The 80x25 VDU Interface Board

Acorn 80 x 25 VDU Interface

"The 80 x 25 character VDU interface is designed for use in professional applications to drive a video monitor. The VDU interface is a 100 x 160mm Eurocard which connects via a 64 way plug and socket to the standard Acorn computer bus. It produces a 1 volt composite signal which drives the 75 ohm input on a monitor.

The screen is mapped to a 2K byte area of memory which usually resides at locations 1000 to 17FF (hex). An MC6845 programmable controller IC provides the addresses for the screen memory, and the horizontal and vertical sync signals.

Accesses to the screen memory is transparent with either a 1 MHz or a 2MHz CPU clock.

A character-generator ROM contains the full ASCII character set, and also produces recognisable symbols on the screen for many of the control codes. If required special character sets may be obtained by fitting a user-programmed 2716 EPROM. A 12 MHz dot clock is used to serialise the data from the character­generator ROM and this is then fed to a video amplifier which provides a composite-video output from the card. The high 12 MHz data rate used to produce 80 characters on a line may be unsuitable for UHF modulator applications, and in order to view the 80 x 25 screen a monitor diagonal of 10 inches or more will normally be required.

Software to drive the 80 x 25 VDU is contained within special versions of the System 3, 4 and 5 Operating System ROMs, and the 80 x 25 versions must be specified when ordering a system."




I have 3 different font ROMs for this VDU, I've called them Font A, B and C:

I have written some python code that generates a simple text file showing each font:

and here is a spreadsheet that lists the font codes:


200.019 Issue 4 [3 System 3]

Acorn 80 x 25 VDU Interface Issue 4Acorn 80 x 25 VDU Interface Issue 4

200.019 Issue 4 [3 System 5]

Acorn 80 x 25 VDU Interface Issue 4Acorn 80 x 25 VDU Interface Issue 4

Data Sheets:

Replica Boards:

I have made two replica versions of this board details of which can be found here:


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