this post was submitted on 19 Apr 2024
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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If you have an unused VGA port, you can use the DDC pins for I2C. Be sure to add ESD protection if you do this. An I2C isolator would be even better.
I2C is really not meant to be used over cables. It has a very limited common mode input voltage range and it can't handle much capacitance on the bus.
Except that in the case of VGA (and DVI, HDMI, and DisplayPort) the i2c interface is intended for use over the cable. All of those ports have a pair of i2c pins and corresponding wires in their cables. The i2c interface is used for DDC/EDID which is how the computer can identify the capabilities and specifications of the attached display. DDC even provides some rarely-used control functionality. Probably the most useful of which is being able to control the brightness of the display from software. I use the ddcci module on Linux and it lets me control my desktop monitor brightness the same way a laptop would, which is great. I have no idea why this isn't widely used.
Edit:
This i2c interface is widely used to control the lighting on modern graphics cards that have RGB lighting. We've spent a lot of time reverse engineering these chips and their i2c protocols for OpenRGB. GPU chips usually have more i2c buses than the cards have display connectors, so the RGB chip is wired to one of the unused buses. I think AMD GPUs tend to have 8 separate i2c buses but most cards only use 4 or 5 of them for display connectors. There is also an i2c interface present on RAM slots normally used for reading the SPD chip that stores RAM module specifications, timings, etc. This interface is also used for RAM modules with controllable RGB lighting.