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From: "John Jardine"
Subject: Re: *standard* VGA sync levels ?
Date: Fri, 29 Nov 2002 19:54:40 -0000
NNTP-Posting-Date: 29 Nov 2002 19:42:44 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
fred bartoli wrote in message
> The fire brigade (I) have to design some protective device (will be a kind
> of dongle) that will be insterted between the video output and the monitor
> cable to prevent a badly designed video output (our nice work) to
> latchup in fields (about 5% every two weeks... eh eh, not so bad).
> I'm OK with the RGB lines (levels are low) but the HSYNC and VSYNC outputs
> are typically 5V levels and I *have* to be ensure clamping under the board
> VCC level (the chip seems pretty sensitive and have no specs) which leave
> room at all if taken abruptly.
> So I need the minimum *standard defined* level a video card has to put for
> sync signals.
> (didn't manage to find anything useful outof google)
> Anyone has a pointer or the values on top of his hat ?
Had a bit of trouble figuring the question ;-).
(thought VGA was -all- logic levels.
There's little VGA info out there but I found this a few weeks ago from
epanorama.net. Don't know if it's relevant ...
More VGA mode information
There are the 3 "standard" VGA modes that each VGA card is supposed to be
able to do:
640 x 350 x 70 is compatible with the old EGA mode, but on a VGA display.
640 x 400 x 70 is the MS-DOS text mode (when the computer is booting !).
640 x 480 x 60 is the default Windows(tm) graphics mode (16 colours !).
Their line frequency is exactly twice that of the NTSC television system, or
almost twice that of the PAL television system. This makes it fairly easy to
implement a VGA input on a television set that uses line doubling for the
television signals so the line deflection already runs on 31 kHz.
The following timings come from a list of 82 different computer timings, and
by now there will be even more. Some video cards even have variable timing
(allowing the user to set width, height and shift...). The only standard is
that there is no standard !
"640 x 350 (EGA on VGA)" "640 x 400 VGA text" "VGA industry
Clock frequency 25.175 MHz Clock frequency 25.175 MHz Clock frequency
Line frequency 31469 Hz Line frequency 31469 Hz Line frequency
Field frequency 70.086 Hz Field frequency 70.086 Hz Field frequency
One line: One line: One line:
8 pixels front porch 8 pixels front porch 8 pixels front
96 pixels horizontal sync 96 pixels horizontal sync 96 pixels
40 pixels back porch 40 pixels back porch 40 pixels back
8 pixels left border 8 pixels left border 8 pixels left
640 pixels video 640 pixels video 640 pixels video
8 pixels right border 8 pixels right border 8 pixels right
--- --- ---
800 pixels total per line 800 pixels total per line 800 pixels total per
One field: One field: One field:
31 lines front porch 5 lines front porch 2 lines front
2 lines vertical sync 2 lines vertical sync 2 lines vertical
54 lines back porch 28 lines back porch 25 lines back porch
6 lines top border 7 lines top border 8 lines top border
350 lines video 400 lines video 480 lines video
6 lines bottom border 7 lines bottom border 8 lines bottom
--- --- ---
449 lines total per field 449 lines total per field 525 lines total
Sync polarity: H positive, Sync polarity: H negative, Sync polarity: H
V negative V positive V
Scan type: non interlaced. Scan type: non interlaced. Scan type: non
Jeroen H. Stessen kindly mailed this information for me to be added to this
document at November 1997.
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