780, Will not detect second monitor

What specific monitor models?

The Optiplex 780 shipped with four different chassis? Which one is yours? Go here, open the assorted PDF's and then tell us which chassis matches yours.

You cannot have more than one monitor active without installing the Intel graphics drivers.

https://downloads.dell.com/FOLDER01026826M/1/Video_Driver_1M8J9_WN_8.15.10.2869_A15.EXE

If that doesn't work:

https://downloads.dell.com/video/VIDEO_DRVR_WIN_R279357.EXE

I am having the same issue and can only mirror the view I can not extend the view onto another screen.  I tried both links and it is telling me that my computer does not meet the minimum requirements for this download?  Any other suggestions?

No such thing as Y adapter for Display port because its digital not analog.

its the same principle for USB no such thing as Y adapter.

You need an active powered hub for USB or display port.

VGA Y Adapter Ebay

In such instances, 1 monitor may only be detected.  I believe I learned that from Dell Community.  Too many "2nd monitor not detected" (and there's "lost 2nd monitor") instances to sort thru to find the exact post.  I tried going in from Google first.

HDMI & DVI Y adapters also available.

USB Y Adapters Ebay  My Dell keyboard has two USB ports acting as a Y adapter and it works great.

(Speedstep - I'm sure you know I'm referencing external adapters.)

More helpful tips:  Dell Community - Detecting Dual Monitors:

Helpful tips are toward the beginning.  Remember GPU cards automatically disable integrated onboard display ports.  As for later in the post, I don't recommend any display Y adapters.

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@Panthrocat wrote:

I am having the same issue and can only mirror the view I can not extend the view onto another screen.  I tried both links and it is telling me that my computer does not meet the minimum requirements for this download?  Any other suggestions?

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I'm guessing we are still talking about some form-factor of OptiPlex-780?

If so, I think your options are:
a. Install and use a dedicated PCIe video card (with 2 or more ports).
b. Connect an USB Video card

@bradthetechnut 

"HDMI & DVI Y adapters also available."

No they are not. This is false.  VGA Y adapters mirror 1 analog signal its not 2 monitors with different displays.

"My Dell keyboard has two USB ports acting as a Y adapter and it works great."

The keyboard has a passive Hub that won't work for high power items like usb flash drives.  A passive hub is not a straight thru Y cable.

No such thing as a passive Digital Adapter for HDMI or DVI.

The same is true for USB there are no passive Y cables for usb only Hubs.

HDMI to DVI and DVI to HDMI is bi directional.

Display Port Multi Stream Hubs are not passive Y cables.

https://www.amazon.com/StarTech-com-Port-DisplayPort-MST-Hub/dp/B00JLRBB8I

Display port is one way aka Display Port >>>>>>VGA

No such thing as VGA >>>Display port

which is why you need a HUB for Display port or USB etc.

No such passive Y cable exists like it does for Power or VGA.

Being able to buy a cable doesn't mean its valid or wont cause damage.

There are USB to firewire IEEE 1394 cables that are total BS but they sell them.

Can You Convert FireWire to USB?

The answer is largely no, for the following reasons:

Both methods use not only different hardware, but different voltage well.
USBs work on a 5v ACK/NAK protocol, while FireWire 12v DMA transfer protocol.
FireWire works at a constant rate, while USB sends data in bursts.

https://www.ebay.com/b/4-Pin-Firewire-To-Usb-Cable/31493/bn_7023396315

The HDMI Device is a hub not a passive Y cable.

"My Dell keyboard has two USB ports acting as a Y adapter and it works great." Again this is a passive hub not a Y cable.

Your references to USB Y cables are a very bad Idea not supported by UL or IEEE or any safety standard.  Not likely EVER working with 2 USB flash drives for example.

The DVI cable is only for Dual Link Support Adapters.

Neither is a passive straight thru cable like a VGA splitter.

People ask about cables like that because they see DMS 59 LFH cables that support 2 monitors.

Which is not the same thing.

DVI Dual Link when its done correctly has 1 analog and 1 digital.

Not 2 digital monitors. DVI doesn't have the bandwidth for 2 digital.

https://www.amazon.com/DELL-IMSOURCING-H9361-DMS-59-Y-SPLITTER/dp/B00LLJYZ8W

Your research is wrong as far as the function and electrical standards.

""HDMI & DVI Y adapters also available."

No they are not. This is false. VGA Y adapters mirror 1 analog signal its not 2 monitors with different displays."

HDMI Splitter on Amazon 

DVI Splitter on Amazon 

@speedstep  You reply to an almost 3 yr. old thread as if I'd forget that I checked before making a certain statement.  I know the Y splitters only mirror display.

USB Splitters and Hubs on Amazon 

At the end of my post:  As for later in the post, I don't recommend any display Y adapters.

@speedstep 

Would you also like to say that USB ports on Dell monitors don't work with flash drives?

Much of your replies seem related to a thread I referenced.  So, if you think somebody in that thread gave incorrect advice, then say so.  Screengrabs below are from the thread I referenced:

"Your research is wrong as far as the function and electrical standards."  Prove it.  Prove you can't plug 2 flash drives into a Dell monitor or Dell keyboard.  Please include screengrabs.

Again, at the end of the post you first replied to:  More helpful tips:  Dell Community - Detecting Dual Monitors  (This the thread speedstep is reacting to.)  (He doesn't just say to not believe another Rockstar.)

Helpful tips are toward the beginning.  Remember GPU cards automatically disable integrated onboard display ports.  As for later in the post, I don't recommend any display Y adapters.  Can you read the bold print?

Dig up an old post to give somebody else a hard time, or don't you do that?  Nobody else to give a hard time?  Stop wasting my time.

I said almost 3 years ago that "Keyboard makes great Y adapter."  I never said anything about it taking flash drives.  My mouse dongle is plugged into it.

So I tested flash drives in it.  I got a pop-up saying "...hub does not have enough power..."  And that's with just 1 flash drive, let alone 2.  So speedstep is right about that.

So, as Dell monitors with USB hubs can take flash drives, may be the monitors supply their own 5v power to the hubs?

There is no such thing as DVI or HDMI or USB splitter.

DMS59 LFH video cards have 2 ports.

DVI and HDMI are single port and bi directional.

DVI Dual Link has  Analog VGA (C1,C2,C3,C4) and Digital DVI

No such thing as HDMI dual link.  LFH cards have 2 vga or 2 DVI ports on the same DMS59 connector.  This is NOT a Splitter or Y cable.

The cables Linked below are NOT Y cables or Splitters.

Use of the term Y cable or Splitter in regards to serial digital data is always wrong.

https://www.dell.com/en-us/shop/c2g-dvi-cable-dvi-i-f-to-dms-59-m-91-in-black/apd/a0751748/

https://www.amazon.com/CABLEDECONN-Monitor-Extension-Adapter-Graphics/dp/B07LGKWTML

DVI Dual Link Pinout

USB ports on Dell Monitors are NOT passive hubs.  They use the power supply for the monitor. 

Even with powered hub those supposed Y adapters for USB will not work with ANY Usb Flash drive so I fail to see your point.

Passive hubs also cannot be daisy chained.

• You can have a maximum network of 127 ports on a USB controller. That includes all the ports used to create the network tree.
• You can have a maximum of 7 tiers, including the root hub and the device at the end of the chain, leaving a maximum chain of 5 daisy-chained external powered  hubs.

All of these restrictions are fully documented.

Y cables that just double the wires to 2 ports are NOT supported ever.  Some dell keyboards have a passive 2 port hub that works with mice and other low power 100ma  MAX devices.

https://usb.org/documents