The MC6847 is a video display generator (VDG) first introduced by
Motorola
Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorol ...
and used in the
TRS-80 Color Computer
The RadioShack TRS-80 Color Computer, later marketed as the Tandy Color Computer and sometimes nicknamed the CoCo, is a line of home computers developed and sold by Tandy Corporation. Despite sharing a name with the earlier TRS-80, the Color Com ...
,
Dragon 32/64
The Dragon 32 and Dragon 64 are home computers that were built in the 1980s. The Dragons are very similar to the TRS-80 Color Computer, and were produced for the European market by Dragon Data, Ltd., initially in Swansea, Wales before m ...
,
Laser 200,
TRS-80 MC-10/
Matra Alice
The Matra & Hachette Ordinateur Alice is a home computer sold in France beginning in 1983. It was a clone of the TRS-80 MC-10, produced through a collaboration between Matra and Hachette in France and Tandy Corporation in the United States.
T ...
,
NEC PC-6000 series,
Acorn Atom
The Acorn Atom is a home computer made by Acorn Computers Ltd from 1980 to 1982, when it was replaced by the BBC Micro. The Micro began life as an upgrade to the Atom, originally known as the Proton.
The Atom was a progression of the MOS Techn ...
, and the
APF Imagination Machine, among others. It is a relatively simple display generator compared to other display chips of the time. It is capable of displaying alphanumeric text,
semigraphics
Text-based semigraphics or pseudographics is a primitive method used in early text mode video hardware to emulate raster graphics without having to implement the logic for such a display mode.
There are two different ways to accomplish the emu ...
and raster graphics contained within a roughly square display matrix 256 pixels wide by 192 lines high.
The ROM includes a 5 x 7 pixel font, compatible with 6-bit
ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
. Effects such as
inverse video
Reverse video (or invert video or inverse video or reverse screen) is a computer display technique whereby the background and text color values are inverted. On older computers, displays were usually designed to display text on a black backgroun ...
or colored text (green on dark green; orange on dark orange) are possible.
It is capable of displaying nine colors: black, green, yellow, blue, red, buff (almost-but-not-quite white), cyan, magenta, and orange (two extra colors, dark green and dark orange, are only possible as backgrounds for alphanumeric text modes). According to the MC6847 datasheet, the colors are formed by the combination of three signals:
with 6 possible levels,
(or
with 3 possible levels) and
(or
with 3 possible levels), based on the
YPbPr
YPbPr or Y'PbPr, also written as , is a color space used in video electronics, in particular in reference to component video cables. YPBPR is gamma corrected YCBCR color space (it is not analog YUV that was used for analog TV, though component ...
colorspace
A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog or a digital represen ...
, and then converted for output into a
NTSC
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
analog signal.
The low display resolution is a necessity of using television sets as display monitors. Making the display wider risked cutting off characters due to
overscan. Compressing more dots into the display window would easily exceed the resolution of the television and be useless.
Signal levels and color palette
The chip outputs a NTSC-compatible progressive scan signal composed of one field of 262 lines 60 times per second.
According to the MC6847 datasheet,
colors are formed by the combination of three signals:
luminance,
chroma and
chroma, according to the
YPbPr
YPbPr or Y'PbPr, also written as , is a color space used in video electronics, in particular in reference to component video cables. YPBPR is gamma corrected YCBCR color space (it is not analog YUV that was used for analog TV, though component ...
color space. These signals can drive a TV directly, or be used with a NTSC modulator (Motorola MC1372) for RF output.
may assume one of these voltages: "Black" = 0.72V; "White Low" = 0.65V; "White Medium" = 0.54V; "White High" = 0.42V.
(or
) and
(or
) may be: "Output Low" = 1.0V; "R" = 1.5V; "Input High" = 2.0V.
The following table shows the signal values used:
Notes:
1) The colors shown are adjusted for maximum brightness and only approximate (different color spaces are used on TV - BT601 and web pages - sRGB).
2) At least on the Color Computer
The RadioShack TRS-80 Color Computer, later marketed as the Tandy Color Computer and sometimes nicknamed the CoCo, is a line of home computers developed and sold by Tandy Corporation. Despite sharing a name with the earlier TRS-80, the Color Com ...
1 and 2, the alternate palette of text modes (actually the text portion of semigraphic
Text-based semigraphics or pseudographics is a primitive method used in early text mode video hardware to emulate raster graphics without having to implement the logic for such a display mode.
There are two different ways to accomplish the emu ...
modes) was dark pink (or dark red) on light pink, of shades not listed here (and no dark orange), whereas the Color Computer 3, with a different chip, made it dark orange on orange.
The first eight colors of this table were numbered 0 to 7 in the upper bits of the character set (when bit 7 was set, bits 4-6 represented the color number), but ColorBASIC's numbering was 1 higher than that in text mode, as it used 0 for black.
Video modes
Possible MC6847 video display modes:
Character generator
The built-in
character generator ROM offers 64
ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
characters with 5x7 pixels. Characters can be green or orange, on dark green or orange background, with a possible "invert" attribute (dark character on a bright background).
An updated version of the chip (MC6847T1) was capable of generating lowercase characters.
See also
*
Motorola 6845, video address generator
*
Thomson EF9345
The EF9345 from SGS-Thomson Microelectronics, Inc., was a semigraphic microprocessor for video image control, encapsulated in a 40-pin DIP and used primarily in the Matra Alice 32, Matra Alice 90 and Philips VG5000 microcomputers.
The EF9345 was ...
*
TMS9918
VDP TMS9918A
VDP TMS9918A
VDP TMS9928A
The TMS9918 is a video display controller (VDC) manufactured by Texas Instruments, in manuals referenced as 'Video Display Processor' (VDP) and introduced in 1979. The TMS9918 and its variants were used ...
*
MOS Technology VIC-II
The VIC-II (Video Interface Chip II), specifically known as the MOS Technology 6567/8562/8564 (NTSC versions), 6569/8565/8566 (PAL), is the microchip tasked with generating Y/C video signals (combined to composite video in the RF modulator) and D ...
*
List of home computers by video hardware
This is a list of home computers, sorted alphanumerically, which lists all relevant details of their video hardware.
Home computers are the second generation of desktop computers, entering the market in 1977 and becoming common during the 1980s. ...
References
{{reflist
Graphics chips
6847