问题1答案:找的好辛苦啊。
For RS-485 receiver inputs between -200mV and +200mV, the output is undefined. That is, if the differential
voltage on the RS-485 side of a half-duplex configuration is 0V, and no master transceiver is driving the line (or
a connection has come loose), then a logic "high" output is as probable as a logic "low." To ensure a defined
output under these conditions, most of today's RS-485 transceivers require failsafe bias resistors: a pullup
resistor on one line (A) and a pulldown on the other line (B), as shown in Figure 1. Historically, the failsafe bias
resistors on most schematics were labeled 560Ω, but to reduce power loss (when terminating one end only) this
value can be increased to approximately 1.1kΩ. Some designers terminate both ends with resistor values
between 1.1kΩ and 2.2kΩ. The trade-off is noise immunity versus current draw.