Video Display Generator - Revision history

Barberd at 19:55, 14 April 2026

2026-04-14T19:55:17Z

← Older revision		Revision as of 15:55, 14 April 2026
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	~~{{Merge\|Motorola 6847}}~~

	===VDG===		===VDG===
	[[Image:Coco2boot.png\|thumb\|320px\|Power-on screen of a CoCo 2]]		[[Image:Coco2boot.png\|thumb\|320px\|Power-on screen of a CoCo 2]]

Barberd: /* VDG */

2026-04-14T19:52:58Z

VDG

← Older revision		Revision as of 15:52, 14 April 2026
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	[[Image:Coco2boot.png\|thumb\|320px\|Power-on screen of a CoCo 2]]		[[Image:Coco2boot.png\|thumb\|320px\|Power-on screen of a CoCo 2]]
	The MC6847 is display generator capable of displaying text and graphics contained within a roughly square display matrix 256 pixels wide by 192 lines high. It can display 9 colors: black, green, yellow, blue, red, buff (almost-but-not-quite white), cyan, magenta, and orange.		The MC6847 is display generator capable of displaying text and graphics contained within a roughly square display matrix 256 pixels wide by 192 lines high. It can display 9 colors: black, green, yellow, blue, red, buff (almost-but-not-quite white), cyan, magenta, and orange.

			It reads data from memory (via the [[SAM]] chip) and displays the contents on screen. The 6847 is used on Coco 1 and 2's only; this function was replaced by the custom [[GIME]] chip in Coco 3 models.

	It is also used in the following computers of the time: [[MC-10]], [[Dragon]], [[Laser 200]] and [[Acorn Atom]].		It is also used in the following computers of the time: [[MC-10]], [[Dragon]], [[Laser 200]] and [[Acorn Atom]].

Barberd: /* See Also */

2026-04-14T19:51:49Z

Barberd at 19:51, 14 April 2026

2026-04-14T19:51:00Z

← Older revision		Revision as of 15:51, 14 April 2026
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	~~{{Merge\|VDG}}~~

	{{Merge\|Motorola 6847}}		{{Merge\|Motorola 6847}}

Barberd: /* See Also */

2026-04-14T19:49:30Z

Barberd at 19:49, 14 April 2026

2026-04-14T19:49:02Z

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 [[Image:Coco2boot.png|thumb|320px|Power-on screen of a CoCo 2]]
 The MC6847 is display generator capable of displaying text and graphics contained within a roughly square display matrix 256 pixels wide by 192 lines high. It can display 9 colors: black, green, yellow, blue, red, buff (almost-but-not-quite white), cyan, magenta, and orange.
 ====Alphanumeric/Semigraphics display====
@@ Line 49: / Line 51: @@
 The 6847T1 may also carry the part number XC80652P; these may have been pre-release parts.
 ==See Also==

Carney: /* Artifact colors */

2025-02-06T18:33:20Z

Artifact colors

← Older revision		Revision as of 14:33, 6 February 2025
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	====Artifact colors====		====Artifact colors====
	The 256×192 two color graphics mode uses four colors due to a quirk in the NTSC television system (see [[~~composite~~ artifact colors]]).		The 256×192 two color graphics mode uses four colors due to a quirk in the NTSC television system (see [[artifact colors]]).
	It is not possible to reliably display 256 dots across the screen due to the limitations of the NTSC signal and the phase relationship between the VDG clock and colorburst frequency.		It is not possible to reliably display 256 dots across the screen due to the limitations of the NTSC signal and the phase relationship between the VDG clock and colorburst frequency.

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	Readers of ''[[~~The Rainbow (magazine)\|~~The Rainbow]]'' or ''~~Hot~~ CoCo'' magazine learned that they could use some POKE commands to switch the 6847 VDG into one of the artifact modes, while Extended Color Basic continued to operate as though it were still displaying one of the 128×192 four-color modes. Thus, the entire set of Extended Color Basic graphics commands could be used with the artifact colors. Some users went on to develop a set of 16 artifact colors{{how\|date=October 2012}} using a 4×2 pixel matrix, giving this set of colors: black, dark cyan, brick red, light violet, dark blue, azure (the blue above), olive green, brown, purple, light blue, orange, yellow, light gray, blue-white, pink-white, and white. Use of POKE commands also made these colors available to the graphics commands, although the colors had to be drawn one horizontal line at a time. Some interesting artworks were produced from these effects, especially since the CoCo Max art package provided them in its palette of colors.		Readers of ''[[The Rainbow]]'' or ''[[HOT CoCo]]'' magazine learned that they could use some POKE commands to switch the 6847 VDG into one of the artifact modes, while Extended Color Basic continued to operate as though it were still displaying one of the 128×192 four-color modes. Thus, the entire set of Extended Color Basic graphics commands could be used with the artifact colors. Some users went on to develop a set of 16 artifact colors{{how\|date=October 2012}} using a 4×2 pixel matrix, giving this set of colors: black, dark cyan, brick red, light violet, dark blue, azure (the blue above), olive green, brown, purple, light blue, orange, yellow, light gray, blue-white, pink-white, and white. Use of POKE commands also made these colors available to the graphics commands, although the colors had to be drawn one horizontal line at a time. Some interesting artworks were produced from these effects, especially since the CoCo Max art package provided them in its palette of colors.

	====Lower case and the 6847T1====		====Lower case and the 6847T1====

Carney: pages down, switched to archive versions

2024-03-01T21:34:19Z

pages down, switched to archive versions

← Older revision		Revision as of 17:34, 1 March 2024
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	The alphanumeric display has two colorsets. The one used by default on the CoCo has black characters on a green background. The alternate has black characters on an orange background. The colorset selection does not affect semigraphics characters. The border in this mode is always black.		The alphanumeric display has two colorsets. The one used by default on the CoCo has black characters on a green background. The alternate has black characters on an orange background. The colorset selection does not affect semigraphics characters. The border in this mode is always black.

	The 6847 is capable of a Semigraphics 6 display mode, where two bits select a color and 6 bits determine which 1/6 of the character box is lit. In this mode only 4 colors are possible but the Colorset bit of the VDG can select two different groups of the 4 colors. [http://www.cs.unc.edu/~yakowenk/coco/text/semigraphics.html Due to a peculiarity of its hardware, only two colors are available in graphics blocks when using Semigraphics 6 on the CoCo].		The 6847 is capable of a Semigraphics 6 display mode, where two bits select a color and 6 bits determine which 1/6 of the character box is lit. In this mode only 4 colors are possible but the Colorset bit of the VDG can select two different groups of the 4 colors. [https://web.archive.org/web/20210214054301/http://www.cs.unc.edu/~yakowenk/coco/text/semigraphics.html Due to a peculiarity of its hardware, only two colors are available in graphics blocks when using Semigraphics 6 on the CoCo (archived)].

	====Additional Semigraphics modes====		====Additional Semigraphics modes====
	By setting the SAM such that it believes it is displaying a full graphics mode, but leaving the VDG in Alphanumeric/Semigraphics 4 mode, it is possible to subdivide the character box into smaller pieces. This creates the "virtual" modes Semigraphics 8, 12, and 24.<ref>[http://www.lomont.org/Software/Misc/CoCo/Lomont_CoCoHardware.pdf Chris Lomont's Color Computer 1/2/3 Hardware Programming]</ref> In these modes it was possible to mix bits and pieces of different text characters as well as Semigraphics 4 characters. These modes were an interesting curiosity but not widely used, as the Semigraphics 24-screen consumed 6144 bytes of memory. These modes were not implemented on the CoCo 3.		By setting the SAM such that it believes it is displaying a full graphics mode, but leaving the VDG in Alphanumeric/Semigraphics 4 mode, it is possible to subdivide the character box into smaller pieces. This creates the "virtual" modes Semigraphics 8, 12, and 24.<ref>[https://web.archive.org/web/20220703160425/http://www.lomont.org/Software/Misc/CoCo/Lomont_CoCoHardware.pdf Chris Lomont's Color Computer 1/2/3 Hardware Programming (archived)]</ref> In these modes it was possible to mix bits and pieces of different text characters as well as Semigraphics 4 characters. These modes were an interesting curiosity but not widely used, as the Semigraphics 24-screen consumed 6144 bytes of memory. These modes were not implemented on the CoCo 3.

	A programmer's reference manual for the CoCo states that due to a fire at Tandy's research lab, the papers relating to the semigraphics modes were shuffled, and so some of the semigraphics modes were never documented. CoCo enthusiasts created experimental programs to try to reverse engineer the modes, and were able to reconstruct the missing documentation.<ref>[http://home.att.net/~robert.gault/Coco/History/Semi24.htm Semigraphics24 for the Coco1&2. Machine language program to create 8 true colors plus text on screen at one time.]</ref>{{Citation needed\|date=January 2008}}		A programmer's reference manual for the CoCo states that due to a fire at Tandy's research lab, the papers relating to the semigraphics modes were shuffled, and so some of the semigraphics modes were never documented. CoCo enthusiasts created experimental programs to try to reverse engineer the modes, and were able to reconstruct the missing documentation.<ref>[https://web.archive.org/web/20090707132337/http://home.att.net/~robert.gault/Coco/History/Semi24.htm Semigraphics24 for the Coco1&2. Machine language program to create 8 true colors plus text on screen at one time. (archived)]</ref>{{Citation needed\|date=January 2008}}

	====Graphics display====		====Graphics display====

MagicFab: /* Lower case and the 6847T1 */ fixed the image

2022-07-24T11:31:20Z

Lower case and the 6847T1: fixed the image

← Older revision		Revision as of 07:31, 24 July 2022
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	====Lower case and the 6847T1====		====Lower case and the 6847T1====
	[[Image:~~Coco2bvdg lc~~.png\|thumb\|320px\|Sample character set display of 6847T1 VDG in true lowercase mode]]		[[Image:Coco2bvdg_lc.png\|thumb\|320px\|Sample character set display of 6847T1 VDG in true lowercase mode]]
	The 6847 is capable of using an external character generator. Several third party add-on adapter boards would allow the CoCo to display real lowercase characters.		The 6847 is capable of using an external character generator. Several third party add-on adapter boards would allow the CoCo to display real lowercase characters.

Diego bf109 at 14:23, 14 April 2020

2020-04-14T14:23:16Z

← Older revision		Revision as of 10:23, 14 April 2020
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			{{Merge\|VDG}}

			{{Merge\|Motorola 6847}}

	===VDG===		===VDG===
	[[Image:Coco2boot.png\|thumb\|320px\|Power-on screen of a CoCo 2]]		[[Image:Coco2boot.png\|thumb\|320px\|Power-on screen of a CoCo 2]]

← Older revision		Revision as of 15:51, 14 April 2026
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	==See Also==		==See Also==
	*[[Motorola 6847]]		*Wikipedia article on the [http://en.wikipedia.org/wiki/Motorola_6847 Motorola 6847]
			*Datasheet http://instruct1.cit.cornell.edu/courses/ee476/ideas/mc6847.pdf

	[[Category:Hardware]]		[[Category:Hardware]]

← Older revision		Revision as of 15:49, 14 April 2026
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	==See Also==		==See Also==
	*[[VDG]]
	*[[Motorola 6847]]		*[[Motorola 6847]]

	[[Category:Hardware]]		[[Category:Hardware]]