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SN65LBC179: Use it with RS485 half duplex mode

Romain DEMAISON
Prodigy 20 points
Part Number: SN65LBC179

Hi,

We have developed a filtering interface for the RS422 communication protocol using the SN65LBC179AD component.

I have done a simplified schematic of the filtering interface :

This box is inserted in the serial communication line. It is working very well for RS422 serial communication protocol. But now, we would like to use them with RS485 half duplex protocol.

Do you have an idea to do that without changing the electrical schematic ? I saw that post which seems to be related to my problem : https://www.avrfreaks.net/forum/conversion-full-duplex-half-duplex

Howerver, It didn't seem to work either with 100 Ohms resistor. Maybe I didn't understand correctly the connections!

Thanks for your help.

  • 0
    TI__Mastermind 25705 points

    Hi,

    If I understand your application correctly, you want to implement bi-directional communication with half-duplex RS485 transceiver. With half-duplex RS485 transceiver, you need to short A to Y and B to Z.  For control pins, please check out this post for the automatic direction control technique. As long as the two sides don't send the data at the same time, the transmitting and receiving can happen one way.

    The post you shared discussed how to use half-duplex to work with full-duplex, which might be not applicable for your case.

    Regards,

    Hao  

  • 0 in reply to Hao L
    Prodigy 20 points

    Thanks for your answer,

    As it is complicated for us to modify the PCB, we tried to connect together A to Y and B to Z with 120 Ohms resistor. It didn't work.

    You can see here one side of the filtering interface with the implementation of the driver and some additionnal components :

    If I understand well after reading the post for the automatic direction control technique, It is mandatory to add some pull-up / pull-down resistor to be able to use bi-directionnal communication ?

  • 0 in reply to Romain DEMAISON
    TI__Mastermind 25705 points

    Can you elaborate what issue was when you connected A to Y and B to Z with 120 Ohms resistor? Was the signal swing too small? Or did the issue happen at the bi-directional communication?

    For the automatic direction control, it's not mandatory to use pull up/down resistors. The drawback is that when the bus is idle, the differential bus voltage becomes 0V. This falls into the undetermined threshold region of the receiver, making its output unpredictable.

    Regards,

    Hao