Gepard – Part Five e): The Hardware: GDC Graphics

The Gepard 80-characters card is able to display also some monochrome graphics, but if you wanted color you had to buy the GDC graphics solution. This solution is rather unique in that it is a modular graphics subsystem centered around the NEC 7220 chip, which is a marvel on its own. But let’s start at the beginning.

A GDC graphics subsystem consists of one Video card and 1 to 8(!) GDC cards. The GDC cards are “daughter-boarded” (or to another GDC card) to the Video card, and only the Video card connects to the Gepard bus, making it a rather bulky PCB block. You can see this on my GDC block consisting of 3 GDC cards on top of the Video card.

IMG_20181203_215534494.jpg

Each Video card cost around 300€ and each GDC card around 500€. This means the maximum configuration cost 4300€, that’s more than an entire (text-capable) Gepard computer…

The Video card has pin header connections for:

  • a monochrome Composite monitor signal (the same as from the 80-characters card)
  • an analog RGB monitor

The GDC card each have a NEC 7220 graphics processor and 128 kB RAM.

If there are more than one GDC card, the cards can share the workload by being responsible only for a part of the color information each.

For example, if you have 8 bits of color information per pixel, you can share it amongst 1, 2, 4 or 8 GDC cards. As a positive side effect you can have higher resolutions as each card has to work less on a single pixel in the restriction imposed by the time a single frame needs to be created (which is given by the time the CRT tube needs to display the single frame).

Depending on the RAM need of a single frame, the number of GDC cards, and some other restrictions, you can have more than a single frame (up to 16) drawn in parallel (then simply switching to another part of the RAM).

You can have up to 256 colors out of a palette of either 2^12 or 2^18 colors (the manual and the price list differ in this point).

The highest achievable resolution seems to be 1024×102 pixels.

It is not possible for me to list here all possible resolutions, color depths, and number of parallel screens as this depends on:

  • the number of GDC cards
  • the type of monitor interface used
  • the jumper configuration on the GDC cards

Also, not all combinations are possible.

The NEC 7220 chip is probably the first integrated “GPU” chip. It was released in 1982 and had the following features:

  • DMA-capable
  • own memory control
  • own command set: lines, circles, arcs, rectangles, characters
  • can address up to 512 kb RAM
  • max. resoution: 2048×2048 pixels
  • can use a lightpen directly

Its importance can also be seen from the fact that Intel licensed the design and produced it under the Intel 82720 designation. Which in turn triggered the Soviet block U82720 clone.

This graphics architecture has the advantage of being

  • very scalable
  • easy to incrementally extend

but also to be

  • very expensive (in 1985, my 3-GDC-card block is more expensive than an entire Atari ST system a year later on)

Therefore, GDC graphics seem to be rather scarce. Out of the 6 Gepards whose configuration I know only mine has GDC graphics, the rest has 80-characters cards.

Also, there are hardly any other computer models based on NEC 7220 graphics that have more than one such chip. The DEC Rainbow 100 (graphics option), Epson QX-10, QX-16, Tulip System-1, EC 1834, A7150, as well as the Number Nine Revolution 512 family of graphics cards all had one NEC 7220 (or equivalent) chip. Only the NEC PC-9801 and the NEC APC family had two, supported by 256 kB RAM.

In later years, GDC cards had 512 kB RAM each.

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One Response to “Gepard – Part Five e): The Hardware: GDC Graphics”

  1. Gepard – An early German 68000 hobbyist workstation | Rare & Old Computers Says:

    […] part 5: GDC Graphics […]

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