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| From Composite Sync to H and V Sync for VGA |
Posted by: MinerAl on 2013-11-08 10:44:35
I'm trying to take the video signal from a 575's wiring loom out to a VGA monitor. The 575 sends out RGB and Composite sync, and the monitor is expecting VGA (RGB + Horizontal and Vertical syncs).
My research (you know, 5 minutes of Googling) has led me to this chip which seems to be a sync separator that can do the job, but the reading in that document quickly goes from dense to Greek to me.
Can one of you EE types tell me if this will do what I'm looking for? If it's possible? If there's a better solution?
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Posted by: NJRoadfan on 2013-11-08 12:21:49
First check and see if the monitor accepts plain old composite sync from the horizontal sync pin, some do even though their data sheet says otherwise.
http://www.epanorama.net/documents/vga2rgb/vgamonitor.html
See "How can I build a circuit which converts composite sync signal to HSYNC and VSYNC ?"
The LM1881 circuit is easy to build, just two caps and a resistor. To get true HSYNC from it, you need to add some additional parts as shown on that page. As before, check and see if the monitor will take a composite signal for HSYNC in addition to the generated VSYNC.
Another thing you can do is cheat and use a Mac2VGA adapter, many of them have sync separators built in (my Belkin does) since many Macs output composite sync and many VGA monitors only take H+V Sync.
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Posted by: MinerAl on 2013-11-08 12:32:54
I will try connecting the Composite signal to the H-Sync. Just in case though...
If I was to use one of those Belkin adapters, to which pin on the Mac side should I connect the Composite Sync signal? H-Sync?
That particular Belkin adapter is easy to find and cheap on eBay.
(Awesome link btw.)
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Posted by: NJRoadfan on 2013-11-08 12:44:16
Composite Sync is pin 3 on all the Mac DB-15 monitor pin outs I have found. The instruction manual for the Belkin adapter omits some information, namely that DIP switches 5,6, and 7 control monitor sync type. The information is printed on the adapter itself.
H+V Sync: DIP 5,6 ON 7 OFF
Composite Sync: 5,6 OFF 7 ON
Sync on Green: 5,6,7 OFF
VGA: 8 ON (I don't think this changes sync, just forces the sense pins in the adapter to tell the machine to output standard VESA refresh and resolution types)
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Posted by: MinerAl on 2013-11-08 12:53:21
Excellent. I went ahead and bought that adapter, just in case 🙂
Here's to hoping it isn't necessary! It'll be a hamned dandy thing to have around anyhow.
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Posted by: NJRoadfan on 2013-11-08 13:13:15
Some more info on the subject: http://www.applefritter.com/node/6301
The adapter may be expecting a power source from some of the pins if you have trouble getting it to work. I haven't cracked open my adapter to see if it uses the diode trick since its glued shut. Said adapter does work on my Apple IIgs despite it not having sense pins or H+V sync output (just composite sync on pin 3).
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Posted by: Trash80toHP_Mini on 2013-11-08 13:42:45
I mentioned this in passing in the "I want to do this" thread, but I'll mention it here for the guys that actually know what they're about.
Could Al's video problems be solved by something as simple as wiring the loom up to the Video-Out board from a Quadra 630 or RoadApple 6XXX?
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Posted by: NJRoadfan on 2013-11-08 21:46:44
If the header on the motherboard has horizontal and vertical sync available on its pins, sure.
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Posted by: techknight on 2013-11-09 11:30:56
I dont remember this C/V/H sync issue when rewiring the old 5XXX series. Maybe only the 575 has this?
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