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SN65HVD485E: RS-485 ground problem

CC Lai
Prodigy 110 points
Part Number: SN65HVD485E

Hello,

I am using SN65HVD485E on the circuit for DMX communication, the installation is track system which has only AC(L/N/G) and DMX data line(D+/D-)

Although it's working fine but when I study lots of articles, they always suggest to have 3rd conductor to avid GPD problem.

In the typical application schematic of SN65HVD485E, there is no 3rd conductor in it.

Is it really necessary for RS-485 communication ?

Can everyone tell me how RS-485 signal current flow back to itself ?

  • 0
    TI__Mastermind 25705 points

    CC,

    This is a very good question. You can find some detailed discussion on section 11 of this app note. RS485 has differential signaling therefore the current flows back by the bus, not by ground. To tolerate ground potential difference, RS485 transceivers work with -7V to 12V common mode range. If the voltage between two communication ends is within this range, the transmission should work. Please let me know if you have more questions.

    Regards,

    Hao

  • 0 in reply to Hao L
    Prodigy 110 points

    Dear Hao,

    Appreciate your answer.

    So if my units exist large GPD, I may need the third conductor we called ground is to tie all RS-485 transciever together to make sure working voltage is within -7V~12V range, right ?

    Because my machines have non-isolated RS485 and independent power supply (110VAC to 24VDC), basically their 24V- are all floating.

    (I checked the power supply, there is a capacitor between AC Earth and 24V-)

    Do you suggest to tie 24V- of all units together to make a same reference ?

    Ryan

  • 0 in reply to CC Lai
    TI__Mastermind 25705 points

    Ryan,

    It's correct if the GPD is too large, we need to do something. I'm not sure it's practical to tie all 24V together. You could only connect the GND of RS-485 together with some limiting resistors, which is the option c in Figure 9 of the app note.

    Regards,

    Hao