The Acorn Visual Display Unit Controller Board connects to the Standard Acorn Computer Bus and contains a memory mapped character storage RAM which is transparently written to or read from by the CPU.
An MC 6845 programmable controller IC provides all the synchronisation signals to drive a 625 line 50 fields per second VDU together with read 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 television; also provided is a 1 volt / 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.
The VDU controller PCB is supplied in kit form with a full set of IC sockets. It is easily assembled using a small soldering iron and useful hints on assembly may be found in the Acorn Micro-computer system 1 Technical Manual. The board operates from a single +5V supply from which it draws not more than 500 mA.
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 or graphics or characters on the VDU. These programs may be loaded and run using the Acorn system 1 Monitor. A new monitor ROM will shortly be available for linking the VDU and an ASCII keyboard to Acorn's 4K Fast BASIC.