Prestel Protocol

Prestel (or Videotex) is a simple terminal protocol, in use in parts of Europe for more than 10 years.
The protocol provides a basic (non-ASCII) character set and some low-resolution graphic symbols. Characters can have seven different colors. The terminal background color is black and can not be changed. However different character background colors can be used.

Screen Size

The width of a Prestel screen is 40 columns, the height is 24 rows.
Originally, characters were designed in a 10*6 point raster.

Character Sets

In different countries, different character sets are in use. Shown below is the British version.
In each language, symbols from three character sets can be combined in one screen. The G₀ set contains normal characters, while there are two different graphic character sets.
Characters are sent to the Prestel terminal as 7-bits values, ranging from 32 to 127.

Remark: even these days, many Prestel hosts use 7-bits even parity communication.


G₀ Character Set	Contiguous Graphics Set	Separated Graphics Set

Remark: the character sets are basically the same ones as used in the Teletext system.

Display Attributes

Display attributes are sent as the Escape character (value 27) followed by a normal character in the range 'A' to '_' (values 65 to 95).
On reception of a display attribute, the cursor position is incremented. So a display attribute itself takes up one position on the screen. Normally it is displayed as a space (in the current colors). When HOLD GRAPHICS mode is active, the previous character is displayed again. Display attributes are effective until a contrary attribute is received, and are only valid on the current row
Following attributes are defined (only the values following the Escape are shown).

Value	Mode
65	G0 character set; red foreground color
66	G0 character set; green foreground color
67	G0 character set; yellow foreground color
68	G0 character set; blue foreground color
69	G0 character set; magenta foreground color
70	G0 character set; cyan foreground color
71	G0 character set; white foreground color
81	Graphics character set; red foreground color
82	Graphics character set; green foreground color
83	Graphics character set; yellow foreground color
84	Graphics character set; blue foreground color
85	Graphics character set; magenta foreground color
86	Graphics character set; cyan foreground color
87	Graphics character set; white foreground color

93	Set background color to foreground color
92	Black background

77	DOUBLE HEIGHT (ignoring characters in next row at same positions
76	NORMAL HEIGHT

72	FLASH
73	STEADY

72	CONCEAL DISPLAY
any	VISIBLE DISPLAY

94	HOLD GRAPHICS (repeat previous character if display attributes is received)
95	RELEASE GRAPHICS

90	SEPARATED DISPLAY (if appropriate, use separated graphics set variant)
89	CONTIGUOUS DISPLAY (if appropriate, use contiguous graphics set variant)

Remark 1: default display attributes are shown in bold.
Remark 2: almost the same attributes are used in Teletext systems. The leading Escape character is not used. The values are offset by -64.

Control Characters

Prestel terminals use a cursor position to define where the next received character should be placed on the screen. The cursor position can be made visible (as a steady rectangle) and invisible by using control characters. Other control characters are used to move the cursor.
Control characters are sent as values between 0 and 31. The following control characters are defined.

Value	Description
8	Move cursor 1 position to the left (go to last column of previous row if already at the left most position)
9	Move cursor 1 position to the right (go to first column of next row if already at the right most position)
10	Move cursor 1 position down (go to first row (same column) if already at the bottom position)
11	Move cursor 1 position up (go to first last (same column) if already at the top position)
12	Clear the screen; move cursor to the top left corner
13	Move cursor to first column (same row)
17	Show cursor
20	Hide cursor
30	Move cursor to the top left corner

Remark: on reception of a normal character or display attribute, the cursor skips to the first column of the next row, if it was at the last column. If it was at the last row too, it will skip to the first row (and first column). This means the screen contents can not scroll.

Control Characters

Extensions have been defined in addition to the original Prestel specifications.
Two display attributes were added to allow the use of a black foreground color.

Value	Mode
64	G0 character set; black foreground color
80	Graphics character set; black foreground color

Further more, an 80 column mode was introduced in which the characters are only half their normal width.
One way of swithing between 40 and 80 columns is by the use of two newly defined display attributes.

Value	Mode
54	80 COLUMN
55	40 COLUMN

I know of two more character sequences that are in use by some Prestel hosts. [Who knows the details?]

Sequence	Mode
31 95 67 112	80 COLUMN
31 95 65 112	40 COLUMN

I beleive these sequences are used in combination with sequences that allow switching between different terminal types [who can confirm this?].

Sequence	Terminal type
27 37 68 96 64	Unknown
27 37 68 97 64	Teletel (Minitel)
27 37 68 98 64	Prestel
27 37 68 104 64	Unknown
27 37 68 105 64	Unknown
27 37 68 126 126 64	ASCII

1996-10-10