Monitor eDID facts (eDCD).            
Or Video Adapter choices  horrors and cures.

Related history of Plug and Pray is here.
Topic is Dead heading PCs, as in no monitors present.  (I use free Anydesk to remote in to PCs)  The free  desktop monitor control program is here.
Others use this trick for projectors that fail or cranky KVM switches that love to fail and block eDID data, or Bitcoin mining malarkey.
(Some older projectors have the wrong eDID data and use the wrong resolution and needs a custom cable to correct this error.)
The below shows how monitor plug and play works (and how to fool VGA and DIV-i , HDMI and DP (display port) video cards,etc)

A modern monitor has eDID chip that talks (data comms) to your PC VIDEO card.
This tells the PC what the monitor can do and can't do both. (in fact lots if information, seen below...)
For the use of a PC dead headed, no monitor present  , used as server and using AnyDesk™ (free) to remote into the dead head I build my own Dummy plug(ghost plug called by some)
If the VGA port is there or DVI-I  we only needs 3 resistors to make it work as seen below.
This simple way just below (3 resistors) does not work on DVI-D (digital only ports) you must use eDID ghost plug seen below.
Some video cards have 2 DVI ports and one has DVI pins C1 to C4 present so use that !. (around the large flat blade ground pin and this is RGB video lines) (RGB = red/green/blue video lines)
If the PC has no video card and has VGA, just just wire up the 3 RESISTORS VGA per photo #2 same way as the adapter.

Photo#1: This same trick works on VGA port with a custom made adapter. (a 60cent connector, and three x 8cent resistors for total DIY cost of 84 cents)
This only works with DVI-e , not DVI-d. (sold for $12,ouch) 86cent adapters are cheaper.
After inserting the resistors I use RTV glue, and glob that on so they can not fall out. (or just on top of resistors to gang them as 1)
When done cold boot the PC so that the GPU card scans and finds these 3 new terminations. (or the PC will be dead remote'd in)

Here is the order of precedence (some PCs have 6 Video ports, 3 on main and 3 on card.)
If the PC has no GPU card, use VGA first then DVD-I next and last buy a  HDMI or DP or DVI-D adapter , that use eDID and cost about $5 to $10HDMI to VGA  sure.(I have one) (mostly technicians tool)
If the PC has a GPU PCI-e card, the main iGPU onboard chips are disabled now, and if that is true the same order above, applies,  (VGA,DVD-I, HDMI, DP, and last DVI-D)  < note the 5 types.
Some PCs have only micro or mini HDMI use a HDMI ghost plug to HDMI micro adapter. (choose sizes carefully there are 3,  full size A, mini and micro  but dummy plugs come on full sized only.)

Theory of Operation (  do not read boring electronics?, skip past it
The basics are simple for eDID connections.
First off this is Plug and Play technology, and has 2 decades of evolution now (VESA spec), and can be big problem mixing old devices with new, of any kind.

All modern monitors VGA/DVI/HDMI/DP  have a chip inside called eDID. (old monitors DDC many variants)
The chip as simple serial port that the PC's GPU can talk to the eDID memory. This memory looks like Photo#2
The chip is an  EEprom flashed tiny serial memory chip in most cases.
The data in said chip is simple Plug and Play data so the PC can figure out that you have plugged in, or even hot plugged. (the reason for this is to avoid, black screen errors, or (out of RANGE errors)
The method of transmission Display to GPU port is I2C . (and works even if the monitor is turned off, because the GPU port sends power the chip via a special pin 9 on VGA 5vdc. (and pin 12/15 for clock/data)
DVI video ports, uses pins 6/7/14 for the same trick.(I2C)
The makers of digital DEAD HEAD  , plugs simply  insert there eDID chip inside and bingo, the GPU is fooled into thinking the PC has  monitor attached. It looks only like plug cut off end a monitor cable, but is not.
If curious:
The best way to see your eDID data is the great and free program called Entech Monitor Asset Manager 2.9 MON INFO, seen here for free.   ( clean!)
No better program can I find and I tried many.
It reads  live date from the chip or the registry echo'd image of  it, actually both seen below. ("real" is called Real-time , below and means directly from the monitors eDID chip)
In my case I only wanted to know the DPMS power features of 1 monitor (others use this for projectors that fail and KVM switches that love to fail and block eDID data.)
Many devices (display, and projectors) have wrong data in the eDID, or the GPU driver , fails to send this data correctly to the OS. (even the driver can be bad)
Common problems are sleep mode, fails,  (either way or ;stuck on or off) (or fails to allow best resolution settings) 
The first problem is about dead head servers , PCs used as Servers with no monitor used at all ,  we remote in to the PC to do work. (
The 2nd problem is bum adapters. here:

The correct parameter for PM is byte 24 decimal.
My hex data read is 78h and the first byte only matters, 7 = 011x in binary.
Bit 7     DPMS standby supported  =0  here for standby means I don't have this feature for DPM
Bit 6     DPMS suspend supported   =1  
Bit 5     DPMS active-off supported  =1  (active off)
Bit4 , don't care or unused.

Run Mon Info to see this data. seen here for free.
The address is in hex format as is the raw data. The canned Samples are really nice too.
This is DVI connected.
Photo#2 It's nice to see the Plug and Play raw data below.  This data fits on Chiclet gum sized chip. The same kind of deal works on each ram stick.  All are hard coded at the factory.

Check out the 2nd page below, see those limit there tagged in RED.

The First DEAD HEAD (a.k.a. headless servers)  cure for DVD-D , HDMI and DP ports are below, and require them more complex eDID chip liar plugs (a.k.a, Ghost plug) 
This needs to bought at Ebay, Amazon or Bestbuy.
The upper port seen top below on any GPU card (or iGPU) of any PC is data only and you need a eDID chip in the dead head plug to  fool the GPU card.
NO VIDEO RGB PINS up top. (buy monitors that have below jack)
The 2nd lower port above,  can be eDID fooled easy with 3 resistors, documented here.

This are sold for all types of headless PC's  (I don't cover thunderbolt)
If by chance you have HDMI or DP? (DP means Display port , full sized or mini)
You  must buy  only this kind of a dummy plug matching your video card , if the card has 3 ports, then do it in this order (just 1 port) VGA,DVI,HDMI , DP)
Both DP and HDMI look near same but are not.  (if mini DP they sell those too)
The eDID chip inside lies to the GPU card the GPU driver and works like charm , and the PC.  (about $5)
DP dead head plug (a liar plug) "eDID inside full of good lies"
Keep in mind the GPU card wants to go to sleep if it thinks the monitor is turned off and dead or unplugged, the GPU card goes to a low power mode or near off.(each maker  of GPU card decides this)
This is GREEN Energy STAR rules, for saving earth from CO
Armageddon, yada yada boo.
History short:
  • In the dark ages of PC 1980s we had no way to tell what monitor is there, and could burn up the CRT if set wrong. (dark ages (burned screen ages?)
  • Then came Multi sync monitors (Sony / NEC and others) "and out of RANGE errors" These smarter monitors simple did the right thing if signals were wrong, unlike the junk above from 1980's.
  • Then came the cute crude, jumper pins on the  monitor plug, a VESA crude way of ID (identification) of what  monitor was there. called DDC (version 0) 1994? (the books cost $100 each so to H3LL with VESA !)
  • Then came DD1 to DDC max and up to early eDDC  this began the good era where the plug  and play is digital message, vastly better than any lame jumper strap logic.
  • and last eDDC with eDID chip, used on all DP/HDMI and DVI only monitors. and even pin 11/12/15 or modern VGA monitors. DVD pin6/7 and HDMI pins 15/16, DP pins 15/17 Aux channel and is complex this.
  • There are many folks wanting to force a badly designed, projector to full resolution and we use a liar chip to do that an  eDID chip that tells the PC the truth of what that projector can do. (I will not cover projectors)

version 4.   5-1-2019, short and sweet.  (added adapters to this page, as the eDID failures can make any adapter fail)