I have two U2715H monitors connected via one mDP->DP cable each to an E-Port Plus docking station (actually a EURO2 Advanced Port Replicator) in which is seated a Latitude E7450 laptop. All items are brand new. The E7450 is equipped with 16 GB RAM and two graphics cards: Intel HD Graphics 5500 and NVIDIA GeForce 840M; selection is automatic, which usually means the 5500 is in operation. It runs Windows 7 Enterprise SP1. The docking station apparently has DP 1.2a capabilities (for daisy chaining) but DP 1.2 is disabled in both monitors.
I have successfully set up the system to have the Windows desktop span both monitors at 2560x1440 resolution, even with the Latitude's internal screen as a third extension at its full resolution (1920x1080). The Main display may be set to either of the two (or three) and I can successfully move the sequence of the monitors around (left, middle, right).
So there is no doubt about the system's capability to support the intended configuration. However, when I dock the Latitude (mostly with the lid shut) and boot it, most of the time it cannot reestablish the two monitor configuration. Some of the symptoms are: One or both monitors go black, indicating cable connected but no signal arriving from the docking station. If there is a picture on one of the monitors, it may or may not contain the login screen. (I can still log in because I know what keystrokes to send.) If the login screen is visible, it is usually in a low resolution. If I have all three screens active, I can move the mouse pointer across the black screen in the middle but it is not visible there.
Both monitors come straight from the box and the ony change I have done is to switch from DP to mDP as input channel. I have reseated the cables and swapped the cables between the two DPs on the docking station. I have turned the monitors off and on. The laptop connection on the docking station is a bit wobbly but I am using the supplied spacer for the E7450 and make sure the connection is correct and stable.
The instructions packed with the monitors had a note about consulting the supplied CD for using the monitor at maximum resolution. However, I don't have an optical drive and I have read all the documentation for the monitor at Product Support without finding anything to illuminate the situation.
Once I have logged in, I can manage to get everything back in order by randomly(?) reducing screen resolution, changing the position of the main display (left-middle-right), deselecting and reselecting mDP as input channel, and forcing other changes. Once both or all three screens are in operation, I can set them all to maximum resolution and start working. However, this is rather bothersome and I would like to have a stable setup.
I should mention than when I undock the E7450, I may be using it in other configurations with or without other external monitors through other ports, before I turn it off an later re-dock it as described. However, for the first month I had two older non-Dell monitors of lesser resolution (on DVI and VGA ports) connected to the docking station and the configuration was stable, and for years I have been used to my older Latitude remembering its peripherals between different hardware setups.
Please, anyone - and Dell support staff in particular?