This jumper can be replaced by a small two-way switch, if required. JP1 can then by used to select the correct operating mode. Since the sync in DOS-modes is often inverted compared to the modes commonly used by Windows, the output of IC1a is inverted by IC1b. VGA-BNC adapter circuit diagramĪn EXOR gate from IC1 (74HC86) combines the separate V-sync and H-sync signals into a composite sync signal. To keep the colour balance the same, the resistors for the other two colours (R1 en R3) have to be changed to the same value as R2. In this case the value of R2 should be reduced a little, but this has the side effect that the brightness noticeably decreases and the load on the graphics card increases. It is possible to get synchronisation problems when the display is extremely bright, with a maximum green component. A pull-up resistor on the green output provides a DC offset, while a transistor (a BS170 MOSFET) can switch this output to ground. These have been added to stop any direct current from entering the VGA card. The RGB signals from the VGA connector are fed to three BNC connectors via AC-coupling capacitors. The principle of operation is very straightforward. As can be seen, the circuit has been kept very simple, but it still gives a reasonable performance. This circuit has been designed with these types of monitor in mind. There are monitors which only have three BNC inputs and which use composite synchronisation (‘sync on green’).
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