Pictured below is a simple arrangement used to read the Temperature sensor from the PC. Standard/Protocol is MODBUS RTU RS-485 Half-Duplex.
I've used this arrangement with both modpoll.exe and Chipkin CASModbusScanner software. Both work fine and report correct readings.
Now, the next arrangement uses the regular CP-2102 USB-to-TTL dongle with XY-K485 Transceiver module (Max3485, automatic direction control) to connect to the sensor:
This arrangement produces the "Reply timeout" error on modpoll, and "Invalid length: short" error on the CAS Modbus scanner.
Now, if I understand correctly, the method the data is sent over the RS-485 is the same as with the RX/TX plain serial: baudrate, parity, start/stop bits etc. except RS-485 uses differential voltage to represent bits, not absolute +5v or +3v3.
So theoretically communicating to the rs485 transceiver using serial from cp2102 to send MODBUS data should work, but it does not. Is my understanding of these principles correct, and why do you thing the second arrangement does not work?
PS: a diagram below shows how I currently understand the communications pipeline: