Monitor resolution problem -- Mac Pro 2014 with BenQ BL3200PT

Hello! You should be using a dual-link DVI cable.

The dual-link cable has to have a power source.. Use USB is used.

13. How is the Apple Mini DisplayPort to Dual-Link DVI adapter different from the Apple Mini DisplayPort to DVI adapter?

The Apple Mini DisplayPort to Dual-Link DVI adapter works best with displays that operate at dual-link resolutions, which are resolutions above 1920 x 1200. For displays that do not support dual-link DVI resolutions (displays that operate at resolutions of 1920 x 1200 or lower) you should use the Apple Mini DisplayPort to DVI adapter instead.

14. Which monitors work with the Apple Mini DisplayPort to Dual-Link DVI adapter?

Use the Mini DisplayPort to Dual-Link DVI Adapter to connect your computer to a 30-inch display that includes a dual-link DVI connector, such as the Apple Cinema HD Display. Note: For such displays, be sure to connect the USB connection on the adapter to the USB port on your Mac; also connect the USB connection from the display to the USB port on the adapter.

15. What can I do if my external display is not working properly with the Mini DisplayPort to Dual-Link DVI adapter?

If you are using an external display that is not dual-link DVI, use the Apple Mini DisplayPort to DVI adapter for best performance.

Try these steps if your dual-link DVI display has video issues while it is connected with the Mini DisplayPort to Dual-Link DVI adapter:

1. Reset the Mini DisplayPort to Dual-Link DVI adapter by disconnecting its Mini DisplayPort and USB connections from the computer for few seconds.
2. Reconnect the cables to the computer to see if the issues persist.
3. If your issue is still unresolved, disconnect the Mini DisplayPort to Dual-Link DVI adapter as described in step 1, then power off the monitor.
4. Reconnect all cables and power on the display.
5. If issues continue to persist, restart the computer.

Apple Mini DisplayPort adapters: Frequently asked questions (FAQ) - Apple Support

Input Connectors

D-sub / DVI / DP1.2 / HDMI / headphone jack / line in

The cheap solution is to skip DVI entirely and use Mini DisplayPort (plugs into ThunderBolt directly) to (Full-Size) DisplayPort (plugs into Display directly). This involves NO CONVERSION, it is just a re-wire.

here are some examples:

http://www.monoprice.com/Category?c_id=102&cp_id=10246&cs_id=1024606

amazon's search is divergent, but the first few are applicable:

http://www.amazon.com/s/ref=nb_sb_ss_i_4_16?url=search-alias%3Daps&field-keyword s=mini-displayport+to+displayport+cable&sprefix=Mini-Displayport%2Caps%2C159.