We want to experiment and learn about controlling MODBUS devices from devices like the Raspberry Pi or Arduino. In traditional process control systems, you use a PLC device costing several hundred dollars, and programmed with "ladder logic". Today single-board-computers are cheap, and are attractive for embedded control systems. Without breaking the bank, I found some ultra-cheap MODBUS temperature sensors made in China, available for about $10, and after some trial and error and more error, I have some success to report. In this post we'll simply go over the device I chose.
With any MODBUS/RTU device it's important to evaluate the setup and use of the device. What serial-line parameters to use (baud rate, parity, stop bits, etc), setting the client ID, and the registers which can be used, are all basic considerations. While this post looks at a specific device, the general outline of the evaluation is what is done for every MODBUS device.