SCART & LM1881 question

Started by tavish, October 19, 2006, 05:49:08 AM

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Here is my situation: I am trying to create a SCART to Commodore 1084 (with a round six pin connector) cable.  I have read through the tutorials and understand how to construct the cable -- putting the red, green and blue and ground lines and using the LM1881 to separate the VSYNC from the composite line.

I just have two questions:

1.  Can the 1084 monitor use the composite sync output from the LM1881 as its HSYNC pin?

2.  What pin from the SCART cable can I use to power the LM1881 (pin 8, Vcc, 5-12V)?  I have narrowed it down to probably being either pins 8 or 16, but I don't know for certain.  Can somebody explain to me how I should do this conclusively.

I should explain that I'm connecting this to a SCART switch so it has to be from the SCART cable and not from a console or exteranally powered.  It has to be a generic solution.

Thanx in advance.



You can probably get away with connecting the just the C-Sync from the H-Sync input and leaving the V-Sync off. You can't reliably source any power from a SCART. You'll have to use an external power source - consider using batteries if there's no power point available.


Amiga monitors are really lax when it comes to sync signals I think. I had dreamcast scart cable that rolled on everything else than my amiga monitor and when I opened it up the sync had actually come off.

Oh, and as far as I know you can just jack the composite sync from the scart directly into the H sync as he said, I never needed custom cables for my 1084s. Then again that one even had a scart plug in the back of it.


So, I would not be able to rely on the voltages coming from the "Video Status" (pin 8) pin?  From what I have read it seems that if the SCART is being used (meaning that a device is sending a signal down the line) then it will be between 5V and 12V.  

So, if there is something (a console, VCR, etc) sending to the monitor then it will be high and the LM1881 will have power.  And, if there is not anything sending then it will be low and the LM1881 will not have power, but that doesn't matter because you're not trying to display anything.

It sounds like this might work, but I have not the experience with SCART to tell.  Can anybody tell me if my logic is sound or if this should work or not?


I use the voltage from a SCART lead frequently.  In fact, my TV won't even ack the SCART input without that voltage, so it's pretty much safe to rely on it.  

I don't know how often you'd find it unavailable.  Not often is my guess.


QuoteSo, I would not be able to rely on the voltages coming from the "Video Status" (pin 8) pin?
Not easily. Here's why....
This video status line is a signal line with a >= 10k input impedance. Apply ohm's law and we can find that when it's driven with 12 volts, maximum ammount of current that will flow from the source to the TV is 0.5mA. A device which conforms to the SCART spec (a rarity among game consoles, but most VCRs, DVDs, etc) will output 12v which is current limited to a few miliamps. So if anyone were to short circuit this line the device isn't damaged. The PAL SNES and the dreamcast are good examples of this.

Another problem, is that often game consoles will just connect their +5v Vcc directly to this signal line (as I think you're already aware of). So sometimes you'll get a +5v, sometimes you'll get (current starved) 12v. You can run an LM1881 on both voltages but if you use > 5v then you'll need a resistor divider on the output(s) to maintain TTL compattibility (monitors don't like 12v peak to peak sync signals!). If you switch to a different device that uses 5v then the divider may attenuate the output signal too much for it to be useful.

It's certainly possible in some cases to power stuff from this line but it definately can't be relied upon. However, If you're determined to go ahead with this then I'd suggest testing the voltage/current source abilitys of everything you're likely to be connecting to your device.