P2415Q, MacBook Pro, mDP not working

@Elangsru  I don't understand the problem here because you're not providing very good information.  First of all, the P2415Q doesn't support actual Thunderbolt, so that wouldn't be involved here.  Second, Thunderbolt 2 used Mini-DisplayPort connectors, whereas Thunderbolt 3 uses USB-C, so when you just say you're using a "Thunderbolt" cable, it's not even clear what connector type you're talking about.

Assuming your MacBook Pro has Thunderbolt 2 ports, which use a Mini-DisplayPort connector, it sounds like you're trying to say that when you use a Mini-DisplayPort to Mini-DisplayPort cable, your display goes to sleep, but when you use a full-size DisplayPort to Mini-DisplayPort cable, things work fine.  Is that correct?  If so, are you actually switching going into the display's on-screen menu to switch it over to its Mini-DisplayPort input when you want to use that connector instead?  The display doesn't automatically switch to another input just because you plugged a cable into a different input.  And just out of curiosity, why do you have to use the Mini-DisplayPort input on the display?  What's wrong with using the full-size DisplayPort input if you already have an appropriate cable to connect that way?  DisplayPort and Mini-DisplayPort work identically.

Yes its a Thunderbolt 2 cable using minidisplay ports in both ends. 

yes i switch to mdp in monitor menu

I want to know WHY it does not work as I have a new mac using the full size DP to USB-c port already. The port is there...then it should work!!

I tried the same 2015 Mac+cable at work on my LG 5k ultra fine display. It worked fine

This is the cable I'm using https://www.apple.com/shop/product/MD861LL/A/apple-thunderbolt-cable-20-m?fnode=8b

Elangsru,

The P2415Q was never tested/validated by Dell with any Apple OS or hardware. So this is all unknown ground. You might want to read through the 181 P2415Q threads on the Apple Forum.

When speaking about cabling from PC to a monitor, you always want to mention the PC out port first, like below.

jphughan,
I think he is saying =
MacBook Pro TB2 out port --> Apple TB2 to TB2 cable --> P2415Q mDP in port (not working)

I have no idea what cable he means here, "Using «ordinary» DP to Thunderbolt cable works fine.". Maybe he means the Dell P2415Q mDP to DP cable?

@DELL-Chris M  I agree with your interpretation of the mDP cable setup.  Apple has an official TB2 cable and TB2 uses a Mini-DisplayPort connector on both ends.  I read "ordinary DP to Thunderbolt cable" as "full-size DisplayPort to Mini-DisplayPort cable", with the former end plugged into the display's full-size DP input and the latter plugged into the Mac's TB2 port.  Those cables are easy to find, and it seems that the OP performed that test just to validate that the Mac could in fact send an image to the display, albeit through a different input.  But the OP also says he's normally got a separate system using the display's full-size DP input through a USB-C to DP cable, which is why he wants to be able to plug the MacBook into the Mini-DP input.

Hi @jphughan, @Dell-chris , @Elangsru,

I have some experience with using a cable mDP to HDMI cable from my Macbook pro 2015 (15", 2.7GHz).

I have a cable with mDP (thunderbolt 2) on one side and HDMI on the other.

When i use the HDMI output port on the mac and connect to the mDP in the P2415Q the mDP input is not recognized by the P2415Q.

Whereas if i connect the cable in the opposite direction, that is, thunderbolt 2 output to the HDMI in the monitor it works fine, HDMI is recognized.

The reasons for this weird behavior are opaque to me.

I came to this thread to check whether mDP to mDP is known to work or not. OP seems to say it doesn't.

I would like to ask if you know of any place where working configurations are posted?

Mario

I have not yet resolved this OR seen a solution for it anywhere.

My problem is similar, only I use the mDP to mDP

The DP (monitor) to mDP (mac) still works fine. Same goes for DP (monitor) to USB-c (on my newer Mac)

@Mario_N  Mini-DisplayPort to HDMI cables (and full-size DisplayPort to HDMI cables) can only be used to connect a DisplayPort source to an HDMI display.  They do not work in the opposite direction.  This comes up here all the time.  The reason is that most -- but not all -- DisplayPort source ports support falling back to native HDMI signaling for compatibility.  This capability is called "Dual Mode DisplayPort", abbreviated DP++.  But HDMI source ports do not support sending native DisplayPort signalling, and displays do not support receiving a native HDMI signal on their DisplayPort inputs.

DP++ is technically an optional capability, but it is widely implemented on systems and docks.  However, it is also NOT very often implemented on specialized adapters like DisplayPort MST hubs and Thunderbolt 3 to Dual DisplayPort adapters.  It is also not possible on USB-C/TB3 ports that support video.  The USB-C to HDMI cables/adapters you see are actually "active" cables that incorporate a converter chip within the cable/adapter itself that switches the native DisplayPort signal over to HDMI.

Note that there are technically a small handful of "active" HDMI to DisplayPort cables/adapters that are explicitly meant to connect an HDMI source to a DisplayPort display, but these are very uncommon.  They accomplish this by incorporating an active converter chip in the cable/adapter.  But it gets even more confusing because there are also "active" versions of DisplayPort to HDMI cables/adapters meant to perform a conversion in the opposite, "traditional" direction, i.e. DisplayPort source to HDMI display.  The reason THOSE exist is to work with DisplayPort sources that do NOT support DP++, as well as to deal with some specialized situations where DP++ is not allowed to be used even if it's supported by the GPU.  AMD EyeFinity setups fall into that category.  So if you get into active cables/adapters, you have to be careful about the direction of the conversion that will be performed.

All that said, I'm not sure why the OP is having trouble.  Using either mDP to mDP or mDP to DP should work just fine with this display.  If it doesn't for some reason, I'm not sure what's going on.

Thank you for the detailed answer, it does clarify many of my doubts. I did try to search for this information, but it's nice to find all in one place. I'll put a mDP to mDP to the test!