I am here to present a simple design, using esp8266.
The firmware is customized to be a Modbus server, where it is possible to drive GPIOs, receive value of ADC, etc.
for more details and to downloads, here is the links:
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https://www.youtube.com/watch?v=8bcJnio0Q9o
https://drive.google.com/folderview?id= ... sp=sharing
thank you!