RS-485 is a good choice for long-distance serial communication since using differential transmission cancels out the vast majority of the electromagnetic disturbances picked up by the RS-485 signal. A simple RS-485 network consists of one master and up to 32 slave devices. Since RS-485 uses halfduplex communication—that is, the same two wires are used for both transmission and reception—some means of controlling which side of the connection can transmit must be built into the system. In this article, we discuss the ADDC (Automatic Data Direction Control) concept and explain how it works.