Why does my Displayport monitor not show anything on the boot drive selection screen?

to get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display.

This query is only sent at certain times:

• at startup

• at wake from sleep

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• on invoking Option-(Detect Display) button in Displays preferences

so try doing some of those things and see if the display comes alive.

Modern Displays with multiple ports are sometimes busy scanning the other ports, looking for an input, and miss the query from the Mac. They need to pay attention to the port you are actually using, or they will miss the query.

Some displays have On-Screen Display settings that can be used to tell the display a computer is attached on a certain port, or a certain port should be highest priority. Changing those may make your display more responsive.

Some displays include their own private "sleep" settings for the display alone. This can allow the display to enter its own sleep mode, on top of the Mac's not sending it data. A display that is sleeping on its own cannot respond to the Mac's query, and will stay dark.

I don't think it's a resolution issue either way. HDMI is "smarter" than DisplayPort in terms of negotiating the video settings with the monitor in my experiencing than DisplayPort, but there's no good inherent reason why DisplayPort shouldn't be working off the bat.

I wonder, have you tried doing an SMC reset while this monitor is plugged in? That might be enough to start getting your Mac Pro to work with it properly on your boot startup screen. Unplug the power to your Mac Pro for 15 seconds, plug it back in, and wait 5 seconds before turning it back on. See if things work properly via DisplayPort at this point.

If not, is sticking just with HDMI a viable option for you since you indicate that's working fine?

The code for that mode is all in ROM. It will use a default resolution, which is extremely low, in hopes of getting something on your as-yet-unknown display.

If your display cannot go low enough in resolution, it may not show a picture.

Default of something like 640 by 480 or 800 by 600 seems likely.

A side question I meant to ask you:

Do you get a display when you restart in Safe Mode (hold shift at Startup)? Safe Mode tries to use defaults for everything, but it may use a more generous screen size than ROM-based Startup Manager (Option-Boot screen).

Safe Mode does a five-minute disk repair as it starts up, so even if you see nothing, try to wait five minutes and see if you get a login screen eventually. Your username and password are required to proceed, even if you normally auto-login.

That looks correct.

The reason I ask is that with the advent of ThunderBolt-3, the rule we have come to rely on:

"If you can plug in both ends of an Apple cable, it will do the Job expected"

... Is no longer true, and now we must work much harder to be certain the correct sort of cable is being used.

Who makes that cable? Do you have a link?

Exactly what cables/adapters are you using to run the display with DisplayPort family?

No. if the cable came with the display, it should work fine (or not at all). It's not likely to be cable problem.

I am not sure what the problem might be.

I do have a boilerplate about multiple-input displays with Macs that MAY have some insights. I will put in a posting below.

The Mini DisplayPort end of it picks up exactly the subset of ThunderBolt 1 or 2 that is DisplayPort (family) signals. It doesn't really convert, it gets DisplayPort family directly from the ThunderBolt port, by design . [Mini] DisplayPort <--> [Full size] DisplayPort is just a re-wire, not a conversion.