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SN65HVD3088E: Failsafe biasing with different voltage from VCC

Oba
Genius 17335 points
Part Number: SN65HVD3088E
Other Parts Discussed in Thread: THVD1450

Hello,

 

I have a question about failsafe biasing.

When using 5V RS485 transceiver like SN65HVD3088E, is there any problem with 3.3V failsafe biasing?

On the other hand, using 3.3V RS485 like THVD1450 with 3.3V VCC, is the any problem with 5V failsafe biasing?

 

Regards,

Oba

  • 0
    TI__Guru 62920 points
    Hello,

    Generally this is not a problem, since RS-485 transceivers are able to operate over a wide bus voltage range in order to accommodate potential ground shifts between nodes. (The RS-485 standard defines transceiver characteristics over a range from -7 V to +12 V for this reason.) Because of this, it is common and acceptable for a node to receive voltages outside of its VCC range.

    One thing you should be aware of is that if failsafe biasing is implemented on both a 5-V node and a 3.3-V node, then there may be some current flow from the higher-voltage node to the lower-voltage node. If this is problematic for an application, then possible solutions could be to use failsafe biasing at one node only (which is generally a good practice to reduce bus loading) or to place a diode in series with the pull-up resistance used on the lower-voltage node.

    Max