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Post: Hello With the RS-485 protocol, the voltage is measured as the difference between the two wires A and B. If A - B is less than -200mV then that is registered as binary 1. If A - B is more than +200mV then that is registered as binary 0. The receiver only measures the voltage between the two wires so where they are relative to ground is not important. The specification for RS-485 and EIA-485 allows the voltage to be anywhere between -7V and +12V. The only important part is the difference between the two wires is more than 400mV. As DMX uses the EIA-485 standard for its physical layer it should be compatible with the RS485 Pi. If the signal output on your commercial DMX USB interface is reversed compared to the RS485 Pi then you should be able to replicate this by swapping the A and B wires around.