Commodore 1940 - Mode Switching Circuit

3.3 Clamping Circuit (Low Light Circuit)
This circuit controls the DC voltage at the CRT's cathodes.
Each cathode has a separate adjustment to allow display of pure white.
RB09 is aligned to get the exact grey
under the minimum brightness.
DB05 is clamping diodes for DC regeneration.
<Fig.B Video Output Circuit>
4. Mode Switching Circuit
4.1 General
This circuit is to control the vertical size, horizontal hold and horizontal.phase with mode changing. The vertical
line mode is determined by the horizontal/vertical sync pOliarity, so the control signal is made utilizing the sync
poliarity.
4.2 Mode Polarity
The relations between the sync polarities and the line modes are as follows:
MO.D~
VGA.··
VGA
VGA
E'JR01~
QaLNTSC
SyNC
35g LinE!~
40pLines 4BO Lines
ECS
AA
..
..
Horizontal Positive Negative Negative Negative
Negative
Vertical Negative
Positive
Negative Positive
Negative
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