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Overview of Modbus RTU protocol for MindConnect Nano

The Modbus protocol is supported for the MindConnect Nano plugin. You can select the protocol type Modbus RTU while adding a new data source in Asset Manager.

Modbus configuration

The Modbus configuration view is only visible after a successful onboarding of the MindConnect Nano. The MindConnect Nano Version must be higher than V3.3.0.2.

The following picture shows the protocol selection in Asset Manager:


For more information about how to create a new data source see chapter Adding a data source and data points for MindConnect Nano.

Modbus RTU data source parameter

In order to connect a Modbus RTU device, you need to select the Modbus RTU protocol and enter the following data source parameters:

Parameter Description
Serial Port Available serial ports:
- COM1
- COM2
Serial Type Supported serial communication standards:
- RS232 (default)
- RS422
- RS485
Baud rate Baud rate for serial communication:
- 9600 (default)
- 2400
- 4800
- 19200
- 38400
- 57600
- 115200
Data bits Data bits for serial communication:
- 5
- 6
- 7
- 8 (default)
Stop bits You can select the number of stop bits for serial communication:
- 1 (default)
- 2
Parity Parity for serial communication:
- Odd
- None
- Even
Termination Onboard termination:
- On (default)
- Off

The following parameters are optional and can be adjusted for collecting data from slow Modbus devices:

Parameter Description
byteTimeout You can set the timeout interval between two consecutive bytes of the same message received from the Modbus device. If the delay between is longer than the given timeout, an error will be generated. Range: 500ms (default) … 2000ms
responseTimeout You can set the timeout interval used to wait for a response from the Modbus device. If the waiting before receiving the response is longer than the given timeout, an error will be generated. Range: 500ms (default) … 60000ms
responseSuspensionTime If a byte timeout or a response timeout was detected, then this waiting time is added before the next request is started. During this time any response from the Modbus device will be flushed Range: 0ms … 45000ms. (default: 1000ms). For Modbus RTU it is recommended to set the value to 10000 ms. If the default value 0 ms is set, any incoming incorrect responses will not be destroyed.
requestDelay You can set the time between the response from the Modbus device and the next request to this Modbus device. Range: 0ms (default) … 10000ms

Pin assignment of Modbus protocols

The following table shows the pin assignment for the Modbus RTU protocol:

Pin RS 485 RS 422 RS 232
1 Data- Transmit / receive data - (I/O) for half-duplex mode TX- Transmitted data - (O) for full-duplex mode Not supported
2 Data+ Transmit / receive data+ (I/O) for half-duplex mode TX+ Transmitted data + (O) for full-duplex mode RxD Received data (I)
3 - RX+ Receive data + (I) for full-duplex mode TxD Transmitted data (O)
4 - RX- Receive data - (I) for full-duplex mode Not supported
5 M Signal ground M Signal ground M Ground
6 - - Not supported
7 - - Not supported
8 - - Not supported
9 - - Not supported

Modbus RTU data point parameter

In order to collect the data of your Modbus RTU device, you must add new data points.

The following picture shows the data point parameter window:


The following table shows the parameter of the Modbus RTU data point window:

Parameter Description
Data Type Data type supported by backend (e.g. INT).
Slave Number Address/Slave number of the Modbus device.
Start Address Start address (offset) of the data.
Quantity Quantity of data to be read.
VariableType You can choose the following variable types of the Modbus data to be read:
1: char
2: unsigned char
3: short
4: unsigned short
5: long
6: unsigned long
7: float
8: double
9: ASCII-Text
Real Type Real type - determines, if the registers of real values have to be processed in reversed order
0: Normal order
1: Reversed order
2: Byte swap order
3: Word swap order
Real types 2 and 3 are available from V03.04.02.01.
Response Address Offset Offset of the data to return in register units.
Response Quantity Quantity of the returned data. Only for string data a value greater than 1 is supported.

The following table shows the parameter of the Modbus RTU data point window for Function Type and Acquisition Type:

Function Type Acquisition Type
The function type depends on the type of the data (see manual of the Modbus device). The acquisition type depends on the selected function type.
1: Read Coils Read
2: Read Inputs Read
3: Read Holding Registers Read
4: Read Input Registers Read
5: Write Single Coil1)
6: Write Single Register1),2)
15: Write Multiple Coils1)
16. Write Multiple Registers1),3) Write
22: Mask Write Register1)
23: Read/Write Multiple Registers1)
  • Currently function codes 1..4 are supported.
  • 1) Supported since MindConnect Nano V03.04.06.00 b003.
  • 2) Used to write values to data points with variable types using a single register (example: short).
  • 3) Used to write values to data points with variable types using one or more registers (example: long or double).


The information is stored in the Modbus Slave device in 4 different tables. 2 tables store on/off discrete values (coils) and two store numerical values (registers). Each coil is 1 bit long and assigned a data address between 0 and 65535. Each register has the size of 1 word(16 bits/ 2 bytes) and has a data address between 0 and 65535.

Coil/ Register numbers Data addresses Function type Table name
1-9999 0 to 65535 1 Read Coils
10001-19999 0 to 65535 2 Read Inputs
30001-39999 0 to 65535 4 Read Input Registers
40001-49999 0 to 65535 3 Read Holding Registers

Coil / register numbers can be considered as location names since they do not appear in the configuration. The Data Addresses (Start Address) are used in the configuration.

Data Types:

Industrial IoT data types:

Data type Length Data range
BOOLEAN 1 bit True
INT 4 bytes −2147483648
LONG 8 bytes −9223372036854775808
DOUBLE 8 bytes -9.9999999999999999999999999999999999999E+125
STRING 1 - 255 bytes

Modbus data types:

Data type Length Data type number
char 8 bits 1
unsigned char 8 bits 2
short 16 bits 3
unsigned short 16 bits 4
long 32 bits 5
unsigned long 32 bits 6
float 32 bits 7
double 64 bits 8
ascii-text 8 bits / char 9


Example Data type Function type Start address Variable type Response Address Offset Response Quantity
Reading the first Holding Register, number 40001, has the Data Address 0. int Read Holding Registers 0 unsigned short or short 0 1
VariableType: 4 or 3 (unsigned short or short)Reading the second Input Register, number 30002, has the Data Address 1. int Read Input Registers 1 unsigned short or short 0 1
Reading a float (32 bits) from Input Register 30108 / 30109, has the Data Address 107. double Read Input Registers 107 float (reads two registers) 0 1
Reading a string. This example describes to read four letters from register 400006 and 400007. Each register contains two characters. string Read Holding Registers 5 unsigned short 0 2 (registers)

Using Modbus register addresses directly

Usually, it is necessary to enter Modbus registered address decremented by 1 as "start address", but it may lead to confusion when comparing the data point's "start address" with the registered address from Modbus device's manual. Therefore, enter a negative value in the "response address offset" field to directly use the registered address from the Modbus device's manual as "start address". In case a negative value has been entered as "response address offset", this value will internally be subtracted from the given "start address" number to read the data from Modbus device. Positive "response address offset" values will be handled to address specific data within the read data if several registers need to be read to acquire the data from Modbus device as data block and the necessary data itself is only a part of that data block.

Supported since:

  • MindConnect Nano V03.04.03.03 b001


Using decremented start address Using address from manual Getting data from a read data block
Registered address from Modbus device manual 303 303 64001
Start address 302 303 640011)
Quantity 1 1 27
Response address offset 0 -1 11
Response quantity 1 1 16

1) Sample start offset as documented in SENTRON PAC manual - can be used without decrementing it by 1.

Technical specification

Protocol Description Value Data rate
Modbus RTU - RS485 Data provider
Max cable length
1200 m

max. 115,2 kBd
Modbus RTU - RS422 Data provider
Max cable length
1200 m

max. 115,2 kBd
Modbus RTU - RS232 Data provider
Maximum cable length
2.5 m
5.0 m
7.5 m
15 m
30 m
60 m
120 m

115,2 kBd
57,6 kBd
38,4 kBd
19,2 kBd
9,6 kBd
4,8 kBd
2,4 kBd


Maximum cable length depends on the cable type used and maximum length can be increased by using repeaters.

Last update: April 16, 2024