Instrument Control (GPIB, Serial, VISA, IVI)

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RS485 half duplex(2 wire)

Hi All, 


I am using NI PXI 8431 for performing RS485 communication with our meter.

I am able to communicate to the device from NI 8421 using Full duplex communication.


 However I am not able to do the same with Half duplex communication.


 I am able to do Full duplex communication between my device and NI PXI 8431(2 port Rs485).


Tx+ -> Rx+

Tx- -> Rx-

Rx+ -> Tx+

Rx- ->Tx-

I am trying to do RS 485 Half duplex communication with the following configuration:


Tx+ -> Tx+;

Tx- -> Tx-

 Gnd -> Gnd


Iam not able to establish communication. 


To check if my device supports RS485 Half duplex, I did the following.


I connected the RS485 half-duplex output from my device to a RS232 converter and able to communicate with it via serial port!!

 (By connecting Tx+ to D+ and Tx- to D-)



 Can any one  help me to repeat to establish half duplex communication with NI RS485 (PXI 8431).

 The communication between my device and NI PXI 8431 is Modbus protocol using the drivers available  here







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Message 1 of 6



Did you set the PXI8431 correctly in MAX (auto 2-wire) ?


Is the RS-485 line terminated at both ends ?



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Message 2 of 6

Hi KC,


  Yes,I have set it to RS485/Wire2-Auto


Iam not using any termination registers.Iam able to do full duplex communication without termination resistors.

Are they mandatory for half duplex ?



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Message 3 of 6

Full or half duplex: a RS-485 line should always be terminated.

Situations with 1 slave, short lines, 'low' baudrate, etc. will work mostly. But to be sure that the absence of terminators is not the cause of your probelm you should add them.



Message 4 of 6

Hi Kees,


The issue got solved.


    For Half Duplex, we need to short (RX+,TX+) and ( Rx-,Tx-) in the DB9 connector side (of RS485)

    D+ will go to (Rx+,Tx+); D- will go to (Rx-,Tx-)





Message 5 of 6

Sorry that one was too obvious for me. Smiley Wink


and reading your first post I thought that you already made this connection.

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Message 6 of 6