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SN65HVD32: Can SN65HVD32 is used as half-duplex transceiver?

Masanori Tachibana
Genius 5350 points
Part Number: SN65HVD32
Other Parts Discussed in Thread: SN65HVD35,

Hi support team,

Our customer is considering RS-485 communication with switching full - duplex or half - duplex.
Therefore I am studying whether full-duplex RS-485 transceiver can be used as half-duplex transceiver.
I understand that the tranceiver with enable control of the driver and receiver such as SN65HVD35 is required.
But a port for enable control can't be prepared .
So I want to try to use the transceiver which is without enable control such as SN65HVD32 to the half duplex transceiver.

Because I think there is a possibility that the driver output may conflict, I am considering the following circuit diagram.
The resisters "Ri" are inserted to driver output of SN65HVD32.

Is there a possibility that this circuit can be used as a half duplex transceiver?
Also, please teach me if it has any precautions on use.

Sincerely,
M. Tachibana

  • 0
    TI__Guru 62920 points
    Hello Tachibana-san,

    In general, any full-duplex transceiver can operate in a half-duplex mode by connecting the differential transmitter outputs to the differential receiver inputs. However, there are some issues associated with trying to operate a half-duplex link without ensuring that inactive drivers are properly disabled.

    The introduction of series "Ri" resistances will help to reduce currents that could flow when the two driver outputs conflict, but will also introduce attenuation to the signals driven by HVD32. Since that device has a minimum differential output of 1.5 V, solving for the equation you provided (which is correct) means that the maximum series resistance value would be 195 Ohms. In actuality, it would be good to have some margin to this value since there may be some loss across the cabling as well. As the series resistance decreases, though, the effective loading that the HVD32 circuit would impose on the bus would increase.

    For a simple point-to-point connection, this scheme should be functional as long as the cable loss is not too high. For a multi-node network, it may become difficult to find a series resistance value that can work. (The presence of these resistances and the associated bus loading will also reduce the number of nodes that can be achieved on the network.)

    Ultimately, it may be simpler to use one of the devices in the HVD3x family featuring driver enable (DE) controls.

    I hope this is clear - please let me know if you have any questions.

    Regards,
    Max Robertson
  • 0 in reply to Miguel Robertson
    Genius 5350 points
    Hi Max-san,

    Thank you for teaching me in detail.
    I was trying to propose the value of the resistors as Ri = 120 Ω, Rt (SN65HVD32 side) = 240 Ω and Rt (opposite side) = 120 Ω.

    However, the customer may consider replacing them with SN65HVD35 with Enable control.
    So I will propose it.

    Thank you very much for helping me.
    Sincerely,
    M. Tachibana