I am trying to use THIS schematic to use TMS9918 wideo chip with SRAM memory in one of my projects. It is based on 8080, but it doesn't matter because it it will work, I am also going to use it with my ongoing 6502 project.
Unfortunately I encountered some weird issue I can't debug myself.

1. There is communication between my system nad TMS9918. I can write and read to and from its wideo memory. I can also perform operation on control registers.
2. Chip generate image and reacts to register writes. I was able to initialize it in text mode, the it showed some gibberish (content of uninitialized memory). i am also able to change colors by writing to REG7.
3. I copied character patterns definitions to wideo memory (0x08000) and then I wrote simple basic loop, which was filling name table (0x0800) with incremented values from 0x00 to 0xFF. It should have resulted with complete sets of characters in order on some parts of a screen and gibberish (uninitialized part of pattern table) on the rest. Unfortunately it is not what I got.
As you can see on attached image, there are groups of eight proper characters, followed by gibberish.

Initially I thought I've swapped some address lines, but then checked my circuit with schematic, using multimeter. Everything looks fine.
Any ideas what can be the cause of this weird behavior?

Swapped data lines?

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
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Thanks for suggestion. Of course I will check it, but honestly - I don't think so.
In that case character definitions should also be scrambled - they are transmitted to the vram by the same data bus. However they look perfectly normal.

Hey, someone is using my repo!

One goofball thing I remember about the TMS9918 is that TI reversed the data lines. Meaning, D7 on the TMS9918 is the same as D0 on everything else in the world.

I assume you wired the databus up correctly?

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Cat; the other white meat.

I used the very same schematic to connect the 9918 and SRAM together and it has been working for a long time now.

I agree it looks like you have managed to write to the 9918 registers properly to set up the display mode - being able to change colour in R7 further confirms that.

But I had problems when getting the 9918 up and running on my set up - some things to try / theories based on my issues..

1) The 9918 responds quicker to register read/write than memory read/write. I had a situation where I was also able to set up the screen mode and colours but still getting a lot of corruption on the screen. Turns out I wasn't giving enough time between issuing write commands. That meant sometimes I would get repeated characters, which granted is not exactly what you are seeing, but can you check how many uS you are waiting between successive writes? Even at 1.34Mhz on a 65c02 I can write faster to the 9918 than it will accept, so I started with a very generous amount of NOP instructions between writes until it worked and trimmed from there (I have since increased the 65c02 clock to 5.36Mhz and with extra NOPs it is working reliably).

2) After setting up read/write memory pointer for a particular VRAM address, successive memory accesses auto increment it which is faster than having to initialise the pointer each time. However, this can be disturbed by anything other than a read (or write - depending on which memory mode you are in). I was having all sorts of weird glitches until I realised that I was setting the 9918 up to issue vertical blank interrupts and then in my interrupt routine, reading the 9918 status register, which took it out of read/write mode. In fact this could occur anywhere where I was sending more than 1 byte to the 9918. The solution of course is to disable interrupts (or don't have interrupts running at all).

I don't think you have swapped data or address lines else you would not be able to even initialise the 9918 to a screen mode reliably... hopefully it's something related to the above and you sort it out, feels like you're pretty close..