Dear
ericthegeek,
Thanks for your time. Attachement is the picture I took from the oscilloscope.
You can see the Frequency is 20Hz, with 5V amplitude(RDM-100 is power by USB). And the Dir Pin of the RS485 Tranceiver IC(SP3075EEN:RE & DE shorted together) also has the same waveform as the RX pin, only with 3.3V amplitude(the CPU is STM32F103). I am thinking of giving up this RDM-Controller and buy another one. Do you have any good suggestion, better with a cheap price,thanks.