Motorola 6847

Lore & Background

The MC6847 was designed for use with television sets as display monitors, with its low resolution necessitated by overscan and the limited resolution of consumer TVs. The chip outputs a progressive scan NTSC signal of 262 lines at 60 Hz. Colors are formed from Y (luminance) and two chroma signals (φA and φB) based on the YPbPr color space, with Y having six possible levels and each chroma signal having three levels. The hardware palette comprises twelve colors: black, green, yellow, blue, red, buff (light gray), cyan, magenta, orange, plus dark green, dark orange, and light orange used in text modes.

The built-in character generator ROM contains 64 ASCII characters in a 5×7 pixel matrix, organized as 64 columns of 35 bits each. Characters could be displayed in green or orange on dark green or dark orange backgrounds, with an invert attribute. Motorola offered customers the option to order the MC6847 with a custom masked ROM pattern, provided on MCM2708 or MCM2716 PROMs or on a floppy disk. The MC6847 also supported an external character ROM, as demonstrated by the Dragon 200-E which used a daughterboard with a 2532 EPROM. The updated MC6847T1 variant included a 96-character ROM with lowercase letters.

Reader's Guide

The MC6847 was notable for its simplicity and direct NTSC output, enabling inexpensive home computers to generate color graphics and text on standard televisions. Its 256×192 resolution and 12-color palette were typical for the era, though the chip's text modes offered only two color pairs (green on dark green, orange on dark orange) plus an inverse video effect. The chip's design reflected the constraints of 1970s television technology, where overscan and limited resolution prevented wider or denser displays. The MC6847's use in multiple systems—from the TRS-80 Color Computer to the Gakken Compact Vision TV Boy—demonstrated its versatility as a low-cost video solution. The availability of interlaced and non-interlaced variants, as well as custom ROM options, allowed manufacturers to tailor the chip to specific needs. Its legacy is tied to the early home computer market, where it provided a functional but limited graphics capability that influenced later video display generators.