Dear reader,
you're likely already familiar with the GBS8200 - a video scaler device somewhat popular in the retro arcade scene. Popular enough that there are hardware mods plenty, including a complete brain transplant (see GBS-control by ramapcsx2). Reader, can you imagine what a disappointment it was to find out, that for all this popularity, no proper schematics have ever surfaced? It might be that I'm just that incompetent at search engines, but, be as it may, holding the scaler board in hand, it occured to me: it's not all too fine-pitched. No BGA, all components on the same side...perhaps, no more than two layers, even? I figured I could probably trace it out with nothing more than a multimeter. And....I think I may just have succeeded.
The GBS8200 feeds off of any 5 V to 12 V power supply. Internally, it obtains 5 V from the input voltage by the means of a XL1583 buck converter (IC11), and then drops that down to 1,8 V by a small LDO (IC5).
The board has several inputs, allowing for RGBHV, RGBS and maybe even RGsB video in the typical VGA connector, or over some JST-like connector. A 3xRCA input is also provided for YPbPr video signals. The RGB levels can be adjusted by trimpots, and there are some other resistors sprinkled about, probably busy with impedance matching or something else. I hope that you might have more of a clue in these matters than I do.
Not much of note hides there - another DE15 VGA output. The RGB signals are routed through a switch (IC4) to permit OSD to peek through. Sync is buffered by IC2.
Where the magic happens - a Tvia TrueView 5725 scaler chip paired with what pretends to be a Samsung K4S643232H SDRAM chip. Mine says "Solution 2X32Y3VTM". Finding a datasheet for that was hopeless, but thankfully, the internet has no shortage of pictures of older (read - less cost optimized) GBS8200 boards. The TrueView 5725 is much more capable than the limitations of the GBS8200 make it seem, but don't take my word for it. Somewhere in this repo you will find the datasheet for this chip, the manufactuer will do a far better job of selling it than I ever could. (Nota bene: I am fairly certain that this datasheet was already sometwhere on the public web, if it's not supposed to be republished like this, please let me know)
The MTV230GMV, which is, with any luck, a close relative of the MTV230M. The datasheets of the latter one float freely around the internet, the former, not so much (id est, I found none). It's an 8051 based microcontroller with some TV-related peripherals onboard. Not much to write home about. It's duty onboard the GBS8200 is reading the buttons, bossing the scaler around over I2C, and generating the OSD. Due to its lacking capabilities and general reclusivity (no readily available dev environment), users have taken to silencing it by forcing it into permanent reset state, and ceding the control to an ESP32 planted atop in a somewhat grotesque manner, not unlike a cordyceps mushroom taking over an insect.
There are pads for three transistors near the VGA output. What are they for? One end is connected to +5 V, the other one to ground. The remaining one goes straight to the video signal. What's that supposed to acheive?
A pin on the controller is connected to VB3 on the scaler chip, a bit in the digital video I/O. All other digital video I/O are, as far as I can tell, unconnected. What is to be made of that?
The I2C lines connect not to the I2C pins on the controller, but two, seemingly random GPIO pins. Does the MTV230GMV have a different pinout from the MTV230? Or are they just, for some reason, bitbanging it?
For the life of me I cannot find capacitors C61 and C62 on the board. C60, then right over to C63.
The schematic is still somewhat rough. It's drawn in a way that roughly resembles the physical layout of the components, which is not neccessarily a good practice, but easier to manage when tracing out a board.
To be honest - I am not at all familiar with the buses and hierarchial features in KiCAD (or other EDA suites for that matter). It is entirely possible that the schematic is completely broken (or for that mattter, inaccurate).
A potential continuation of this project would be dumping the firmware of the MCU and the contents of the EEPROM chip, and/or recreating the PCB itself, too.
What do all of these points have in common? I'm not planning to do any of them, this thing has been living in my brain without paying rent for long enough. I severely need to touch grass instead.
As of writing this, no license has been selected, simply because I have no clue which one would be approperiate. Help would be much appreciated. There are also several datasheets included in the repo, which I have nothing to do with. If hosting them in my repo like this could pose any legal problems, please let me know.
