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DS89C21 reflects back an inverted version of DI to RO

Nick Wernicke
Intellectual 775 points

Other Parts Discussed in Thread: DS89C21

I am using a DS89C21 for RS422 serial communications. It appears that when I send data to its "DI" pin, I see the same data inverted, coming out of the "RO" pin. The output Rx and Tx pair pins are not connected to each other. The differential output correctly appears on the DO pair, and the RI pair stays neutral until data is sent from the other terminal.

Signals go to and from the chip properly when testing using a Moxa 4 wire USB-RS422 port. The terminal window on the Moxa sends and receives data just fine, the terminal window on the DS89C21 also sends and receives, but it is strange to also see the results of each character inverted and reflected back.

So, the RS422 side is working, I just don't know why the DS89C21 reflects back inputs to the ttl side. Is this expected behavior, or is the part broken? If it is expected behavior, what is the reason for it? It would seem that this chip can only be used in half duplex. I don't know a lot about RS422, so perhaps this is normal?

  • 0
    TI__Expert 3175 points

    Hi Nick,

    The DS89C21 is a driver and a receiver on one package. The driver and receiver are separate, so I am surprise you mentioned there was intereaction from DI at the driver to RO of the receiver. The only thing I could think of is any possibility of severe crosstalk from the transmit diff pair to the receive diff pair of the cable? Are the diff pair terminated at the receiver end? If you can send me your schematic, I can take a look.

    Suggest to use two differential probes simultaneously monitor the receive and trasmit pairs at the far end of the cable. This can tell if the signals are really echo back through crosstalk, or caused by something else.

    regards,

    TK Chin