Model 30 Video Interface Schematic (page 7 physical)
Model 25 Technical Reference (Video Subsystem - page 49 physical)
The MCGA "Multi-Color Graphics Array" subsystem is present on the planar
board of the original Models 30 and 25. It was never made available in form of
an adapter card. The only known "clone" that implemented MCGA was Epson Equity
Feature-wise it's a superset of the
older CGA "Graphics Adapter" standard, but it uses analog video output
(VGA compatible) and adds two new video modes:
- 320 x 200 graphics (APA) with 256 colors
- 640 x 480 graphics (APA) with 2 colors
Note: MCGA does NOT provide
compatibility with EGA "Enhanced Graphics Adapter".
The MCGA subsystem consists of:
- Video Memory Controller gate array (72X8300)
- Video Formatter gate array (72X8205)
- 64 KB of multi-port video DRAM
- 8 KB of SRAM for character generator
- 256 x 18-bit color palette with three 6-bit DACs (Inmos IMSG171 RAMDAC)
- Support ("glue") logic
MCGA Block Diagram
Video Memory Controller
P/N: 72X8300 (old) or 11F8028 (new)
Gate Array: Seiko-Epson SLA6430J
Die Photo (21808x21778px! 154 MB) online zoomable version
Courtesy of John McMaster.
Gate Array: Seiko-Epson SLA6330J or IBM GA (early samples only?)
The Video Memory Controller gate array responds to I/O addresses 304h and 305h.
The Video Formatter gate array responds to I/O addresses 308h through 30Fh.
The color palette is programmed through the video formatter at addresses 3C6h through 3C9h.
See the Technical Reference
(page 57 physical) for more information.
MCGA and VGA Design Teams
I was working in Boca in 1985-1987 .. happy times. ... MCGA
and VGA were two design teams working in different buildings, separated by a
parking lot but shared a cafeteria. One interesting feature was MCGA used a
VRAM design, which was a little more sophisticated than VGA's plain ol' DRAM