9871 half-duplex configuration

[tomherron]

I am trying to configure an NI 9871 4-port 485 serial module for half-duplex communication with no-echo.  My problem is I can't figure out how to select the transceiver mode.  Table 3 of the manual (http://www.ni.com/pdf/manuals/372277e.pdf) shows four different modes but it's a mystery to me how I configure the mode.  The module is currently in 4-wire mode where the receiver is always active (if I tie TX & RX lines together, I get back what I send.)   I would like to be in 2-wire Auto mode.   Can anyone point me in the right direction?

BTW:  I'm using a cRIO-9075 in scan mode and using Labview 2012 SP1 (windows), In case that matters.

Thanks.

[ks30]

Hi Tomherron,

Since you are using the Scan Mode then you should refer to the following KB for directions on configuring the module and making changes  to your individual ports in LabVIEW using a VISA Configure Serial Port VI or a VISA Property Node (Serial Settings>>Wire Mode) :

Using the NI 987x Modules in Scan Mode

http://digital.ni.com/public.nsf/allkb/16989C708E5AF2C1862578A90067D9DC

Alternatively, if you use the FPGA interface then you would use the FPGA I/O Property Node as outlined in the following document:

NI 9871

http://zone.ni.com/reference/en-XX/help/370984R-01/lvaddon11/crio-9871/

Regards,

Kyle S.
Applications Engineer
National Instruments
http://www.ni.com/support

[tomherron]

Thanks for your reply, Kyle.  However, unless I'm missing something, that article does not have anything to say about transceiver configuration.  Are the different transceiver modes not supported in Scan Mode?

[ks30 · Accepted solution]

Hi Tomherron,

If you are using Scan Mode then you can set the Transceiver Mode using a VISA Property Node. The process would be:

1) Place a VISA Property Node on the block diagram of your application.

2) Left-click on the Property Node and select: Select Property»Serial Settings»Wire Mode. This will allow you to set or get the wire mode for a particular port. In addition, this allows you to have different wire modes for different ports.

3) Right-click and select Change All to Write.

4) Right-click the Wire Mode and select Create»Constant.

5) Click the constant to select which transceiver mode you want to use.

Two of the options are RS-485/Wire2-EchoDTR and RS-485/Wire2-Auto. Are those what you are looking for?

Regards,

Kyle S.
Applications Engineer
National Instruments
http://www.ni.com/support

[tomherron]

OMG!  I can't believe I missed that...right under my nose.  Thank you for being so patient.

[ks30]

Hi Tomherron,

I’m glad that seems to work for you!

Regards,

Kyle S.
Applications Engineer
National Instruments
http://www.ni.com/support

[jtana]

Hello Gentlemen:

We are working on a NI9871 too, however, it just woks on full duplex, we will follow your instructions, thank you for share.

My question is: Could you  submit the diagram for the connection on half duplex on the DB9 connector, please?

Do we need to link RXD+ and TXD+ (3,8) and RXD- and TXD- (9,5)?, are there any other electrical connection required?

Are the terminal resistors (120 ohm) required?, Our NI9871 is running as the sole slave,

Thank you

[ks30]

Hi Jtana,

Please refer to the following KnowledgeBase article for the wiring diagram for the configuration that you requested:

What Pins Are Needed for 2- and 4- Wire Transmission with RS-485 Serial Communication?

http://digital.ni.com/public.nsf/allkb/5FFDF323C35C1B5386256F1C00622596?OpenDocument

For your question regarding termination resistors, please refer to the following tutorial:

Termination and Bias Resistor Information

http://www.ni.com/support/serial/resinfo.htm

Regards,

Kyle S.
Applications Engineer
National Instruments
http://www.ni.com/support

[tomherron]

jtana,

Here are some additional links that I found helpful:

http://www.ni.com/support/serial/resinfo.htm - shows set up for bais resistors.  This is useful in half duplex over long distances -- helps suppress noise when no module is driving the line.

http://digital.ni.com/public.nsf/allkb/12BB6CD900B2D0EF862564980050F5A7 -- Shows NI's solution to termination.  Also shows bias resistors must be added externally.

Tom Herron