The 40x25 VDU (Teletext) Interface

Acorn 40 x 25 Teletext VDU Interface 

"The Visual Display Unit Controller Board connects to the Standard Acorn bus and contains a memory mapped character storage RAM which is transparently written to and read from by the CPU.

An MC6845 programmable controller IC proves all the synchronisation signals to drive a 625 line 50 fields per second VDU, together with refresh addresses for the character RAM. Characters are then fed to an SAA5050 character generator IC which produces the necessary dot patterns to create the characters to refresh the VDU.

The SAA5050 produces Teletext standard characters and has Red, Green and Blue drive outputs giving coloured characters or graphics.

The RGB, and sync outputs may be used to drive a colour encoder and modulator for a UHF TV; also provided is a 1V/75 ohm composite sync and video output which can directly drive a monochrome monitor on which the different colours will appear as different scales of grey.

Also provided are listings for programs which set up the MC6845, display 25 instructions in hex on the VDU. (With double or treble byte instructions on a single line) and allow the drawing of graphics or characters on the VDU. These programs may be loaded and run using the System 1 Monitor. Acorn systems 2, 3, 4 and 5 are usually supplied with operating system software which controls the 6845 and outputs characters onto this VDU.

The VDU controller PCB can be supplied in kit form with a full set of IC sockets. It is easily assembled using a small soldering iron. The board operates from a single +5v supply from which it draws not more than 500 mA."


Note: both versions of the schematic have an error at IC2: pin 2 should be tied to 0V and not +5V as shown.




200.002 Issue 3 [0]

Acorn 40 x 25 Teletext VDU Interface Issue 3Acorn 40 x 25 Teletext VDU Interface Issue 3


 200.002 Issue 3 [3 System 4]

Acorn 40 x 25 Teletext VDU Interface Issue 3Acorn 40 x 25 Teletext VDU Interface Issue 3


Data Sheets:

Replica Boards:

Details of two replica boards can be found here:


Back to Acorn System Hardware

Acorn Home Page