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On the IO Board why is the VGA designed in this way?
Posted: Mon Nov 23, 2020 11:43 am
by andrezheng
Hi Guys
Could you please let me know why the VGA display port designed in this way?
(1) Why there's an capacitor C6 on the G color line?
(2) What is the functionality of the circuit highlighted in red rectangle? R1, R2, Q1, SOG?
Thank you so much.
Re: On the IO Board why is the VGA designed in this way?
Posted: Mon Nov 23, 2020 7:47 pm
by eflanery
C6 appears to AC couple the green signal, likely to provide DC isolation between it and the sync signals.
R2 drops VSync to an appropriate level for Q1's base.
Q1 appears to modulate the HSync signal with the VSync signal, to create a composite sync signal.
SOG (the dashed box) is a switch, to select the composite sync signal generated by Q1 ('Override'), or a 'SOG' composite sync signal generated (or left floating) elsewhere ('Auto').
R1 drops the selected composite sync to an appropriate level for injection onto the green signal.
Re: On the IO Board why is the VGA designed in this way?
Posted: Mon Nov 23, 2020 11:21 pm
by andrezheng
Thank you so much for your detailed explain. I'll digest these comments one by one.
eflanery wrote: ↑Mon Nov 23, 2020 7:47 pm
C6 appears to AC couple the green signal, likely to provide DC isolation between it and the sync signals.
R2 drops VSync to an appropriate level for Q1's base.
Q1 appears to modulate the HSync signal with the VSync signal, to create a composite sync signal.
SOG (the dashed box) is a switch, to select the composite sync signal generated by Q1 ('Override'), or a 'SOG' composite sync signal generated (or left floating) elsewhere ('Auto').
R1 drops the selected composite sync to an appropriate level for injection onto the green signal.