Thunderbolt/Mini displayport requires a VGA connection to work???

[juquinha]

Hi!

So, i got a strange situation here. I have an old Asus ROG G751JM laptop connected to an old Samsung monitor (Model S20C301L) through a VGA connection.
This monitor supports both DVI-D and VGA inputs, while my laptop can output through VGA, HDMI or Thunderbolt/Mini DIsplayport. I wanted to upgrade to digital signal, so i bought an Thunderbolt/Mini DIsplayport to DVI cable.

At first, it was a nightmare. There was absolutely nothing i could do to get it to work. I spent hours troubleshooting, but the monitor just could not detect any signal, only VGA one. But then this happens.
I was testing with only one connection at once and i got tired of plugging and unplugging the cables every time. So i had the idea to keep both cables plugged in and change the source detection in monitor's menu when i wanted to. So, with the TB/MiniDP cable already connected, i've then connected the VGA cable (while DVI was selected as input source). The display started flashing. After 2 seconds, voilà, it was working. And i was like, "what the f***???" I've then unplugged the VGA cable, signal lost. Plugged it again, got digital signal.
Well, i'm happy that it's working now, but i simply don't understand what's happening here. Why is that i need both connections simultaneously to make it work? WIll that still happen if i use a HDMI to DVI cable?
Fun fact: the image displays only after windows startup. So, if i want to enter BIOS setup for example, i need to switch back to VGA as input source.

Sorry for my english.

 

Update: Can't run games with It, i get bsod.

[Michael]

Doubt it's the monitors fault, pretty sure it's some sort of issue with the Asus. You on the latest bios and video drivers? 

[juquinha]

Sadly i don't have any other devices to test where the problem is. But i make the same bet as you, the monitor is fine, the issue is with the laptop. And yes, everything is up to date.

[juquinha]

Update: Manage to run games with it. I found out that the problem causing it was an useless Thunderbolt config app that i installed while troubleshooting.
BUT, it's awful... I have massive amount of stuttering and significantly less fps compared to VGA only.

[juquinha]

Bought a HDMI to DVI cable. No sucess...

Some kind of signal is being detected by the monitor, but it tells me to "check" it. No image is displayed this time, even with VGA cable connected. And windows won't recognize the digital connection with the monitor as well.

[Bulvaui]

I tbought thunderbt and the mini diaplay are 2 different things