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• New read-edid

Old version of read-edid

read-edid elucidates various very useful informations from a conforming PnP monitor; I wrote it to find out the scan line and frequencies of the 85Hz 1024*768 mode of my monitor.

XFree86 4 (but not version 3) integrates this functionality on some video cards, so read-edid isn't needed then.

If you can't find the data to set your monitor up under XFree86, then read-edid is for you; it should auto detect all necessary info and should even provide a few modes with better refresh rates/resolutions than the standard VESA ones. As of now it won't do everything for you; you'll have to edit XF86Config yourself.

If read-edid doesn't work and you're sure it's the latest version, please contact me. Questions, comments, suggestions are all welcome!

The specifications for the systems used by this program are kept by the VESA people, more or less locked up: access is only to members, and membership is prohibitively expensive for most open-source work. The way that I got them was by begging. A public text seems to have sneaked out since then.

Instructions

There are two stages: 1) getting the EDID from the hardware, 2) parsing it.

Simple recipe

1. Download read-edid.tar.gz the source tarball for x86 linux/DOS.
2. Untar.

   % tar zxf read-edid-$VERSION.tar.gz
       

3. Compile.

   % cd read-edid-$VERSION/
   % make
       

4. Try to read the EDID from the hardware under linux. You must be the superuser (root) because this stage interfaces directly with the hardware, a somewhat risky business.

   root# make read-edid
   	

5. If everything ran smoothly, a XF86Config style monitor description should have been printed to stdout. You could put it in your XF86Config or use the information to generate more modelines (see XF86Config(5) and the XFree86-Video-Timings-HOWTO).

Trouble shooting

Technical details

get-edid (linux or MS-DOS version) uses a VESA VBE 2 interrupt service routine request (the VBE/DDC extension, which is only optionally supported by a VBE implementor) to read a 128 byte EDID version 1 structure from your graphics card. The graphics card has (previously?) asked for this data structure from your monitor, via the Data Display Channel. parse-edid parses this data structure and outputs data about the monitor suitable for inclusion into a XF86Config.

parse-edid I think supports EDID version 1 revisions 0, 1 and 2. Version 1.3 is not supported yet. If you get such an EDID, mail me! My monitor doesn't give me an EDID longer than 128 bytes so it doesn't support long EDIDs. If you have one, mail me!

Deciphering broken EDIDs

EDID format data is supposed to start with 0x00, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00, if that's any help. Send your EDID to me if it seems broken, and I'll see what I can do.

Requirements

Your VBE implementor (graphics card manufacturer) must implement the VBE/DDC extension. Your monitor must support DDC (to communicate with the graphics card) and EDID (to have something to say). The get-edid program first checks for this but does not abort if it seems your hardware is inadequate.

Consequent problems

• You might not be able to get the EDID data from your card at all, if it's more than a few years old.
• Some monitor/video card combinations output a valid EDID which basically contains no information, I guess so they can put the DDC logo on the box without actually having to implement it.
• Some monitors give good monitor details but then a bunch of 1x1 video modes; possibly this is a bug in parse-edid.

Error messages

Fixes

• Can't get the EDID data from your video card under linux, or EDID is corrupt.
  1. In this case download the DOS binary of get-edid get-edid-dos.zip. The source is the same as the linux get-edid. The DOS version can be made by compiling with DJGPP.
  2. Unzip get-edid-dos.zip and then

     C:\GET-EDID.EXE EDID-OUT
     	    

     This obviously can't be done under DOSEmu in Linux; you'll need to boot to MS-DOS/MS Windows where the true VESA interrupt interface is available.

  3. GET-EDID.EXE writes the EDID data to the file EDID-OUT.
  4. Boot back to linux and cd to the read-edid directory. Run parse-edid on the EDID-OUT file output.

     % ./parse-edid /mnt/dos/EDID-OUT
     	    

  5. You might now get the proper output.
• If your monitor description (from parse-edid) contains no modelines, all hope is not lost. See Issues below.
• Everyone seems to want a 1024x768 85Hz mode. Here's one from my monitor. Hopefully it won't fry yours (it seriously might) - good luck!

  	Mode "1024x768" 
  		DotClock 94.500000 
  		HTimings 1024 1072 1168 1376 
  		VTimings 768 769 772 808 
  	EndMode

• To stop XFree86 4 from trying to read the EDID from your display (and have it honour your monitor description and modelines instead), add an

     Option "NoDDC"
  	

  line to the Device section in your XF86Config.
• If nothing works, get the EDID (if any) from the monitor. For example, using compiled get-edid, in the read-edid-$VERSION directory:

  root# ./get-edid > edid-file
  	

  Send the "edid-file" to me as an attachment, (with, obviously, a description of the problem and your monitor and graphics card details).

Known hardware problems

Platforms other than x86

get-edid will not compile or work as it interfaces with the x86 BIOS. OTOH on powerpc Linux you can run parse-edid on

/proc/device-tree/pci/{video-card}/EDID

Multiple video cards

People with more than one video card in their system (one card is not connected to a monitor) have reported problems getting the EDID (no-one has been successful, try the support builtin to XFree86 4). There is untested support for cards with more than one monitor, but not for more than one card in the same machine. In fact, the VBE 3 standard says that it is impossible to access a second device.

Specific problematic video card/monitor combinations

	ModelName "MAG DX700T-2"

	HorizSync 30-70
	VertRefresh 50-120

        Identifier "MAG XJ796"
        VendorName "Unknown"
        ModelName "MAG XJ796"
        HorizSync 30-96
        VertRefresh 50-160
# Max dot clock (video bandwidth) 200 MHz
        Mode    "800x600"
                DotClock        56.250000
                HTimings        800 832 896 1048
                VTimings        600 601 604 631
        EndMode
        Mode    "640x480"
                DotClock        72.850000
                HTimings        640 680 752 864
                VTimings        480 481 484 527
        EndMode

Issues

• I'm sure that the image on my monitor is shifted slightly to the right in X; in the same mode in MS-Windows it is centered exactly (because I adjusted my monitor with the OSD to this end).
• Established Timing fields in the EDID data are not parsed. Normally the EDID contains a list of standard modes that the monitor can support. parse-edid does not use this information, because you can presumably generate them (and better ones) with a tool. Try KVideoGen if you have KDE. There is also a program called videogen on sunsite for the console. See also the XFree86-Video-Timings-HOWTO. If I understand correctly, XFree86 4 has the standard modes built in and can be told to use the best one, according to the monitor's capabilities (which parse-edid does output).

Download

New versions of read-edid are at polypux.org. Thanks to the Poly-poly man!

All files are small (<100kB)

• Obsolete read-edid-1.4.1.tar.gz: C source tarball.
• Obsolete get-edid-dos.zip DJGPP compiled binary of above to get the EDID under DOS

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