Modbus Master

Purpose

The Modbus Master plugin reads and writes Modbus TCP, RTU, and ASCII devices.

Open the plugin

Open Development Configuration → Collection Configuration, create a device, and select Modbus Master as the collection plugin.

Create the channel and device through Collection Configuration. This page lists only Modbus-specific settings, addresses, and debug actions.

Supported channels and data types

Channel type	Check before configuration
TCP client	Confirm the device IP, port, and network reachability from the gateway.
TCP server	Confirm the local listening address and port, and that the field device can connect to the gateway.
Serial port	Confirm that the port is not in use and that baud rate, data bits, stop bits, and parity match the device.
UDP	Confirm the bind address, port, firewall rules, and packet source.

Supported data types: Boolean, Byte, Int16, UInt16, Int32, UInt32, Int64, UInt64, Float, Double, String, and Decimal.

Plugin properties

Modbus Master also inherits the common collection properties. Channel type, remote/local address, serial settings, TLS, connection timeout, retry count, read/write duty cycle, write priority, data format, string byte reversal, and maximum concurrency remain available in the device editor; see Collection Configuration for those shared fields and their channel dependencies. The tables below list Modbus-specific settings.

Protocol

Setting	Description
Modbus type	Select ModbusTcp, ModbusRtu, or ModbusAscii to match the field device and channel.
Default station	Station used when the variable address does not specify one. The default is 1.

Request packing

These limits control how many points are placed in one request. Keep the defaults unless the device manual specifies a smaller limit.

Setting	Range / default	Description
FC1 maximum coils	1–2000, default 1600	Maximum coil bits read in one FC1 request.
FC2 maximum discrete inputs	1–2000, default 1600	Maximum discrete-input bits read in one FC2 request.
FC3 maximum holding registers	1–125, default 100	Maximum holding registers read in one FC3 request.
FC4 maximum input registers	1–125, default 100	Maximum input registers read in one FC4 request.
FC15 maximum multiple-coil write	1–1968, default 1968	Maximum coil bits written in one FC15 request.
FC16 maximum multiple-register write	1–123, default 123	Maximum registers written in one FC16 request.

DTU connection

Setting	Description
DTU ID	Identifier used to match a registered DTU connection when the channel is a DTU service. Leave it empty for ordinary direct connections.

When the channel type is a DTU service, DTU ID must exactly match the identifier registered in DTU Service. Leave it empty for direct TCP, serial, or UDP channels.

Address rules

Use the address area that matches the Modbus function:

Address range	Area
00001 onward	Coils
10001 onward	Discrete inputs
30001 onward	Input registers
40001 onward	Holding registers

Examples:

• 40001 is the first holding register.
• 40001;s=2; reads station 2 instead of the default station.
• 40001;w=16; uses function code 16 when writing the value.
• F=3;100 explicitly uses function code 3 at protocol offset 100 for a holding-register read. With F=, the address is a zero-based offset rather than a 40001-style area address.
• Combine options with semicolons, for example S=2;F=3;100. The options apply to this variable address only.

W= accepts a positive write function code; 15 (multiple coils) and 16 (multiple registers) are the usual values. When it is omitted, the write function follows the variable data type and protocol defaults.

Use the address together with the common variable settings in Collection Configuration. Verify the address against the device point list before importing many variables.

Functions

Function	Purpose
Read and write	Read a confirmed point and write only to an approved test point.
Messages	View sent and received frames, protocol fields, and error details.

Device debug

Open Development Configuration → Collection Configuration, select the Modbus device, and click More Functions, then open Debug.

Read and write

Read a confirmed safe address first. Before writing, confirm the variable permission, station, function code, data type, and field safety conditions.

Messages

Use the message view to check sent and received frames, connection failures, and protocol parsing errors.

Verify the configuration

1. Read one confirmed field address.
2. Confirm that the returned value, data type, byte order, station, and function code match the point list.
3. For a writable point, write a safe value and read it back according to the field procedure.
4. Only after the single-point check passes should you import the complete point list.

Troubleshooting

Symptom	Check
Address read fails	Check the address area, station suffix, data type, and device point list.
Serial device does not respond	Check port ownership, baud rate, data bits, stop bits, parity, and channel type.
TCP device does not respond	Check the remote or bind address, firewall, station, and Modbus type.
Values are shifted or incorrect	Check data type, byte order, array length, and the address offset.
Batch read is rejected	Reduce the corresponding FC1–FC4 limit and compare it with the device manual.
Batch write is rejected	Keep FC15 between 1–1968 and FC16 between 1–123, then compare the configured limit with the device manual.

Related links

• Collection Configuration: Channels, devices, variables, import, and common debug entry.
• Plugin Manual Index: Other collection and data-forwarding plugins.