Why VGA to HDMI adapters fail with DOS text mode

Overview

Planning to connect your retro PC to a modern HDMI monitor? You might hit a wall when booting into DOS. In this video, I explain why many VGA-to-HDMI adapters show “No Signal” when displaying DOS 80x25 text mode (720x400 @ 70Hz), and which video cards can work around this limitation by using 640x480 instead.

Key Moments

• Understanding VGA text mode: 720x400 vs 640x480 resolution differences
• Testing multiple VGA-to-HDMI adapters with different retro PCs
• ATI Rage XL using 720x400 (doesn’t work with adapters)
• S3 UniChrome using 640x480 (works perfectly with adapters)
• Why encoding chips in adapters reject non-standard resolutions
• Practical workarounds and recommendations

Full Transcript (Edited)

Hello everybody. So today I want to show you some compatibility issues that I’m seeing with some of these VGA to HDMI adapters. These are some little adapters that you can get on Amazon that let you plug in a modern monitor with an HDMI cable to the VGA connector of an older computer.

If you’re not familiar with it, VGA is the standard connection that computers from the ’90s until very recently—until maybe 10 years ago—have been using to connect to monitors. So that’s an analog signal. It’s got RGB in it. These adapters allow you to convert the analog signal to a digital signal that can then be sent to a modern LCD monitor. These are USB powered.

So let me show you a little bit about it. On this end you see that you have the VGA plug, and that connects to the computer. In the back you have the HDMI output that goes to your monitor with an HDMI cable. Then usually you’ll see a USB port on one side of the unit or in the rear like in this one. That is for the circuitry inside of here to be powered up. Then you’ll also see a 3.5 millimeter headphone jack as well, and what this allows you to do is you can get the audio out of the computer. This will mix in the audio into the HDMI digital stream so that the speakers on your monitor or on your HDTV will play the sounds coming out of your computer.

So that’s awesome if you want to have a retro machine that is connected to your TV. If you’ve seen these, you need to get something that is powered, right? This has active circuitry in there that does the encoding of the analog signal to a digital stream. If you see some of these that don’t have any power or anything like that, they’re likely designed to work with a specific device and not a general-purpose device like this that can plug into any VGA source and encode it to a digital HDMI stream.

I have this one. I like it, but it has a right-angle power connector. So when you’re getting this, make sure that you understand how the wires come out of it. Because you can see here, if I plug this one into that, the USB cable here will interfere with the power plug. It will work, it will fit in there, but you risk damaging things.

The other ones that I have here are a little bit better for the setup that I’m going for. This is the output where the HDMI cable plugs into, and then it has the same setup—it has a USB power input and audio input here. On the other side now I have a little bit more freedom, more space around the connector, so I can just plug that into the VGA port and it doesn’t interfere with any other cable because all the bulky connections are on the other side here.

All right, so now that you are familiar with the HDMI adapter itself, I want to show you something. I want to show you a couple of machines and I want to test this out. I want to show you some behavior that I want you to be aware of, which is: this won’t work with every computer in every scenario. Let me explain what that means.

These older computers, if you have an old 386 or an old 486 that you want to use with your HDTV or your monitor and you want to use one of these adapters, you need to understand that these adapters generally are going to work with very standard resolutions like 640x480 or 1024x768, 800x600. But the moment you start feeding it different resolutions like I’ll show you here—this is the text mode that this particular ATI Rage XL wants to use, and this is 720x400 at 70 hertz—that will not work with these. You’ll get a blank screen. I’ll show you that in a minute.

Let me go ahead and stop here, hook up the HDMI adapter to my HDMI monitor here, and I’ll show you what I mean.

Okay, so I’ve gone ahead and set up this adapter with my little portable HDMI monitor right here. I plugged in the VGA into the computer and I’m going to turn on the power right now. I’m going to show you what happens. In a moment now we will see the splash screen because that is being shown with a standard VGA resolution. But as soon as that is shown and we switch to text mode, it’s not going to show anything. It says “No Signal.”

But I’m going to go ahead and press Enter on the keyboard here because I know that it’s displaying a startup screen that I have set up for this computer. Let me switch over to VGA on my standard LCD monitor. As you can see there, I have it. It was showing text. This is 720x400 still on this UI, so even that doesn’t show up.

But I’m going to go ahead and plug in the HDMI again, the HDMI adapter. And as soon as I make it to the desktop, see, nothing is going to show yet. But as soon as I make it to the desktop and this switches to 1024x768, I’m going to see the desktop image right here. And that is because these devices must be only compatible with more standard VGA resolutions.

Text mode on a lot of these devices is sent in 720 by 400, which these encoders don’t like. So that’s unfortunate because a lot of machines do use that resolution for text mode. You can see here the adapter does work really well when the resolution is a standard VGA resolution. But if you ever bought one of these things and it doesn’t work for you, it might be that you’re sending in text mode from your 386, 486, Pentium PC, and you’re blaming this device rightly because it is not doing the encoding. But you’re scratching your head—why? Why does it work with a standard monitor and not with this?

Well, now you know. The encoding system that these things have doesn’t support every possible resolution that would come out of a VGA port. You might definitely need to use something much more advanced and probably more costly if you want to capture every possible resolution. I’ve seen other YouTubers using the RetroTink or other devices that are a little bit more advanced and more purpose-built to capture all the timings, all the resolutions, and everything that comes out of the VGA port regardless of what’s being sent. They’re a lot more forgiving. This one just says “No Signal.” So it’s something to keep in mind there.

Okay, so now I’m going to show you that some computers actually use a standard VGA resolution for text mode, and in those cases you will be able to see text using these devices. Let me show you that in a second.

Okay, so now I have another computer here. This is another little thin client, and this one has an S3 UniChrome, one of these embedded GPUs that is usually only found on these thin clients or VIA chipset motherboards. And this one actually shoots out—I’ll show you right now. Look, I’m powering it up with the HDMI adapter, and any moment now I will see the output of the screen. And check it out—I’m seeing text! Why is that? Why does this one work and this one doesn’t?

Well, I’m going to go ahead and plug in my video monitor so I can inspect what resolution is this thing sending out. All right, so there you go. That one is showing the text now. And if I go here—ah, this GPU decides to use 640x480 at 60 Hz, which is a more standard, typical VGA resolution that everybody’s familiar with. And that’s probably why this device works perfectly fine with that.

Whoever implemented the encoding algorithms in this ASIC, in the chip inside of this, decided to only support certain more standard 4:3 ratio or other types of VGA resolutions that they consider to be part of a standard list—like 640x480, 800x600, 1024x768, all these resolutions that when you think of VGA, you think of. That’s probably what they built into this chip. And at the moment you send something that it doesn’t recognize, it just gives up and says, “Well, I can’t do anything with this because this is not part of my approved list of resolutions.”

So yeah, that’s unfortunate, but it’s something to keep in mind. If you have a machine that is sending out some kind of text mode and you’re hoping to use one of these adapters, try to see if your monitor will give you the information that you need to determine whether you’re going to actually have a good experience or not. Because if you see a weird resolution when you’re displaying text—weird in the sense that it’s not like 640x480 or 800x600—you know, because text is coming out at 720x400 in many of the older machines, so it’s not weird for that, but it is weird for this. It doesn’t expect that resolution.

So yeah, I just thought that was an interesting thing because I was actually scratching my head. I thought I had some kind of broken adapters or some were better than others because I randomly ran into this issue and I didn’t spend the time to actually see if it was the computer or adapter that was causing the weirdness that I was seeing. But now I see that the adapters are all kind of the same. They probably all use the same internals, and the machines are the ones that are deciding to either use a standard VGA resolution to send out text like this.

This chipset must be doing its own encoding of text onto VGA resolution. So this one must be doing something non-standard because the standard I think is this, which is 720x400, and this one is doing something non-standard which happens to work with this device.

So yeah, just be careful out there if you spend some money on one of these adapters. Be sure that you can return it in case things don’t work. All right, well, I hope you found this interesting and useful and it saved you some money and time. If you like this kind of content, please subscribe and like the video, share it. I’m trying to grow this channel up and provide some more useful content like this that might help people out there. Let me know in the comments what you think about this, if you’ve run into this, if you found this useful. And I’ll see you next time. All right, have a good one!