Mac-to-VGA monitor adapter struggles

Elapsed time: 122 hours from design concept to finished product in my hands.

What a world we live in. They’re beautiful!

Hey, it works! Self-powering works, the CSYNC and VSYNC detection LEDs work, and LM1881 sync separation works. This is awesome! You can bet I'm going to be running around Mactoberfest Meetup plugging one of these into every Mac and Nubus video card I can find, making a list of which ones output CSYNC, separate H+V sync, or both.

The self-powered supply voltage (diode rectified from the sync signals) results in a VCC about 4.09V. That seems to be high enough, but we're in uncharted territory for the LM1881 and it may not work correctly at this voltage. The Schottky diodes used here should drop about 0.3V, but there's also a 180 ohm series resistor on each of the Mac's sync outputs that will result in additional voltage drop. I estimate the adapter is pulling about 3.3 mA from each of the three sync signals, 10 mA total.

The red LED is being fed slightly less than 1 mA of current, but is still easily visible.

The LM1881 outputs a rational-looking VSYNC signal, but the high part of the signal is only about 2.65V. If I recall, this signal was about 3.6V when the circuit was powered from 5V, so being lower than VCC is probably normal. I think 2.65V is still high enough to be a valid high voltage seen at the monitor, but it's cutting it close. If I were going to stay with this design, I'd probably add a buffer here to bring this signal all the way up to VCC.

In this adapter I changed the LM1881 RSET resistor from the 680K reference value down to 510K, because I thought it was closer to what's needed for Mac-speed video signals. This should have led to a shorter VSYNC pulse width, but it's actually substantially longer. The mess-o-wires prototype had a VSYNC period of 10 lines, but this one is 15-17 lines. I'm not sure if that's because of the lower supply voltage, or because my component value is wrong, or what.

What's especially strange is that the VSYNC pulse width is variable, by about 2 lines. The pulse always starts at exactly the right spot relative to CSYNC, but sometimes it lasts longer than others.

So does it actually work for separating HSYNC and VSYNC? The answer is yes, sort of. On my Dell 2001FP it works, but the picture jumps up and down vertically by a couple of lines, probably due to the problems with VSYNC. I just received a Dell EL151FP, which does not work with the IIci normally, but does work via this adapter. However once again there's some vertical jitter. The VG900b doesn't work. The Viewsonic 6 CRT, which does not work with the IIci normally, works via this adapter and the picture looks stable.

I can try using a 5V external supply instead of self-powering, and see how things change.

I think I'm going to pursue the microcontroller sync splitter concept instead of continuing with this LM1881 concept, but this is still a useful device for sync detection and it works well for CRTs at least. It also proves that self-powering is possible, although the resulting VCC is lower than I would have hoped. The self-powered VCC also depends a lot on the value of the series resistors on the sync signals, inside the Mac, and those may be different for other Mac models and video cards.

All in all, this is pretty darn good!

bigmessowires said:

It also proves that self-powering is possible, although the resulting VCC is lower than I would have hoped. The self-powered VCC also depends a lot on the value of the series resistors on the sync signals,

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Would a voltage boost circuit be feasible? There are some efficient little circuits out there for running 5v from 3v that are in the 90s for efficiency. I integrated one in a battery powered board I did a while back... I could dig it out.

I’m having some adapter struggles too. I’ve got a IIci newly recapped and up and running - and have been trying to get a Griffin MacPNP adapter to work with an Apple Multiscan 15AV display. I’ve got the DIP switches set to 6 + 8 which should be correct according to the manual. But I get no output.

I assume the monitor isn’t compatible with the IIci, is it a sync-on-green thing?

croissantking said:

I’m having some adapter struggles too. I’ve got a IIci newly recapped and up and running - and have been trying to get a Griffin MacPNP adapter to work with an Apple Multiscan 15AV display. I’ve got the DIP switches set to 6 + 8 which should be correct according to the manual. But I get no output.

I assume the monitor isn’t compatible with the IIci, is it a sync-on-green thing?

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Can we see the instructions?

Phipli said:

Can we see the instructions?

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Here’s the manual I’ve been referring to:


This is how I’ve had it set up:

croissantking said:

Here’s the manual I’ve been referring to:

https://www.pccables.com/pdf/MonitorAdapter_MacPnP.pdf

This is how I’ve had it set up:

View attachment 63448

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Generally you need a ten switch for a IIci. For sync reasons.

I don't know what your style of adapter does wrong, but the way it does Sync probably assumes the computer outputs separate horizontal and vertical sync.

Also, using two adapters is more likely to cause issues because you need both to be wired correctly.

Phipli said:

Generally you need a ten switch for a IIci. For sync reasons.

I don't know what your style of adapter does wrong, but the way it does Sync probably assumes the computer outputs separate horizontal and vertical sync.

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Ah, I’ll try and get hold of a 10 switch adapter.

croissantking said:

Ah, I’ll try and get hold of a 10 switch adapter.

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Note I updated my last post.

Try NO adapters.

Phipli said:

Note I updated my last post.

Try NO adapters.

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Nope, that doesn’t work. I assume that the IIci won’t enable its onboard video if it doesn’t see a compatible monitor.

croissantking said:

Nope, that doesn’t work. I assume that the IIci won’t enable its onboard video if it doesn’t see a compatible monitor.

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Hum. I grew up with a IIci and I had a MultiSync 15" Apple monitor connected. I can't remember if I had it on the 4•8 card or the built in, but either way, it was not MultiSync compatible and it just worked at 640*480. The setup should just default to 640*480...

Are you sure the monitor and computer both work?

croissantking said:

Nope, that doesn’t work. I assume that the IIci won’t enable its onboard video if it doesn’t see a compatible monitor.

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I just tested on my IIci by setting an adapter to be a 14" MultiSync and it doesn't work on built in video. I must have had it plugged into the 4•8 display card as a kid. Shame.

croissantking said:

Nope, that doesn’t work. I assume that the IIci won’t enable its onboard video if it doesn’t see a compatible monitor.

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Oh, no, not quite, It /does/ work as a 14" MultiScan. I got mixed up and forgot I had the adapter set to 0110 not 1001 on the first few switches because I was looking upside down.

So, your monitor SHOULD work on your IIci with no adapters.

Phipli said:

Oh, no, not quite, It /does/ work as a 14" MultiScan. I got mixed up and forgot I had the adapter set to 0110 not 1001 on the first few switches because I was looking upside down.

So, your monitor SHOULD work on your IIci with no adapters.

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Both Mac and monitor work, although I should note that the cable end of the monitor isn’t in very good condition. On my other Macs I have to wiggle it around to get the image to not be red, blue or green tinted. So maybe this has something to do with it?

So you are using your multiscan with no adapters? I’m confused as you mention an adapter but then say it should work without one

croissantking said:

Both Mac and monitor work, although I should note that the cable end of the monitor isn’t in very good condition. On my other Macs I have to wiggle it around to get the image to not be red, blue or green tinted. So maybe this has something to do with it?

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Yeah, it genuinely should just work because the MultiSync sense wires are effectively 1 and 4, which means that a computer that doesn't know better, like the IIci, just thinks it is a 13" Apple monitor and drives it at 640*480. A more modern computer can identify the diodes used to spot that it is MultiSync capable. This was done intentionally by apple.


The adapter is set to Mode 2, composite sync, 14" MultiSync.

croissantking said:

So you are using your multiscan with no adapters? I’m confused as you mention an adapter but then say it should work without one

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I don't have a MultiSync 14/15" screen, so I'm spoofing the sense wires that those monitors have using my adapter. I have my adapter set to the setting that pretends to be your monitor. Make sense?

Phipli said:

Yeah, it genuinely should just work because the MultiSync sense wires are effectively 1 and 4, which means that a computer that doesn't know better, like the IIci, just thinks it is a 13" Apple monitor and drives it at 640*480. A more modern computer can identify the diodes used to spot that it is MultiSync capable. This was done intentionally by apple.

The adapter is set to Mode 2, composite sync, 14" MultiSync.

I don't have a MultiSync 14/15" screen, so I'm spoofing the sense wires that those monitors have using my adapter. I have my adapter set to the setting that pretends to be your monitor. Make sense?

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Ah, it does make sense now.

So, I shouldn’t need an adapter. Interesting that the screen won’t come out of standby.

croissantking said:

Ah, it does make sense now.

So, I shouldn’t need an adapter. Interesting that the screen won’t come out of standby.

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Something isn't right with the monitor, cable or computer.