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• Got video - Sort of :/

  Dave • 04/30/2026 at 11:51 • 0 comments

  So I have managed to get something on the screen. Shame it's a bit random. Hsync and Vsync are just fine. The image should be alternating bars of green and black 8 pixels wide. But after a reset, I get something different each time. Probably got something wired upside down, causing corrupted dot and colour data. Oh well, time to dig out the logic analyser and DSO.
  And in some good news, someone was kind enough to lend me his original Excalibur 64 PCB!!! This particular one is rev 5, the last version. There are some reported errors still, but nothing major. This would be great for verifying the wiring for the video. Having dodgy schematics (has crossed wires and missing labels) and a dodgy technician is not a good mix, so actual hardware will be a great boost for this project.
  I'll be creating a new project for the original board as I will be recreating the PCB, then populating the original and returning it to its owner.
  My plan of attack with the borrowed board involves:
  

  • Visual Inspection: Cross-referencing trace paths with my messy schematics to solve the mystery labelled lines.
  • Video Probing: Using the DSO to capture working waveforms from the functional dot and colour data lines.
  • Logic Analysis: Comparing the data stream states before and after a hard reset to pinpoint where my own system is corrupting the signal.
  • Layout Mapping: Documenting the physical layout thoroughly so I can accurately reproduce the PCB artwork before returning the borrowed board.

• Video board 2 of 2

  Dave • 04/22/2026 at 05:51 • 0 comments

  Now that the CRT board (housing the CRTC, SRAM, and character ROM) is finished, it’s time to serialise the dot and colour data. This next stage involves the "Colour, Video & RF Circuit" board. Its job is to convert parallel data into EGA-style signals, composite video, and RF.

  While the final build will include an RF modulator, I’m leaving it out for now. I haven't been able to find a new one yet, so I may have to salvage one from a C64. An authentic, static-filled picture will have to wait!

  The board features a 15-pin D-sub connector with HSYNC, VSYNC, and two pins for each RGB channel. Although I haven't found official documentation, it’s a safe assumption that this is EGA (specifically 6-bit 'Enhanced' RGB). My plan is to hook this up to a GBS-8200 (EGA-to-VGA converter) first, as getting the composite video working adds a layer of complexity I’d like to avoid for now.

  • Clocking: At the bottom of the board, a 74157 divides the main board’s clock frequency and switches between low and high resolution.
  • Serialisation: A shift register handles the pixel data serialisation, while an octal buffer drives the 6331 Colour PROM.  The 6331 acts as a Colour Look-Up Table (CLUT). Since these are one-time programmable and no longer available, I’ve replaced it with an ATF16V8 (GAL).
  • Timing: A 17.734 MHz master clock and 74S04 inverters form a Pierce oscillator for video timing. This provides the high-frequency "dot clock" and is divided down to create the precise PAL colour burst signal. This frequency is exactly four times the PAL subcarrier (4.43361875 MHz).
  • Logic: The top row of ICs contains 74156 dual 2-to-4 decoders/demultiplexers with open-collector outputs. These handle the colour mixing, functioning as a DIY Digital-to-Analogue Converter (DAC). They take digital data from the EGA lines (Ra, Rb, Ga, Gb, Ba, Bb) and decode them into specific voltage levels. A resistor ladder is connected to their outputs to create the various voltages representing different colours for the composite and luminance signals.

  Installing that ladder was a massive pain—so many different values to keep track of! But finally, the signal passes through an amplifier and buffer stage before exiting via a dodgy-looking RCA connector (I accidentally chose the wrong footprint on the PCB 🫤). So what’s Next?

  Next up is testing the EGA output. I didn't actually need to install half of the components for the EGA to work, but the board looked so bare without them! In the next update, we’ll see what happens when I run it through the GBS-8200. I’ll just turn it on and squint my eyes a little—I'm sure it’ll be fine.

• Video board 1 of 2

  Dave • 04/17/2026 at 06:48 • 0 comments

  CRT board done, this contains the MC6548 CRTC, SRAM and Character ROM. Provides the dot data and colour data to the video_RF board, which is next.

• New (old) chips

  Dave • 04/16/2026 at 05:35 • 0 comments

  Just got a bunch of ICs from a fellow Hackadyer ( is that the right term?). He sent me 8251 UART, 8253 TIMER, 8255 PIO,  Z80 DMA, 6522 VIA, AY-50-1015 UART, Z80 DART and 6845 CRTC. Thanks :) and will test and put to good use.

• Side quest - run some programs using MAME

  Dave • 04/11/2026 at 04:20 • 0 comments

  While this project is about the recreation I have added some screengrabs using MAME. These are all images loaded up with MSBASIC. The booty one is particularly interesting as it has colour. There more but some are too rude, even at 320x200 monochrome.

• Progress updates

  Dave • 04/05/2026 at 12:24 • 0 comments

  Currently it has working, audio and  serial and parallel ports. It has a monitor ROM so I can do basic tests and more importantly transfer test programs without having to burn the EEPROM all the time. I have since learnt about the OneROM project on GitHub would have used that if I knew earlier. Oh well I now have some ready for the next project.

  Also  in this picture is a real Excalibur 64 keyboard kindly donated by an owner of a couple of machines and had and extra keyboard. 

  Next step is getting the video going. This is going to be the hardest bit, so have split the video into the processing side which includes the 6845 CRTC, SRAM and other bits to process the colour and dot data. A second board will take this data and serialise it for RGB ( I think its a form of EGA ? ), composite video and maybe even RF. Probably steal a RF modulator from a old C64 board for that.

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