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SN65HVD1791: Using SN65HVD1791D as Half duplex RS485 instead of Full Duplex

Part Number: SN65HVD1791

Could SN65HVD1791D be used as Half duplex RS485 by simply shorting

A-Y ----> Then using this as 'A' terminal for Half Duplex RS485

B-Z  ----> Then using this as 'B' terminal for Half Duplex RS485

or is it not designed to work in Half duplex mode?

I am seeing some issues operating this in Half Duplex mode but instead of shorting 

A-Y & B-Z

If I connect 100R between A & Y 

& another 100R between B & Z

then it seems to work but not reliably and I am not sure if I should really be adding 100R between the two lines for each pair!?

  • N S,

    It is OK to directly short the TX and RX pairs together to operate as half duplex. 

    From what you describe I bet the issue is with the enable controls.  In a half-duplex application only one driver circuit should be enabled at a given time - otherwise if multiple drivers send opposite bit values they could corrupt one another.  You should make sure that "DE" is high only when this node needs to transmit data and low on all other nodes.  And, you should make sure "DE" is low and "/RE" is low when this node needs to receive data on the the bus from other nodes.  Please let me know if this isn't clear. 

    If that doesn't solve your problem, let's look at the signal waveforms at the device's various IO pins to see if it sheds any clues on what is wrong.


  • Thank you Max. Yes it was related to De and re enable lines in half duplex mode .

    Its working now :)