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Revision as of 19:58, 6 November 2017 by Casandro (talk | contribs) (Technical Details)
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Description We'll bring Bildschirmtext back to live as a more distributed and more playable system
Has website
Persons working on
Tags retrotechnology, onlineservices, retrocomputing
Other projects...


What is Bildschirmtext?

In the 1970s and 1980s advances in microelectronics made it possible to build fairly cheap terminals connected to the telephone network. Since those terminals were either dedicated machines or modules integrated into your TV set, they didn't have to deal with the low resolution of NTSC and therefore provide high resolution images. The German BTX, for example 480 content pixels per line, and 250 lines. This allows for 40x24 lines of 12x10 pixel characters as well as an additional status line. There is support for user defined characters as well as 32 colours out of a palette of 4096. User defined characters are also available in colour! This is the lowest level of CEPT.

On the telephone side of this, there was a simple V.23 modem, using the 1200 bps channel for downstream, and the 75 bps channel for upstream. Later devices supported faster transmission rates as well as 64 kbit ISDN.

Technical Details

Character sets

There are multiple standard character sets.

Character set G0: ($20-$7f)


Character set G1: ($20-$7f)

Btx g1.png

Character set G2: ($40-$7f)

Btx g2.png

Character set G3: ($20-$7f)

Btx g3.png

There are also 93 DRCS characters

Combining characters

Use the "ss2" command, then select a charactter from G2, then have your main character.

Example: "ss2"Hu gives you an ü.


Octet Name Description
$08 APB Active Position Back (left)
$09 APF Active Position Forward (right)
$0A APD Active Position Down
$0B APB Active Position Up
$0C CS Clear Screen
$0D APR Active Position Return (to left most column, keep line
$0E SI Shift In
$0F SO Shift Out
$11 CON Cursor On
$12 RPT Repeat last character n times. n in next octet


Because we can!

Also, CEPT is an already defined standard, we can use. There's hardware out there, but it can also be emulated using cheap Wifi modules and 320x240 TFT displays. Since everything is page oriented and page selection happens via a numeric keypad, this might be a cool function for a future badge.

What can I do?

First of all, you can play with the terminals provided.

If you want to do something more technical, you can set up an TCP server speaking the CEPT protocol and I can make the terminals connect to it. I'll also publish the modem source code as soon as it's ready.

Planned things

Bildschirmtext terminals

We'll bring probably one terminal connected to the POC network

Dial-in points

We will have a dial-in number for the POC network which speaks v23 as well as the data link layer protocol for error-free connectivity.

If you want to have your own service, set up a TCP-server which speaks CEPT and I'll be able to hook that up.