2013 Mac Pro Maximum Resolution

This is the definitive article of the Mac Pro 2103 and multiple displays. It says it supports three 4K displays, or more.

Use multiple displays with your Mac Pro (Late 2013) - Apple Support

That Apple article you cited is for adapters. you are not using any adapters, so it does not apply to you.

A fundamental problem users are encountering is that the Mac -- unlike PCs -- depends on the stated resolutions returned in a query of the display using the EDID channel. PC's simply set the resolution in a custom driver for that display.

If the display does not report it true abilities, the Mac can not use them. This has been a problem with a few Dell displays, because Dell just found any old EDID information from a less-capable display and gave it to the newer, more capable display, and called it a day. Since PC users just get a driver with hard-coded resolutions, their customers did not complain -- until Mac users started to use the advanced displays and could not access full resolutions.

So what I suggest is that this display may not be correctly reporting its top resolution, and unless someone can procure or create an appropriate "Override" file, that is all you can get out of it.

Be sure to ask the display manufacturer if they have an appropriate firmware update or Override file for their display and/or why you can not access its full marketed resolutions over Displayport.

I have not seen any articles suggesting that different Mac Pro 2013 configurations i.e. D300 vs D500 vs D700 support different video resolutions or different numbers of screens. I would therefore lean towards believing that whilst the performance will obviously be different the resolutions and quantities will be the same.

Originally the Mac Pro 2013 supported a maximum of three 4K displays. It now supports also three 5K dual-link displays e.g. the Dell UP2715K.

See - https://support.apple.com/kb/SP697?locale=en_GB

See also - https://support.apple.com/en-gb/HT206587

See also - https://everymac.com/systems/apple/mac_pro/specs/mac-pro-quad-core-3.7-xeon-e5-gray-black-cylinder-late-2013-specs.html

The built-in HDMI port is only HDM 1.4a so yes it can do 4K but only at 30Hz which is not supported by most computer monitors. If you get an active Mini Displayport to HDMI 2.0 adapter this supports 4K at 60Hz. See - https://www.club-3d.com/en/detail/2339/mini_displayport_1.2_to_hdmi_2.0_active_adapter/

You need to be careful which combination of Mini Displayport/Thunderbolt and HDMI connections you use. See - https://support.apple.com/en-gb/HT202801

As indicated by Kurt the fact you are only seeing 1920x1080 is typically due to using an HDMI connection, altohugh it could be due to it providing incorrect EDID information.

According to https://www.benq.eu/en-uk/monitor/photographer/sw2700pt/specifications.html your monitor has three connections.

• HDMI 1.4
• Dual Link DVI
• Displayport (full size not the mini one)

You should therefore use a Mini Displayport to Displayport cable like this one - https://www.amazon.com/gp/product/B00YOQYQWY/

did you use the option key for scaled resolution in the Displays pane?

That is simply bizarre you're getting the opposite results for the cables in use.

Maybe there is something weird about the D300 graphics card. Apparently, Apple didn't use it for very long.

Yes, they did:

Introduction Date:October 22, 2013* -- Discontinued Date:April 4, 2017**

That resolution suggests you're using an HDMI cable. That is TV resolution and puts out a non-RGB signal (can't think of what it is at the moment).

Apple made the decision back in about Mountain Lion to stop using HDMI as a monitor output. The idea is for video users to have an RGB signal for the work monitor (where all of your palettes and such for Premiere Pro would be), and a separate HDMI connection to a standard HD TV. This eliminates the need for a breakout box.

In short, you need to use a DisplayPort connection to your monitor in order to use the higher resolutions. If your monitor doesn't have a DisplayPort connection, you'll need to purchase one that does.

Funny, I recall reading the Display Port cable was the way to go.

The display port cable IS the way to go.

It gives a true "monitor" timing and RGB color information, highest possible resolutions, and drops the "heartbeat" refresh used by CRT displays. It has the highest possible refresh rates. When the display picture stops changing, the cable goes quiet. This means it runs cooler as well, and effectively has more bandwidth than any "legacy" type such a HDMI or DVI.