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RS485 driver without separate power supply

John Shen1
Prodigy 60 points

Hello 

Do you have a RS485 driver that does not need a separate power supply? It draws power from the RX and TX lines, instead?

Thanks

john 

  • 0
    TI__Guru 62920 points

    Hi John,

    No, I don't know of a device currently available capable of doing this. In some cases it is possible to distribute power over the RS-485 lines using a coupling inductor; please see for an example implementation. Note that this requires AC coupling the RS-485 data as well, so it works best if the data is high in bit rate and encoded to have DC balance (equivalent average number of high and low levels).

    Regards,

    Max

  • 0 in reply to Miguel Robertson
    Prodigy 60 points

    Hello Max,

    Thank you for quick response. 

    Our product wireless transmitter is going to connect to a water meter through RS485. The two are just 5m away. Both wireless transmitter and water meter are powered by battery and the battery has to last for 10 years. So, it is important to keep the power consumption low. Do you think the method you mentioned  (AC coupling) will fit our application? will that method waste too much power since you have to modulate the signal to higher rate? BTW, our data rate is normally set to 9600 Baud, but could be changed to 19200. 

    Do you have a better way for our application?  

    Thanks a lot.

    john 

    Best regards

    John

  • 0 in reply to John Shen1
    TI__Guru 62920 points

    Hi John,

    If both sides are powered by batteries, couldn't the battery voltage supply the RS-485 transceivers?  It would generally be OK for the two sides to operate off of independent power supplies.  (The exception to this would be if there were something that could force some offset voltage between the ground reference potentials of each node.  RS-485 signals can handle offsets of about 7 V but above that may lose data if specialized transceivers or signal isolation are not used.)

    Max