How to Build Your Own EGA to VGA Converter Circuit
If you have an old computer that outputs EGA graphics, you might want to connect it to a modern VGA monitor. EGA stands for Enhanced Graphics Adapter, and it was a standard for 16-color graphics at a resolution of 640x350 pixels. VGA stands for Video Graphics Array, and it is a standard for 256-color graphics at a resolution of 640x480 pixels or higher.
However, you cannot simply use a cable to connect an EGA output to a VGA input. There are two main problems that need to be solved: the horizontal frequency mismatch and the TTL to analog signal conversion.
Horizontal Frequency Mismatch
The horizontal frequency is the rate at which the electron beam scans across the screen horizontally. EGA uses two different horizontal frequencies: 15.7 kHz for low-resolution modes and 21.8 kHz for high-resolution modes. VGA uses a fixed horizontal frequency of 31.4 kHz. If you connect an EGA output to a VGA input, the monitor will not be able to sync with the signal and will display a distorted or blank image.
One solution is to use a monitor that can handle different horizontal frequencies, such as a NEC MultiSync 1970NX[^1^]. This is a somewhat cheap LCD monitor from the early 2000s that can accept a wide range of frequencies, including EGA's frequencies. However, not every multisync monitor can do this, so you need to check the specifications before buying one.
Another solution is to use a circuit that can convert the horizontal frequency from EGA to VGA. This is more complicated and requires some electronic components and skills. You can find some examples of such circuits online[^2^], but they are not very common or easy to build.
TTL to Analog Signal Conversion
The other problem is that EGA uses TTL (transistor-transistor logic) signals for its RGB (red-green-blue) output, while VGA uses analog signals. TTL signals are either high (5V) or low (0V), while analog signals can have any voltage between 0V and 0.7V. If you connect an EGA output to a VGA input, the monitor will not be able to display the correct colors and will show only black, white, or shades of gray.
The solution is to use a circuit that can convert TTL signals to analog signals using resistors. The basic idea is to use two resistors for each color: one connected to the LSB (least significant bit) pin and one connected to the MSB (most significant bit) pin of the EGA output. The LSB pin determines whether the color is on or off, while the MSB pin determines whether the color is bright or dark. By combining these two signals with different resistances, you can create four levels of intensity for each color: 0%, 33%, 67%, and 100%. Then you connect the output of the resistors to the corresponding color pin of the VGA input.
Here is a schematic of how to do this for one color (red):
You need to repeat this circuit for green and blue as well. You also need to connect the ground pins and the sync pins of both connectors. Here are the pinouts of EGA and VGA connectors for reference:
You can use any values for the resistors as long as they follow this ratio: R1 = 2 * R2. For example, you can use 1kΩ for R1 and 470Ω for R2[^3^]. You can also use a breadboard or a project box to make the circuit more neat and stable.
By using either a multisync monitor or a frequency converter circuit, and by using a TTL to analog converter circuit, you can build your own EGA to VGA converter and enjoy your retro computer on a modern display.