[Technical tips] Run a modbus RTU example on NX8MM-35

Tools you need:

1. i.MX series products like NX8MM-35 / PN8M-090T / EBOX-IMX8MM
   • In this tips, we use NX8MM-35 as an example.
2. Yocto Linux 4.0 sample image with libmodbus: Image / Guide
   • In the default recipe, you will need to add "libmodbus-dev" to IMAGE_INSTALL:append in local.conf before you build your own image.
3. Modbus test tool sample code file: Download link
4. NX8MM-35 with the test fixture connectoing 2xRS485
   

1. Boot up the board and compile the modbus test tool

First, boot up the board, and plug in a USB disk which contains the modbus test tool sample code file and open the terminal.

1. Access the USB disk and input the code below:
    

    $ gcc modbus_test_v2.c -o modbus_test_v2 -lmodbus 

   
2. Move the compiled application to the your appointed file in the system and access it (In this case, we make a file called modbustest.)
    

    $ mv modbus_test_v2 /modbustest
    $ cd /modbustest

   

Use the command below to check the function description:

 $ /.modbus_test_v2 

 Usage: ./modbus_test_v2 [-s|-c] [-r gpio_index] serial_port
 -s : server
 -c : client
 -r : rts-gpios : dm446c rts-gpios numbers: ttymxc0=128, ttymxc3=9

We use COM1 (ttymxc0/GPIO128) and COM4 (ttymxc3/GPIO9) on the NX8MM-35 for RS-485 communication, performing point-to-point Modbus RTU testing between the two ports.
(Be sure to short J39 to switch the COM port to RS485 mode.)

2. Start to perform Modbus RTU testing on NX8MM-35

1. Open another terminal and input command below to set COM4 to server mode:
    

    ./modbus_test_v2 -s -r 9 /dev/ttymxc3

   
2. Use the existing terminal to set COM1 to client mode and send the signal to server(COM4)

   ./modbus_test_v2 -c -r 128 /dev/ttymxc0

   

After executing the command above, the server (COM4) successfully receives the Modbus RTU signal from the client (COM1), marking the completion of the test.