Modbus TCP Acquisition

Description: Depending on the transmission medium, Modbus can communicate over serial ports or network ports (including fiber). RTU uses serial communication, while TCP uses network ports (Ethernet). The Modbus protocol follows a master/slave communication sequence. The edge gateway’s Modbus driver operates as a Master client [master device], actively sending queries to Modbus slave devices. Each slave prepares a response based on the received request and sends it back to the gateway.

1. Create nodes

(1) Open Digital Twin in CommBridge, then select Device Connectivity on the left.

(2) Drag the Modbus Client [Master], Data Processing, and MQTT Upload plugins to the plugin workspace to create three nodes.

(3) Connect the three nodes in this order: Modbus Client [Master] → Data Processing → MQTT Upload.

2. Create a scheme

Select Modbus-TCP as the protocol.

(1) Double-click Modbus Client [Master], then click Add Scheme.

(2) Enter the connection settings, then click Next.

(3) Click Add Group, enter the group settings, then click Confirm.

(4) Click Add Data, enter the point settings, then click Confirm.

(5) Click Finish to add the acquisition scheme.

(6) Enable the scheme, then click Save.

3. View results

(1) Click Deploy in Device Connectivity to deploy successfully.

(2) Switch to Digital Twin and hover over the Modbus Client [Master] node to display its information list.

(3) Click the scheme to view real-time and historical point data for its groups.

Acquired data (before processing):

After processing (view the scheme in the Data Processing plugin):

4. Write data

Data can only be modified when the point’s read/write mode is rw or wo and acquisition is operating normally.

(1) Click Write for a data item, configure its data, then click OK.

(2) View historical data.

5. Configuration details

Connection settings

Parameter	Description	Format	Example
Scheme name	User-defined name for the scheme	Nonempty	Slave acquisition
Protocol	Modbus protocol	Modbus TCP, Modbus RTU	Modbus TCP
Synchronous acquisition	Whether to use synchronous acquisition: Yes or No	Yes, No	YES
Acquisition interval	Polling interval for reading data: the interval between register reads for each group in the scheme	Number, range: (100,2147483647]	2000
Acquisition freshness	All points in a group must be acquired within this period, including frame delays; otherwise the group’s data is discarded	Number, range: (0, acquisition interval]	1000
Timeout	Maximum duration of each acquisition for each group in the scheme; data from a read is discarded if this time is exceeded	Number, range: (0, acquisition interval]	1000
Maximum batch register count	Maximum number of registers per batch when acquiring consecutive points	Natural number, range: [1,120]	120
Frame delay	Minimum interval between acquisitions; the default is 0. Increase it if acquisition frequently times out; otherwise leave it unchanged	Milliseconds; number, range: [0, 2147483647]	0
Write frame delay	Additional wait when writing data, required by some devices with slow write operations; 0 means no extra delay	Milliseconds; number, range: [0, 2147483647]	0
Write register function code	Function code for writing holding registers	Auto, 06, 10; see below	Auto
Change-based acquisition	Acquisition mode. No: no change-based filtering; SingleData: transmit only points that change; WholeGroup: transmit the entire group when any data in the group changes	No, SingleData, WholeGroup	SingleData
Change-based acquisition reset time	Forces a data submission when data remains unchanged for longer than this period. Set to 0 to never trigger it	Seconds; number, range: (0, 2147483647]	0
Failure backoff	Extends the time before the next acquisition according to the backoff factor after a failure, up to acquisition interval × maximum backoff factor	Yes, No; see below	Yes
Backoff factor	Time multiplier on failure; must be greater than 0 and not equal to 1	Number	2
Maximum backoff factor	Maximum time multiplier; once reached, this multiplier remains unchanged	Number	16

Synchronous acquisition:

1. “Yes” uses a common acquisition interval, timeout, and acquisition freshness setting for all groups in the scheme. These settings apply to all point data in the scheme.
2. “No” allows each group to configure its own acquisition interval, timeout, and acquisition freshness.

Write register function code: Auto uses 06 when writing one register and 10 when writing more than one register; 06 uses 06 for all register writes; 10 uses 10 for all register writes.

Failure backoff: for an acquisition interval of 1 second, a backoff factor of 2, and a maximum backoff factor of 16, successive acquisition failures produce waits of 1s, 2s, 4s, 8s, and 16s. Similarly, for an acquisition interval of 10 seconds, a backoff factor of 0.5, and a maximum backoff factor of 0.1, successive failures produce waits of 10s, 5s, 2.5s, and 1.25s. A successful acquisition restores the initial default acquisition interval.

When the backoff factor is greater than 1, the maximum backoff factor must be greater than the backoff factor. When it is less than 1, the maximum backoff factor must be less than the backoff factor.

Add a group

Parameter	Description	Format	Example
Group name	User-defined name for the group	Nonempty	Group 1
Slave ID	Modbus device address: the slave address of the device being polled	Number, range: (0,255]	1
IP	IP address of the Modbus device	Address in dotted-decimal notation	172.21.78.157
Port	Port number of the Modbus device; defaults to 502	Number, range: (0,65535]	502
Acquisition interval	Data polling interval: the interval between register reads for this group	Number, range: (100,2147483647]	2000
Acquisition freshness	Validity window for acquiring all points in this group. If all point data is not read within this period, all point data in the group is discarded	Number, range: (0, acquisition interval]	1000
Timeout	Maximum duration of each acquisition for this group; data from the read is discarded if this time is exceeded	Number, range: (0, acquisition interval]	1000

Note: When Synchronous acquisition in the connection settings is No, these settings apply to all point data in the group. Modbus limits the number of registers that can be read at once. A single read supports at most 120 consecutive Holding Register or Input Register points, or 2000 consecutive Discrete or Coils points.

Add data

Parameter	Prerequisite	Description	Format	Example
Data alias		Point name, unique within the group	Nonempty; cannot start with a digit or underscore, or contain spaces	M
Address	Register address, unique within the group	Register address, unique within the group	Number, range: (0,2147483647)	0
Register type		Modbus operation code	Holding Register, Input Register, Discrete, Coils; see below	Holding Register
Data type	Determined by the register type	Raw data type of the register	uint16, int16, uint32, int32, float, boolean, combine, string, hex string; see below	uint16
Byte order	Determined by the register type and data type	Storage order in memory for data types larger than one byte	AB, BA, ABCD, CDAB, BADC, DCBA; see below	AB
Register count	Determined by the register type and data type	Number of registers occupied by the data type	See below	1
Read/write mode	Determined by the register type	Supports ro (read only), rw (read/write), and wo (write only)	ro, rw, wo	ro
Calculation		User-defined formula based on the data alias		{M}/10
Offset	Data type is bit	Bit offset from high to low after byte-order adjustment	Number, range: [0,15]

For example, if the temperature property on your device is at address 1, set the register address to 1 when testing that property.

1. For Discrete and Coils registers, the data type defaults to boolean.
2. For uint16, int16, and combine, the available byte orders are AB and BA. For uint32, int32, and float, they are ABCD, CDAB, BADC, and DCBA. Other data types have no byte-order setting.
3. For Discrete and Coils, the register count defaults to 1. For uint16, int16, and boolean, it defaults to 1. For uint32, int32, and float, it defaults to 2. For combine, select 1 or 2. For string and hex string, enter the register count manually.

16-bit order:

1. AB: a one-word register, with the high word before the low word and the high byte before the low byte within the word (big-endian).
2. BA: a one-word register, with the high word before the low word and the low byte before the high byte within the word (little-endian).

32-bit order:

1. ABCD: two words, with the high word before the low word and the high byte before the low byte within each word (big-endian).
2. BADC: two words, with the high word before the low word and the low byte before the high byte within each word (big-endian with byte swap).
3. CDAB: two words, with the low word before the high word and the high byte before the low byte within each word (little-endian with byte swap).
4. DCBA: two words, with the low word before the high word and the low byte before the high byte within each word (little-endian).

6. Modbus protocol parameter details

The register type determines the data type, register count, and read/write mode.

Register types

0x, 1x, 3x, and 4x are address ranges defined by the Modbus protocol.

Register type	Name	PLC register address	Protocol register address	Read/write access	Description
Holding Register	Holding register (4x)	40001-49999	0000H-FFFFH	Read/write; read: 0x03, write: 0x06	Output or stored parameters: parameters configured while the controller is running
Input Register	Input register (3x)	30001-39999	0000H-FFFFH	Read only, 0x04	Input parameters obtained from external devices while the controller is running
Discrete	Discrete input status (1x)	10001-19999	0000H-FFFFH	Read only, 0x02	Input ports whose states are changed by external settings
Coils	Coil status (0x)	00001-09999	0000H-FFFFH	Read/write; read: 0x01, write single coil: 0x05, write multiple coils: 0x0F	Output ports whose output states can be set and read

Data types

The data type is determined by the raw data type, whether the high and low bytes within registers are swapped, and whether the register order is swapped. Each letter represents one byte.

1. Raw data types: uint16, int16, uint32, int32, float, boolean, combine, string, hex string.
2. Swap high and low bytes within a register: swap the first and last 8 bits of the 16-bit register data. For 16-bit data, only normal order (AB) and reverse order (BA) exist. High-order first is normal order; low-order first is reverse order.
3. Swap register order: swap the 16-bit halves of 32-bit data, for example ABCD and CDAB.

Raw data types:

Name	Description	Register count
uint16	16-bit unsigned integer	1
int16	16-bit signed integer	1
uint32	32-bit unsigned integer	2
int32	32-bit signed integer	2
float	32-bit floating point	2
combine		1, 2
boolean	Boolean value	1
string	ASCII string	Configurable
hex string	Hexadecimal string	Configurable

High/low order:

Value	Description
AB	A register containing 2 bytes, with the high byte before the low byte (normal order)
BA	A register containing 2 bytes, with the low byte before the high byte (reverse order)
ABCD	2 words, with the high word before the low word and the high byte before the low byte within each word (big-endian)
BADC	2 words, with the high word before the low word and the low byte before the high byte within each word (big-endian with byte swap)
CDAB	2 words, with the low word before the high word and the high byte before the low byte within each word (little-endian with byte swap)
DCBA	2 words, with the low word before the high word and the low byte before the high byte within each word (little-endian)