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Old 02 April 2017, 12:51   #1
jarp
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Amiga VGA sync signals?

Hi there!

Would anybody happen to understand how sync signals sent by Amiga via RGB port do work? Oscilloscope would tell but I do not have one I've always assumed Amiga's H/V sync signals are VGA compatible; kept normally high and low pulse means sync. Like described in this article (last picture):

http://martin.hinner.info/vga/vga.html

I've done buffered VGA adapter for myself which routes sync signals via two inverting buffers and it works well with my monitor. But then I started to google around and some people are talking about that Amiga's sync signals would need to be inverted because they are the reverse - kept normally low and high pulse means sync.

Does anybody know for sure? I am asking because I plan to get few RGB to VGA boards manufactured and I would like to make them as compatible as ever possible.

Thanks!

Edit: My question is kind of flawed I realize it now after I've read a bit about this topic. Seems that "VGA" uses both positive and negative pulses depending on resolution - actually this was used by monitors to detect resolution back in the days. Amiga's resolution, on the other hand, is not VGA compliant anyways. So to be most compatible one would have to use switch so user could select sync polarity, no?

Last edited by jarp; 02 April 2017 at 13:09.
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Old 02 April 2017, 13:31   #2
Daedalus
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Some schematics for VGA adaptors I've seen seem to have inverters on the sync lines, but I've never used one and any monitor I've tried with a thraight-through adaptor hasn't had a problem with them.

The official Commodore adaptors (as supplied with the A4000), has buffering on the sync lines but no inversion, so that looks like the official line regarding compatibility at the time.
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Old 02 April 2017, 16:00   #3
idrougge
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Buffering has been absolutely necessary for my Microvitek 1438 screen to work with the Amiga — or rather the opposite: the Amiga refuses to boot without a buffer circuit.

I use 74HCT08 for buffering in my cables like the mainstream Commodore adapter.
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Old 02 April 2017, 23:40   #4
patrik
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Quote:
Originally Posted by idrougge View Post
Buffering has been absolutely necessary for my Microvitek 1438 screen to work with the Amiga — or rather the opposite: the Amiga refuses to boot without a buffer circuit.

I use 74HCT08 for buffering in my cables like the mainstream Commodore adapter.
Is your computer an A4000? I have an AT M1438S which works fine with the A1200, but my A4000 refuses to boot with it connected.
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Old 02 April 2017, 23:43   #5
TheDarkCoder
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@patrik: I also have experienced this, A4000 needs the C= silver adapter to work with Microvitek 1438 (which I suppose it's more or less the same as AT M1438S
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Old 03 April 2017, 00:46   #6
idrougge
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Quote:
Originally Posted by patrik View Post
Is your computer an A4000? I have an AT M1438S which works fine with the A1200, but my A4000 refuses to boot with it connected.
Indeed it is.
However, I can't recall my old AT M1438S not working with the A4000 – it had an integrated D23 connector instead of a VGA connector.
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Old 03 April 2017, 01:42   #7
xArtx
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Buffered doesn't mean it has to be inverted.
Most circuits including Commodore's own use 74HCT08.
Does your circuit use two inverters in series?
That woul invert the signal, and then invert it again.
The result of that would just be a slightly delayed
but normal signal.
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Old 03 April 2017, 09:41   #8
idrougge
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No, I just use one HCT08 and never said it inverted anything. That's what a '04 does AFAIK.
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Old 03 April 2017, 09:51   #9
jarp
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Exactly, 08 is an AND gate, 04 is an inverter.

I currently have adapter build on veroboard based on 74HCT14 which is an schmitt-trigger inverter. Not sure if there is any point to use schmitt-trigger inverter instead of inverter, but when using it my display (or rather VGA-HDMI adapter, namely the OSSC) syncs to all display modes and without it it only syncs to PAL mode, nothing else.

In any case, I am just about to submit my boards to PCB house. Hate those veroboards so much...
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Old 03 April 2017, 18:03   #10
mark_k
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I think most monitors should handle either positive or negative sync polarity. See https://courses.engr.illinois.edu/ec...vga_timing.pdf

That mentions original VGA modes: 640×350@70Hz using hsync +ve, vsync -ve. 640×400@70Hz hsync -ve, vsync +ve. And 640×480@60Hz both hsync & vsync -ve. So back in the day, VGA-compatible monitors could have required those polarities to set the correct vertical picture size. But nowadays (and since the Amiga output doesn't match any specific VGA mode) any polarity should work I think.
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