On the IO Board why is the VGA designed in this way?

andrezheng
Posts: 31
Joined: Tue Aug 25, 2020 3:10 pm
Has thanked: 1 time
Been thanked: 6 times

On the IO Board why is the VGA designed in this way?

Unread post 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.
Attachments
无标题.jpg
无标题.jpg (452.28 KiB) Viewed 2143 times
eflanery
Posts: 4
Joined: Mon Aug 17, 2020 7:56 pm
Been thanked: 1 time

Re: On the IO Board why is the VGA designed in this way?

Unread post 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.
andrezheng
Posts: 31
Joined: Tue Aug 25, 2020 3:10 pm
Has thanked: 1 time
Been thanked: 6 times

Re: On the IO Board why is the VGA designed in this way?

Unread post by andrezheng »

:D :D :D 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.
Post Reply