STEbus Advanced Colour Graphics

Based on the Hitachi HD63484 ACRTC

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The controller chip included a microprocessor and firmware so it could perform graphic operations by itself. The board used many PAL chips as address multiplexers and programmable-length shift registers. All clocked very fast, they got pretty warm.
Designed by Mike Spinks, and described in his book.

The SG84 is a fast, high resolution colour graphics board. It contains an advanced display controller and a powerful graphics-optimised processor in a single chip, the Hitachi 63484 ACRTC. 

The graphics processor uses hardware and microcode to perform very-high-speed pixel operations, including lines, arcs, ellipses, filling, and block transfers.

The display controller provides an upper, lower, and middle base screen, one window, and zooming, directly from hardware. The cursor can be a block-cursor, or up to two cross-hairs, as chosen by software.

The resolution is 640 by 400, in either 16 or 256 logical colours, from a palette of 262144 physical colours. This allows a range of special effects such as instant colour changing, or producing ‘false colour' images. Colours can also be made to flash. The SG84 supports two frame access modes. In single access mode, user can choose between the flicker-free display priority mode, where drawing is stopped during the display period, or the faster drawing priority mode where drawing cycles may occur during display periods. In dual-access mode, display and drawing cycles are allowing fast drawing at any time without disturbing the display. This is only available to the displays. The 256K of on-board memory provides either one 256-colour screen, or up to two 16-colour screens. Any memory not used for display may hold text fonts, icons, windows etc.

The SG84 provides and analogue level video signals with selectable sync polarity. Genlock capability will allow the display to be synchronised with an external video source.

Power consumption:

(from STEbus): 0.8A typical at 5V


  • STEbus: slave interface occupying 6 I/O locations within a 16-byte boundary.
  • Fast pixel manipulation
  • 640 × 400 resolution
  • 256 Kbyte RAM
  • 16 or 256 logical colours
  • palette of 1262144 physical colours
  • RAM based text font
  • Monochrome output option
  • DMA down STEbus

SG84 manual in html form

Zip Archive - 5.08 MB - 07/12/2021 at 13:09


HD63485: Memory Interface HD63486: Video Attribute Controller HD63487: the two chips above combined into one This board used PAL chips to replace these chips.

Zip Archive - 3.16 MB - 02/14/2021 at 02:21



This is the nearest equivalent open-source design, by Steve Ciarcia for Byte magazine. It uses the HD63485 and HD63486 chips which are probably hard to get, but they make for easier design. They were expensive back then, which was why they were designed out of the SG84.

Adobe Portable Document Format - 1.51 MB - 01/04/2022 at 21:25



Part 2 is about hardware. There is 512K of DRAM, as sixteen 64K by 4-bit dynamic RAM chips, giving a 64-bit word output. 32 bits into each of the two GVAC chips.

Adobe Portable Document Format - 7.05 MB - 01/04/2022 at 21:25



Part 3 is about software.

Adobe Portable Document Format - 7.95 MB - 01/04/2022 at 21:25


  • Similar board, from Control Universal catalogue 1987

    Keith10/30/2021 at 13:18 0 comments

    Celeste/484 High Resolution Colour Graphics


    • Hitachi HD63484 Advanced Graphics Controller
    • 512 KBytes on-board video memory
    • 32 MHz INMOS G170 colour palette option
    • Powerful on-board graphics primitives
    • Resolution up to 1600 pixels by 1300 lines
    • Display rate up to 32 million pixels/sec
    • Fast plotting of complex lines, curves and area fill
    • Multiple screens and windows
    • Hardware zoom up to 16 times
    • Memory accessible from bus under DMA
    • Any number of windows and/or character fonts can be stored in non-displayed video memory

    Celeste/484 is the powerful graphics processor card of the Control Universal range of STE cards. The Hitachi 63484 controller has three processors, one to manage the screen display, the second to manage all CRT timing functions, and the third to perform all the drawing operations. Commands supported directly via simple parameters include circle, ellipse, polyline, arc, paint, etc. A choice of screen access modes allows either full resolution (by drawing only during flyback) or higher drawing speeds by interleaving screen display and drawing times. Up to 1600 pixels can be displayed horizontally with standard 15.75 kHz line rate monitors. Other resolutions are obtainable under software control or by changing the pixel clock frequency.

    Version A has the INMOS colour palette allowing a choice of 256 colours from 262,144. Selected colours may be flashed at variable rates. Different palettes may be loaded from the host CPU. Output drives analogue or TTL monitors.

    Version B has no colour palette and drives TTL monitors only.

    External Connectors:

    Version A: 5 SMB 'subclic' coaxial connectors for analogue monitors. 
    Version B: 9-way standard mini "D" type connector 
               providing RGBI and Sync for TTL monitors.

    Board Dimensions: 100 × 160mm to STE IEEE P1000 specification.

    Board Power Consumption: +5V, typically < 1000mA

    Celeste/484A - With INMOS colour palette	SE485AA		£765
    Celeste/484B - No INMOS colour palette		SE484AA		£595

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Keith wrote 05/04/2022 at 20:10 point

I think it was a purely graphic system. There were already many text based boards around. The SG84 software draws characters from a stroke font. As for scrolling, this is fast because you simply update a register that selects the start of the displayed data. You can also have a window within a window.

  Are you sure? yes | no

Marten Feldtmann wrote 05/04/2022 at 19:28 point

The reason I ask is, that the performance of character drawing is to that much. Scrolling and so own is pretty slow. So the best solution would be to have a combined character display and graphics display triggered by the 63484 - so one has a fast character terminal and the possibility to have graphics commands

  Are you sure? yes | no

Marten Feldtmann wrote 05/04/2022 at 19:26 point

As I noticed, you did not connect the 63484 CHR signal ?

  Are you sure? yes | no

Marten Feldtmann wrote 05/04/2022 at 19:19 point

I have a little question - do you know how to realize the character display together with the 63485 or 63487 ?

  Are you sure? yes | no

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