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Acorn 80 x 25 VDU Interface (front)
Acorn 80 x 25 VDU Interface (back)
The Acorn 80x25 VDU interface produces a 1v composite signal to drive the 75ohm input on a monitor. This card came from a System 5. The part no. is 200,019 issue 4.
The screen is usually mapped to a 2K area of memory at 1000 to 17FF. An HD68B45SP-2 programmable controller provides the addresses for the screen memory and the horizontal and vertical sync signals. A character generator ROM contains the full ASCII character set (this ROM has a white label which reads "80CHVDU 035B"), special character sets may be obtained by fitting a user-programmed 2716 EPROM. a 12MHz dot clock is used to serialise the data from the character generator ROM and this is then fed to a video amplifier which providesa composite video output from the card. The software to drive the card is contained in the System OS ROM.
Here is the 80 character VDU card manual.