New design for an 82720-based colour graphics generator

Abstract

A low-cost Multibus-compatible graphics generator has been designed for a resolution of 512 × 512 pixels with a 3-bit colour code per pixel. The heart of the system is an 82720 graphics display controller (GDC). Usual designs of colour graphics generators use three different memory planes, each containing information corresponding to one of the three primary colours of the cathode ray tube (CRT). However, this approach requires a large PCB area and a large number of memories and transceivers. Further, the design does not use the full speed of present-day dynamic RAMs, as the three planes are read in parallel during display. In the scheme presented in this paper, the memory is partitioned into segments, three of which store graphics information corresponding to three colours. The three segments are read sequentially within each GDC read cycle, using the full speed of the dynamic RAMs. Moreover, memory segmentation reduces the chip count and PCB area considerably. The system can produce a character display of 40 rows with 64 characters in each row, and can magnify any part of the display by a factor of up to 16.