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Post: Hi Edi RS485 uses a differential signal for communication so the receiver should measure the difference between the two wires to determine if the signal is high or low. According to the standard, the difference has to be at least ±200mV but that can be anywhere in the voltage range of –7V to 12V so a low signal of 0V and a high of 2.2V would be within the standard as the difference is more than 200mV. Similarly, your RS485-USB dongle is also within the standard as -2V to +2V is more than a 200mV difference. If the Modbus device conforms to the RS485 standards then it should recognise signals from the RS485 Pi or the USB dongle. Does your Modbus device have a terminator resistor fitted? If not then reflections on the wire could stop it from working.