I can now send and receive data, and yep, it was a dumb oversight on my part. The acronym “RTFM” comes to mind!
Bottom line, use XCTU’s I/O Settings section to turn off the UART DIN & DOUT pins in the firmware and turn on the SPI pins as shown below (see the lines with the blue triangles at the end.) This might vary if you’re using a device other than the 900Pro.
@ScruffR - thanks again for your feedback. This application will be using multiple SPI devices, so I added that code back in, as you suggested. Plus it’s very helpful to have someone with your level of experience to help with a sanity check!
Steve