mrb's blog

The 5-second VGA Dummy Plug

Keywords: amd gpu hack hardware nvidia

VGA dummy plugs were made popular by folders. They are electronic devices used to fake the presence of a monitor attached to a VGA or DVI output port so that the graphics driver can be forced to initialize the corresponding GPU in order to be able to run GPGPU programs on it —Folding@Home being one of the most popular. A dummy plug works by simulating a resistive load of 50-150 Ω on the analog red, green and blue VGA signals. Many people build pretty complex dummy plugs, simulating load on all 3 signals, soldering resistors, and encasing them in VGA connectors.

I prefer a much simpler technique: simulating load on one signal, without soldering, or any extra hardware. I simply insert the resistor —75 Ω is the ideal spec— in the DVI output port to make contact between C1-C5 (analog red - analog ground), or C2-C5 (green - ground), or C3-C5 (blue - ground).

[Update 2011-07-08: added schematic]. The photo below demonstrates a resistor in C2-C5. I have tested the method successfully with ATI HD 4850 and ATI HD 4850 X2 cards.

Simple resistor used as a VGA dummy plug

It made sense for drivers to not enable a GPU when no monitor was attached to it. But since the rise of GPGPU applications, graphics vendors AMD and NVIDIA realized that this behavior was inconvenient. At least with the most recent Linux drivers, dummy plugs are not necessary anymore. [Update 2011-02-22: However Windows still requires dummy plugs, as of February 2011.]

Comments

Brad wrote: This is awesome!! THANK YOU!!! 21 Jul 2011 18:53 UTC

Keng Vang wrote: is that still a 75 ohm resistor? 05 Aug 2011 10:55 UTC

mrb wrote: No, the one in the picture is 100Ω. 06 Aug 2011 01:32 UTC

anon wrote: I just wanted to let you know that a 68 ohm resistor works fine as I tested it on a Nvidia Geforce 8400GS, and a GTX 280. 27 Jan 2012 01:38 UTC

sam wrote: works like a charm on macmini with debian :-) 15 Sep 2012 10:37 UTC

Benedikt wrote: Even works with a simple paper clip. The folding/mining Software sees the card now, MSI Afterburner and Open Hardware Monitor don't see the card.

Do these tools (or other overclocking tools) work on your system?

Thank you for posting your idea!
31 Mar 2013 15:33 UTC

Allan C. K. Kipfer wrote: HI, cai use 94 Ooms(2 x 47 Oms in series)
to do this? just weld 2 in series, if u say that shoud work with my HD7950 XFX i will use it, i think it should work, cause u set between 50-150 Oms so 94 is little above the ideal, but way far for 150 :D so i think is 'almost perfect'

also, do i need to do this on my 2HD7950?
3 of those on each card? i'm using my iGPU for browsing, and wanna use this method for mining while still use cpu for browsing/light games aka Point Blank ¬¬

also with this method, i will be able to overclock my HD7950?
06 May 2013 03:10 UTC

SimplyFun wrote: Be forewarned. As well as this method works, if you short the resistor you are basically frying your card ultimately rendering it as an expensive paper weight. If you utilize another similar method using VGA/DVI adapter plugs, the only thing you'll fry is the plug itself if you accidentally short it. 09 May 2013 12:25 UTC

mrb wrote: SimplyFun: that is not true. "Shorting" a resistor (eg. replacing it with a paper clip) actually still makes the card work. The current and voltage flowing through the analog lines is not high enough to cause damages. 11 May 2013 04:11 UTC

Qnfauf wrote: This worked great. This is much better than using "dummy plugs." 24 May 2013 04:26 UTC

DRU wrote: Hi!
I try to solve a Problem i have with a dvi-d dual link connector on my 7750.
i try to use it for bc.mining. because of my small case i have to use a low Profile graphics Card. i cant use the vga connector for my Monitor.

do you have a solution for this dvi-d dual link connector?

sry for my bad english, i am from Germany........
24 May 2013 06:16 UTC

devman77 wrote: Thank you for sharing this valuable information!
It works on my 7850, but only when I've plugged all resistors — C1-C5 and C2-C5 and C3-C5 ! ( 3 x 82 Ω )
17 Dec 2013 12:48 UTC

mysterylectricity wrote: While it's probably true you can't hurt the output driver transistor(s) by shorting them to ground (they should have a 75 ohm internal impedance) , common sense dictates that the resistor(s) used shouldn't look exactly like the one in the photo. Stick the long lead into the grounding bar and keep both leads as short as possible: just long enough so the short lead bottoms out just prior to the resistor smacking the connector: maybe about 1/4". 29 Jan 2014 22:14 UTC

msyterylectricity wrote: .. less likely to fall out or be brushed out. Trying to think of a good glue that would help ruggedize it but also snap off the connector cleanly when you upgrade/sell your video card... maybe a small spheroid of hot melt before the glue gets very hot (and starts to flow into the holes), I don't know because I don't use hot melt very often. 29 Jan 2014 22:22 UTC

CrayCrayCrypto wrote: I had 220 ohms and found while card will run with 220 but with no access to Catalyst drivers for OC'ing. So put two in parallel thus 110 ohms and got the performance tab back thanks! 19 Feb 2014 19:15 UTC

Max wrote: For me it worked completely without any resistors. I used a DVI-VGA Adapter and connected at the VGA-End the Pins 1 6, 2 7 and 3 8 with some short thin wires. 01 Apr 2016 12:14 UTC

cjm2477 wrote: THE PICTURE IS A DVI CABLE, RETARD 12 Sep 2016 16:44 UTC

mrb wrote: cjm2477: watch your language or I will ban you from my blog.

Firstly, it is not a DVI cable but a DVI port. Secondly, "VGA dummy plug" is a term generally used to describe a dummy plug that plugs into any graphics port regardless of type: VGA, DVI, etc.
12 Sep 2016 19:57 UTC

husncarter wrote: How can make like this for DVI-D please help me.
husncarter23@gmail.com
28 Feb 2017 20:17 UTC

mrb wrote: husncarter: I am not sure how to design this for DVI-D... Maybe a 1K Ω pull-up resistor on the hot plug detect pin? http://philtechnicalblog.blogspot.com/2013/03/fixed-dvi-pin-16-hotplug-dilema.html 11 Mar 2017 22:48 UTC