Chapter 8	Di spl ay  Uni t


	The teletext standard V.D.U. in the system produces coloured alphanumerics and graphics as well as some other features in response to some control codes.  These codes are listed in decimal as follows:-
	129	Red	alphanumerics	145	Red Graphics
	130	Green	"	146	Green  "
	131	Yellow	"	147	Yellow "
	132	Blue	"	148	Blue   "
	133	Magenta	"	149	Magenta "
	134	Cyan	"	150	Cyan   "
	135	White	"	151	White  "
	136	Flash		152	Conceal display
	137	Steady		153	Contiguous Graphics
	138	End Box		154	Separated Graphics
	139	Start Box
	140	Normal	Height	156	Black background
	141	Double	Height	157	New background
				158	Hold graphics
				159	Release graphics



These characters affect the characters displayed to the right of them on a line, and provide the following options:-


ALPHA (COLOUR)	causes following characters on the line to be in the colour specified.


GRAPHICS (COLOUR)	In Graphics mode each character space displayed is divided into 6 cells.  Each cell is illuminated, in the specified colour, if a corresponding binary bit in the code stored at that location is set.  The bit assignments are:-




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Bit 5 in the byte is always set for a graphics character, if bit 5 is clear then the upper case, alphabet and characters

		,	,   ,   ,# , @ are available in the same colour.


FLASH	causes the following characters on the line to flash.


STEADY
terminates the flash option on a line


END BOX & START BOX



DOUBLE HEIGHT

Are options for using the circuit to superimpose text onto a normal TV picture.


Must appear on two consecutive lines followed by the same characters in which case the characters fill the two lines specified.


NORMAL HEIGHT	Clears the double height option on a line.


CONCEAL DISPLAY	Switches off the characters on a line. These can only be revealed by applying a signal to the character generator chip (or deleting the CONCEAL character).


CONTIGUOUS GRAPHICS	In this mode the graphics cells in a character are joined up.


SEPARATED GRAPHICS	In this mode the graphics cells are separated by one character bit width.


NEW BACKGROUND	Sets the background to the colour of the last colour specifying character.


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BLACK BACKGROUND HOLD GRAPHICS




RELEASE GRAPHICS

Resets the back ground colour to black.


Causes the last graphics character to be displayed when a control character is entered.


Causes a space (Background colour) to be displayed when a control character is entered.



The character generator assumes an initial setting at the start of each line of:ALPHA WHITE, STEADY, END BOX, NORMAL HEIGHT.

CONTIGUOUS GRAPHICS, BLACK BACKGROUND, RELEASE GRAPHICS.






























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	The characters produced by the V.D.U. and their decimal codes are tabulated here:-
	32	space	64	@	96
		4@9633!	65	A	97	a
	34	''	66	B	98	b
	35	£	67	C	99	c
	36	$	68	D	100	d
	37	%	69	E	101	e
	38	&	70	F	102	f
	39	'	71	G	103	g
	40	(	72	H	104	h
	41	)	73	I	105	i
	42	*	74	J	106	j
	43	+	75	K	107	k
	44		76	L	108	l
	45		77	M	109	m
	46		78	N	110	n
	47	/	79	0	111	0
	48	0	80	P	112	p
	44	1	81	Q	113	q
	50	2	82	R	114	r
	51	3	83	S	115	s
	52	4	84	T	116	t
	53	5	85	U	117	u
	54	6	86	V	118	v
	55	7	87	W	119	w
	56	8	88	X	120	x
	57	9	89	Y	121	y
	58		90	Z	122	z
	59	;	91	<-	123	l
						/
						4
	60	<	92	_	124	11
	61		93	->	125	3
						/
						4
	62	>	94		126	/
	63	?	95	!=









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