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ISO35: Interfacing to half duplex slaves

Part Number: ISO35

I am developing a PCB in which I have ISO35 which is communicating well with RS485 Full Duplex slave but its not communication with half duplex slaves. In half duplex whatever master is sending is being received at slave but whatever slave is responding is not received at master. For half duplex interfacing I have shorted Y with A, Z with B at the end (J8pin2 with J9pin1, J8pin1 with J9pin2 -> refer to schematic attached below). In my circuit, R(t) of 100 ohm between A&B. RE pulled down with 10K. R & DE pulled up with 10K. With this configuration, I suppose master should receive whatever master is sending but its not happening too.

Could you please suggest where I am going wrong.

sch485.pdf

  • Hi Nabeen,

    Thanks for reaching out and sorry to hear about the issue.

    I have reviewed the schematic and it looks good. Could you please remove R97 and test again to see if ISO35 is receiving the data at R which was transmitted at D?
    I also see that there JP3 and JP4 jumpers in the schematic which short FLOW_R pins to FLOW_T pins and create half-duplex mode for ISO35. Did you also try shorting these jumpers and testing again?

    Please do also share the PCB layout to review for any issues, thanks.


    Regards,
    Koteshwar Rao

  • Hi Nabeen,

    I missed to notice that DE pin is hard pulled-up to VCC through 10kΩ resistor and MCU is not controlling DE pin. This is perfectly fine for full-duplex because transmit and receive bus terminals are different while for half-duplex, this is not going to work.

    Since the transmit and receive bus terminals are connected together for half-duplex, the driver needs to be turned ON only during transmission and to be disabled when receiving data from other nodes. If driver is not disabled, then the driver will end up holding the bus a state that its driving to. When multiple nodes drive the bus, the voltage on the bus could create either states based on how strong the nodes are driving.

    Please make this change and the device should work fine for half-duplex. Thanks.


    Regards,
    Koteshwar Rao

  • Thanks Mr. Rao. I appreciate your help. After adding DE to one of the controller pin and activating only during transmission resolved the issue. The two jumper I originally placed for hardware configuring my RS485 port as per requirement (FULL or HALF). But now instead of this I need to dedicate one GPIO for this configuration. 

    Regards,

    Nabeen