JTAG Emulation and Debugging Unit: This unit includes a Jlink emulator for downloading and debugging programs. It features a TYPE-C interface, allowing students to use a mobile phone charging cable for connecting the computer and the experiment board, while also providing power to the board ( Li et al., 2021 ; Wang et al., 2017 ) ( Figure 1 ).

USB-to-Serial Communication Unit: Facilitates serial communication between the computer and the experiment board.

Main Control Chip Unit: Incorporates the STM32F103RC, a high-performance 32-bit MCU based on ARM, with up to 512 KB of flash memory, USB, CAN, 11 timers, 3 ADCs, and 13 communication interfaces.

Display Unit: Features a 0.96-inch OLED screen with a resolution of 128 × 64.

Analog Joystick Unit: Includes a 3D model airplane joystick potentiometer, with dual potentiometers and a switch.

Keyboard Unit: Utilizes an EC11 rotary encoder.

Additional Units: The experiment board also consists of a display unit with 8 LEDs, a DS18B20 temperature sensor unit, a buzzer unit, and 16 IO expansion pins.

In microcontroller experiment courses, each student is required to solder their own experiment board. PCB boards and components are distributed prior to class. Students solder the board following the circuit diagram, learning surface-mount soldering techniques and gaining a deeper understanding of circuit principles ( Zhu et al., 2017 ).

Once the circuit board soldering is completed, it needs to be debugged, during which many issues may arise. Guiding students to resolve these issues based on the actual conditions helps them develop problem analysis and resolution skills. This experience is invaluable for both further academic pursuits and employment, providing lasting benefits ( Figure 2 ).

The Modbus bus hardware structure follows a master-slave configuration, as depicted in Figure 3 .

In this setup, the master can be a computer or a touch screen, serving as the host device. Industrial applications commonly use the RS485 bus to connect slave modules. The master communicates with the modules based on their address numbers.

The Modbus data model includes four definitions:

Discrete Input: Represents digital input, read-only, with function code 02 for reading.

Coil: Represents digital output, both readable and writable, with function code 01 for reading, 05 for writing a single coil, and 15 for writing multiple coils.

Input Register: Represents analog input, with function code 04 for reading.

Holding Register: Represents analog output, with function code 03 for reading, 06 for writing a single register, and 16 for writing multiple registers.

Since the holding register model can be flexibly adapted to replace the other three models, this experiment uses the holding register model with function codes 03 and 06.

The data frame definition is illustrated in Figure 4 :

Address Field: Represents the module’s address number.

Function Code: Used for reading and writing operations on the data model.

Data: Represents the data of the register.

Checksum: Used to verify the correctness of the data

Modbus data frames come in two modes: RTU and ASCII. The RTU message frame uses the binary raw value, while the ASCII message frame converts the binary raw value to ASCII code, with a frame header “:” and a frame tail consisting of a carriage return and line feed.

This experiment uses the RTU message frame.

The microcontroller defines an int type array, illustrated as the central memory array in Figure 5 , serving as a bridge for communication between the host and the microcontroller via the protocol stack, and for the microcontroller’s IO operations.

In this setup, the host reads the array using function code 03 in the left loop and can also modify the array using function code 06. In the right loop, the microcontroller updates the array with results from peripheral interactions. For instance, the microcontroller connected to 2 temperature sensors and 8 LEDs stores the temperature values in units 40,001 and 40,002. The host can retrieve these temperature values using function code 03 through the protocol stack. Additionally, the host can modify the value in unit 40,003 using function code 06, and the microcontroller can drive the LEDs by monitoring changes in the value of unit 40,003 ( Feng et al., 2021 ; Hou & Gu, 2017 ).

The setup involves a computer converting USB to TTL serial via the FT230XS chip to connect with the microcontroller’s serial port. An OLED display unit is used to show information, an analog joystick simulates voltage changes for ADC sampling, and 8 LEDs are used for digital output ( Feng et al., 2014 ; Shen et al., 2021 ) ( Figure 6 ).

The microcontroller program setup is done using STM32CubeMX for configuring the clock and peripheral ports, generating the initial program framework, which can be compiled and debugged with IAR or Keil ( Mo et al., 2016 ; Sheng et al., 2017 ).

Within this framework, the following steps are taken:

In ADC.c, the function uint16_t GET_ADC (uint32_t CH) is added to read the ADC conversion value, and it is declared in ADC.h.

The UART interrupt service function is modified to store data from the UART receive register into the receive array, reset the UART receive time variable, and set the receive flag variable to 1.

The timer interrupt service function is configured to trigger every 1 ms, incrementing the UART receive time variable each time.

The Modbus protocol stack files modbusRTU.c and modbusRTU.h are added to the project.

The main.c main function first initializes the working states of the system clock, timer, I/O, serial port, OLED display, etc.

In the infinite loop while (1), it first checks if the receive flag variable is 1 and if the serial receive time variable is greater than 10 ms (RTU messages are determined by the interval time to judge the received frame). If a frame of data is received, it verifies the data, checks the address, and parses the function code. If the function code is 03, it returns the data in the memory array shown in Figure 5 to the host computer based on the starting address and quantity of the registers. If the function code is 06, it writes the data sent by the host computer to the specified memory array.

Next, it collects the voltage values of the two potentiometers of the analog joystick through the ADC port and places them into the 40,001 and 40,002 memory arrays. It also checks for changes in the 40,003 memory array value, and the 8-bit binary value of its low byte corresponds to the on/off state of 8 LEDs, thus achieving control by the host computer.

Lastly, the OLED displays the values of the 40,001 - 40,004 memory arrays to facilitate debugging.

First, Modscan software is used for integration testing with the experimental board to give students a preliminary understanding of the Modbus communication protocol ( Figure 7 ).

Next, to deepen students’ understanding of the application of the Modbus communication protocol, KingView software is used. Students use KingView software to define Modbus device drivers, set data dictionary variables, configure the interface, connect variables, and run integration testing with the experimental board, providing them with an introductory knowledge of industrial control ( Figure 8 ).