B&B Electronics labels the data lines on RS-422 and RS-485 with an "A", for negative or "B" for positive, to indicate the positive and negative relationship between the two data lines. It is important to label the data lines because this positive and negative relationship between the lines must be maintained when connecting one device to another.

Some manufacturers of RS-422/485 equipment label the data lines on their devices with "+" or "-", Signal and Inverted Signal, or an "A" and "B" that are opposite to other converters or devices in the network. This can add confusion when making connections or troubleshooting a network with devices from different manufacturers. This application note addresses some of these differences, show how they've come about, and try to relate them to each other so that the system designer can interconnect devices from different manufacturers.