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SN65HVD1781: RS422/485 Termination under Software Control

Michael Klos
Intellectual 310 points
Part Number: SN65HVD1781


Working on a semi-universal serial design that we want configurable by our processor.  We want one design that we can fully populate and select transceivers and termination via software.  We will have two ports each RS232, RS485/422 and CAN.  The RS232 will each be full DB9 DTE or DCE.

We would like to have the termination for the RS485/RS422 under software control.  That's where we are hoping you can help.

No parts have been selected yet.

  • 0
    TI__Guru 62920 points
    Hi Michael,

    The following reference design shows a couple of different approaches for implementing software-controlled termination:

    www.ti.com/.../TIDA-01238

    It uses CAN as an example, but the same approaches could work for RS-485 or RS-422 as well.

    One method is to place a FET (or pair of FETs) in series with a termination resistance and then control the gate voltage of this FET in order to enable or disable the termination. The challenge here is getting a control voltage that can always be reliably above or below a VGS threshold (depending on if you want the termination on or off) when accounting for the operating voltage ranges that CAN/RS-485/etc. need to support on the bus. This means that you may need to generate a higher-voltage rail for gate control, limit the operating voltage range of your interface, or both.

    The other method presented removes this restriction (at the expense of cost) by introducing isolation between the bus and the control voltage domain using a solid-state relay (SSR). A logic-level control line is used to turn an internal LED in the SSR on or off, and then this enables or disables a conduction path on the field/bus side accordingly.

    I hope this helps - please let me know if you have any questions (or if you come up with any better solutions).

    Regards,
    Max