About ten years ago I came across a simple DIY screen line generator on the Internet. The idea of “ruining” a picture on an LCD monitor for the sake of hard nostalgia seemed funny to me and I wanted to try such a device in practice. However, at that time I was busy with other problems and put off the implementation until later and eventually forgot about it. A month ago, I again remembered my old plans and I found time to implement them.
Now such a simple device can be bought on Aliexpress, but I don’t like having any external modules connected to the computer and with manual switches on them. Therefore, I developed my own scanliner, connected to the VGA Feature Connector and with the ability to turn it on/off programmatically.

In comparison with the option available on Aliexpress, there are the following differences:
The possibility of selecting even/odd lines is excluded, since I don’t know why this is even needed.
As unnecessary, the need to select the polarity of the sync pulses depending on the resolution is eliminated. Any resolution available to the video card is supported.
Support for doubling scan line thickness has been removed. In my opinion, the scan line generator makes sense only in simulating CGA and EGA monitor modes; you can also use VGA modes 320x200 and 320x240. With other resolutions, loss of information in the image is inevitable. Double line thickness spoils all modes.
In order to simplify the device and reduce its size, the ability to adjust the brightness of blocked lines is excluded. The brightness of the lines depends on the brightness of the replaced pixels, the color changes from black to gray.
Added the ability to programmatically turn it on/off using the COM port.
It is possible to manufacture scanliner in the form of an external module.

Particular care was taken during design to ensure that the scanliner does not introduce significant capacitive loading on the signal lines in order to avoid any noticeable blurring in the OFF state. The capacitive load is determined mainly by the junction capacitance of Schottky diodes at a reverse applied voltage of 3.5V. I used the BAT48 Schottky diodes I have; in this case the load is about 12 pF. If you use other diodes (necessarily Schottky) you can achieve a lower capacitance. However, 12 pF is a fairly small value. I conducted tests on the OTI087 card at the maximum available resolution of 1200x1024, which has excellent image quality, at least for this class of video cards, live by connecting and disconnecting the scanliner and did not find any noticeable blur. Scanliner is powered by lines BLANK, VSINK, HSINK and consumes no more than 100 µA.
The version with connection to VFC requires a small, non-destructive and easily removable modification of the video card. It is necessary to connect the pins unused in VFC with three wires to the DAC outputs before the output filters. In this case, as a rule, the minimum length of wires is achieved. In the case of SMD filters, this is not easy to do; then you can connect the wires to the pins of the VGA connector on the back of the video card.

I did not make a version with an external module, but the performance of the solution was tested on a breadboard.
If anyone wants to replicate the VFC version, I can provide the file in Sprint-Layout 6.0 format. However, the PCB is designed to be made at home and may need to be completely redesigned.

For program control, a simple non-resident slg.com utility has been developed, running in the DOS environment. The utility accepts two parameters: E for enable scan lines and D for disable scan lines. The COM2 port is initially used, but can be configured to other ports using a hex editor. The utility is located in the slg.zip archive.

I apologize for the low quality of the photo, what I can’t do is what I can’t do.
If you have any questions, ask.

To be continued.