Page 42 of the user manual starts to document how to expose the additional UARTS.
http://www.ni.com/pdf/manuals/375466a.pdf
Those UARTS are implemented in the FPGA, so the TX and RX pins can be routed to any of the FPGA DIO lines. That is defined via the sbRIO CLIP generator tool installed with LabVIEW and implemented by selected a socketed CLIP RMC plug-in in the LabVIEW project.
Page 24 of the RMC design guide also gives further detail:
http://www.ni.com/pdf/manuals/375536a.pdf
Also, this NIWeek presentation walks through the RMC design process and SW tools for configuring additional peripherals, such as UARTS.
https://decibel.ni.com/content/docs/DOC-43347
Regards,
Spex
National Instruments
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