, local bind address, port, firewall, maximum connections, and client permission list.
Value has wrong byte order	Data format, string-byte setting, target data type, and external master format.
Write is rejected	Target RPC write, variable RPC write, client permission, and the external master's function code.
Serial server has no response	Port ownership, baud rate, data bits, parity, stop bits, flow control, and frame timing.

Related links

• Data Forwarding: Forwarding groups, triggers, cache, redundancy, and common target operations.
• Plugin Manual Index: Other collection and data-forwarding plugins.