Introduction ModBus protocol and transplanted to STM32 microcontroller
https://blog.csdn.net/silent123go/article/details/92440091
Modbus and Modbus Slave Test Tool ModbusPoll use
https://blog.csdn.net/byxdaz/article/details/77979114
modbus slave tool
http://www.cr173.com/soft/21410.html
modbus poll tool
http://www.cr173.com/soft/21405.html