Automatic transceiver & conversion RS-485 interface circuit and testing circuit

Published:2011/6/19 8:37:00 Author:TaoXi | Keyword: Automatic transceiver, conversion, interface circuit , testing circuit | From:SeekIC

The interface circuit is as shown in the dotted line box of the figure 1, by analysising the true value table, the sending and receiving process are: when the sending port DI=O, the DE/RE=1 sends the 0 electrical level, the receiving port RO=O; when the sending port DI=1, DE/RE=0, VA=VB=2.5V, the receiving port RO=1 with the function of the pull-up resistor. If you add the 1kHz TTL square wave at the TXo port of the interface circuit to test the circuit. The interface chip pin 485-A and pin 485-B all have the 50μs voltage changing process when you have not add the 120Ω port resistance, as the figure 2 shows. The receiving port Ro waveform's rising edge has the 30～40μs obvious delay to cause the large transmission error; when you add the 120Ω port resistance, the delay decreases, it is about 3μs.



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