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THVD1400: The power supply voltage of the receiving and transmitting is different

Part Number: THVD1400

Hi, Team,

Customer want to if THVD1400 powered by 3.3Vcc, connecting with SN65HVD3082 powered by 5Vcc. Is there anything wrong using like this?

  • This will work. The RS-485 bus voltages are not related to the logic supply voltages.

  • Hi Dillen,

    THVD1400 at VCC = 3.3V can communicate on the differential bus (A and B pins) with the SN65HVD3082 at VCC = 5V. So it will work as long as the data rate is capped to 200kbps or less since the SN65HVD device is capped to 200kbps - that is the upper limit of data rate  of the system. 

    RS-485 requires the following (this is not an exhaustive list):

    1. Input voltage range of -7V to 12V on the A/B pins regardless of VCC.

    2. When RS-485 receiver is active a differential input (from A to B) of 200mV or greater is read as "high" and a differential input (from A to B)  of -200mV or less is read as "low". This is also is required regardless if VCC = 5 or VCC = 3.3V. 

    3. RS-485 devices, regardless of VCC (so either 3.3V or 5V), need to be able to output 1.5V across 54 Ohm resistor between A and B

    Since both of these devices are RS-485 compliant, then they have to meet the three requirements listed above - the 5V part may output a higher differential voltage typically, but it won't be out of range of requirement 1 and will meet the other two requirements. The 3.3V part still can meet the minimum output voltage required of RS-485 as well so there should be no issues on the differential bus.

    Please let me know if you have any other questions!

    Best,

    Parker Dodson