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Original question:

SN65HVD75: Question of RS-485 mode

SN65HVD1780: SN65HVD1780 enable

Vidhya Jeyapaul
Intellectual 780 points
Part Number: SN65HVD1780
Other Parts Discussed in Thread: THVD1406

Hi Team,

We are using SN65HVD1780 part in our design. Refer to the circuit below.

1. DE and RE are always enabled and are not controlled by MCU. Will there be any issue in transmitting and receiving half duplex communication?

2. If there is any concern, drop in replacement to full duplex parts are available? without design change? 

Please revert.

Thanks,

Vidhya

  • 0
    Guru 243120 points

    As long as DE is high, this device will transmit on the bus, and no other node can send anything.

    A full-duplex transceiver has separate differential bus signals for sending and receiving; this would require two additional pins (or pins in place of the DE/RE pins). So a P2P change is not possible.

  • 0 in reply to Clemens Ladisch
    Intellectual 780 points

    ok. will there be a concern if there is one to one communication(with only one driver and one receiver)? 

    What if DE is left open? With RE enabled, only receive operation can be performed?

    Please revert.

  • 0 in reply to Vidhya Jeyapaul
    Guru 243120 points

    As long as DE is high, the only possible communication is from this driver to other receivers.

    When DE is open, the driver is always disabled, and only receiving is possible at this node.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 47895 points

    Hi Vidhya,

    For a half-duplex implementation, it is common for DE to be controlled by the MCU so that the direction of communication can be changed. This way DE can be driven high when the local node is transmitting, then DE driven low when receiving. The receiver can be left enabled in both cases to always reflect the bus state, or the /RE line can be shared with DE so that it is only active when the node is not actively controlling the bus. 

    Alternatively, a full-duplex device may be used to allow the driver to remain active persistently. However, like Clemens mentioned, this requires a separate differential pair for the unidirectional signals, and thus a different pinout. 

    Let us know if you have any more questions.

    Regards,
    Eric Schott

  • 0 in reply to Vidhya Jeyapaul
    Guru 243120 points

    You might be interested in the upcoming THVD1406 device with automatic direction control.

  • 0 in reply to Eric Schott1
    Intellectual 780 points

    Thanks Clemens, Eric.

    So I can configure as 3rd one with only DE controlled from MCU and RE kept always enabled (active low)?

    Kindly confirm.

    Thanks and Regards,

    Vidhya

  • 0 in reply to Clemens Ladisch
    Intellectual 780 points

    Hi Clemens,

    with THVD1406 still SHDN pin need to be controlled by MCU right?

    How is it automated? Kindly clarify.

    Thanks,

    Vidhya

  • 0 in reply to Vidhya Jeyapaul
    TI__Guru 60042 points

    Vidhya,

    In the THVD1406, if D is left open/high for longer than tdevice-auto-dir, the receiver will automatically be enabled. So, on the THVD1406, you can tie both SHDN and RE high and control the driver and receiver using the D pin connected to the MCU. When being driven by a toggling waveform, the D pin will keep the device in driving mode. If D is left high meaning no messages are coming through, the THVD1406 will automatically switch to receiver mode.

    Regards,

    Eric Hackett