The teletext standard VDU 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
bit in the ASCII code stored at that location is set. The bit assignments are:
Bit 5 in the byte is always set for a graphics character, if bit 5 is clear then the upper case, alphabet and characters ¬, ½, ®, (up arrow), #, @ 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 | Are options for using the circuit to superimpose text onto a normal TV picture. | ||||||
| DOUBLE HEIGHT | 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 character 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 . | ||||||
| BLACK BACKGROUND | Resets the background colour to black. | ||||||
| HOLD GRAPHICS | Causes the last graphics character to be displayed when a control character is entered. | ||||||
| RELEASE GRAPHICS | 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.
The characters produced by the VDU and their decimal codes are tabulated here:-
32 space 64 @ 96 - 33 ! 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 ¼ 60 < 92 ½ 124 || 61 = 93 ® 125 ¾ 62 > 94 126 ¸ 63 ? 95 #
Symbol 94 is "up arrow"