CMU mapping to Target Display
Below are some trouble shooting solutions for a blank target display.
Verify if the monitor settings matches to the SDI input signal: resolution, 444/422, RGB/YCbCr, single/dual/quad link, etc.
If you're using iCMU
The second SDI channel of your I/O card is defined to the mapped output of iCMU. Make sure there's the second channel in your I/O card and the 'Dual SDI output' function has been enabled in Resolve project setting pane.
If you're using eCMU
Check the status of your CMU first. This is something you should always keep in mind whenever you find problems of target display.
Is the CMU in 'black-out' mode? If yes, switch it to normal mode.
Is the CMU in HDMI tunneling mode? If yes, switch it to SDI mode.