RS-485/RS-422标准电路实用指南

The main reason why RS-485 can communicate over long distances is the use of differential or balanced lines. A com-munication channel requires a dedicated pair of signal lines to exchange information. The voltage on one line equals the inverse of the voltage on the other line.

TIA/EIA-485-A designates the two lines in this differential pair as A and B. Line A is more positive than Line B (VOA > VOB) on the driver output if a logic high is received on the input of the transmitter (DI = 1). If a logic low is received on the input of the transmitter (DI = 0), the transmitter causes Line B to be more positive than Line A (VOB > VOA). See Figure 1.