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RS485 question

Question asked by frankd on Jul 13, 2015
Latest reply on Aug 16, 2015 by ChawCS

HEllo Engineering Zone Community:

 

Please refer to the attached scope shot.  The yellow line is the R on the RS-485 which is connected to the UART Rx and the blue line is the D on the RS-485 which is connected to the UART Tx.  The redline is the DE(D Enable) and /RE(Not R Enable) which are tied together and are connected to the uController I/O pin to control the receive and transmit data on the RS-485 chip. 

 

  As you can see, when we go to try to transmit making DE and /RE hi, the R pin starts to go low, but not all the way down low which in turn makes the UART Rx firmware begin receiving data, but since there is no data being sent at that time by the other guy, the data received in the UART receive register is just garbage.  This part of it is just detail for you, but the main question, we’re wondering about is whether or not when we try to transmit by making the DE and /RE line hi, should the R line go low like it is in the scope shot.  If so, then we will ignore this data in software.  I am looking for verification that the bus should exhibit this behavior.  If it shouldn't, can you provide any insight as to why this is happening?

Thank you,

Frank

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