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PXI-8431 not going into receive mode with VB6

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I am using a PXI-1045 chassis with a PXI-8431 RS422/485 card and communicating via a VB6 program


The output of the PXI-8431 RS485 module works OK when it is in send mode, the signal levels are approx 1v and 3v for high and low, but when it is in receive mode the outputs remain at 1v and 3v instead of going into passive mode and both lines floating at approx 2v with approx 300mV differential as they should. This means when the remote devices send, the amplitude of the signal is approx half as they are competing with the PXI module, and that the centre point of the two signals are at 1v and 3v respectively. Our software does not receive the signals from the PXI module which is consistent with my theory that it is still in send mode and taking control of the lines, or maybe it has changed it to 4 wire mode.


I have swapped over the PXI card and the remote cards with our good system and the problem does not move.

I have eliminated ALL the hardware on the new ring by replacing it with a 67 Ohm resistor on the pins of a 9 way D connector plugged onto the PXI card directly. With a scope I can see that the new system is not going into passive mode, but with the same connector the old system works OK.


The only difference I can see between the 6 old rigs we have that do work and the 2 new ones that do not work is the new ones use the the NI-XNET-PXI-8513 and the old ones use the PXI_8464 CAN card. Could this software be conflicting? Is there any other updates to the NI software that could be causing it?

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I've had a check to see if we have any known software conflict issues with those cards but it all seems fine, it could be that the Windows Driver layer is causing some issues. If you nip into Device Manager and go to the properties of the 8431 you should be able to change it to 2-wire mode from those settings then re-run the program.


Also does the program throw any errors at all in terms of device resources when trying to move from send to receive mode?


What versions of NI-VISA and NI-Serial do you currently have installed?


Kind Regards,

Applications Engineer
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It is set to 2 wire mode in "Measurement & Automation" and Windows "Device Manager"


There are no error messages in the VB program. We are using mscomm which does not have the facility to change 2 wire/4wire mode, but this has worked well upto now. (is there a more up-to-date method we should be using - I havent found any other example programs on your website?)


On the bad system NI-VISA version 4.6.1, NI-Serial 3.5.1


On good systems NI-VISA versions 4.4.1 and 4.3, NI-Serial versions 3.4 and 3.3.0




Philip handscomb

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Accepted by topic author philhan



Serial 3.5.1 did have some issues related to serial read/write when in 2 wire mode, I'd recommend either matching the driver versions with your working system or upgrading to the latest serial driver.



Applications Engineer
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