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DETAILS ON SN65HVD78DR RS485 IC

Chandan kumar
Prodigy 130 points

Hi ,

I am designing a custom board and i have a requirement of RS485 Transceiver IC for data communication

So i have selected SN65HVD78DR IC

I have also attached my circuit .

I have following few questions as i have not worked much on RS485 

1)What is the co relation between A, B Differential Signals with + & -ve Differential signals. I have gone through many links and in which few suggests that A is considered as  -ve signal and B is considered as +ve signal .But in few reference designs it was routed in the opposite way . Please relate me these terminologies to avoid

2) The differential voltage level for the bus that i used here is 3.3V .What if the other connected RS485 device differential voltage levels is different (say 5V/10V) .Will the communication happens? and if so at what v/g level?

Thanks,

Chandan

  • 0
    TI__Intellectual 2885 points
    Hello Chandan,

    The convention that all TI parts use for the AB lines is that A is +ve and B is -ve. As for the differential signaling, it will not matter if you are using a 3.3 V part in conjunction with other voltages. RS-485 only looks for a differential signal of ± 200 mV. The RS-485 spec for input voltages is -7 V to +12 V. Any voltage within that range will work.