Hi Charlie,
When you are using the module NI 9871 in the scan mode you can short the RXD+ with TXD + and RXD- with TXD- pins using some kind of jumper cables so that your Rx and Tx are shorted for the loopback.
In the Scan Interface mode, you can write a simple code for a serial loopback using VISA read and VISA write.
Basically you can refer to the Simple Serial.vi in the Example Finder of LabVIEW and with some small modifications in the configuration you should be able to do the serial Loopback in the Scan Mode.
For the FPGA mode, The 9871 does not support the FPGA I/O node. So once you add the module in the FPGA mode you will get different ports and you can then use the FPGA I/O property and I/O methods to read or write on the port. For more information on how to use the method and property nodes please refer to this link.
http://zone.ni.com/reference/en-XX/help/373197B-01/lvaddon11/crio-9871/
You can also refer to this example in the example finder.
labview\examples\CompactRIO\Module Specific\NI 987x\Serial Loopback\NI-987x Serial Loopback.lvproj
Hope this helps !
Regards
Archan Mudwel
AE, NI-India
