"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.
Documentation:
or see this section from the System 5 Manual System 5 Handbook Part 2 Section 4 80 x 25 VDU Board [2]
ROMs
I have 3 different font ROMs for this VDU, I've called them Font A, B and C:
80x25 VDU Font A.rom (this ROM has a £ symbol for character code 96 and the lowercase j is lowered)
80x25 VDU Font B [3 system 3].rom (this ROM has a left apostrophe ` for character code 96 and the lowercase j is raised)
80x25 VDU Font C [3 system 5].rom (this ROM has a left apostrophe ` for character code 96 and the lowercase j is lowered)
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:
Photos:
200.019 Issue 4 [3 System 3]
200.019 Issue 4 [3 System 5]
Data Sheets:
HD6845R (new part number for HD46505SP)
Replica Boards: