VGA sync signals

I'm doing a VGA sync generator in a Virtex II and can't decide how to go about driving the HS and VS signals to the connector. Does anyone have any experience / ideas about this that would be helpful please? Presumably I could:

1) Simply drive the signals from the FPGA (LVTTL signal level) and not worry about it. 2) As above but put some protection diodes on the lines. 3) Put series resistors in the signals (I've seen this on an old FPGA board). 4) Use a buffer chip e.g. LS07.

TIA Rog.

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Roger
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"Roger" schrieb im Newsbeitrag news:M_Nce.8045$ snipped-for-privacy@newsfe6-gui.ntli.net...

worry

Use series resistors of about 50 ohms. this will give you roughly 75 Ohms output impedance, well matched to the cable.

Regards Falk

Reply to
Falk Brunner

Falk,

So the Sync signals are 75 Ohm terminated as well? I thought it was just the RGB lines. I thought the Syncs were TTL.

Rog.

Reply to
Roger

Falk,

Looking into this a bit more, I've found monitor input impedances in the low KOhms range i.e TTL type inputs.

Rog

Reply to
Roger

Roger,

Driving with 75 ohms and terminating with a relatively high impedance, for your case, is exactly what is needed for those LVTTL signals that need their signal integrity maintained. For the video and sync signals, this is important.

If you generate a signal with a source impedance equal to characteristic impedance of the associated transmission line (cable or trace), and then terminate the end into a high impedance, then the end will receive a full voltage (LVTTL level) swing -- and it will not have any signal-distorting reflections contained in it. This technique is called "source termination".

For this "source termination" technique, what happens is that when you drive a transmission line with a source of equal impedance value, the signal is "launched" into the transmission line at half of its source voltage (V/2). When the signal hits the end of the transmission line, the signal will double in value (in the forward direction) -- thus regenerating its original voltage level (V). A reflection heads back toward the source at level -V/2 (note the minus sign). When it hits the originating end's source impedance the reflected wave is completely absorbed, so it doesn't reflect back to the destination end again.

It is possible to end terminate LVTTL, but then it must be driven into the transmission line at full V level. This implies that the source impedance of the driver be very low. In practice, this is difficult to achieve. That's why source termination is the most popular technique used for LVTTL (and like) signals that need terminating.

Bob

Reply to
Bob

Roger, what Falk suggested is called series-termination. It matches the drive impedance to the cable. That means the signal starts out at half amplitude, but it doubles up when it gets reflected at the unterminated end ( kilohm is very high impedance = unterminated.) So the receiver sees a perfect full-amplitude signal, and all further reflections die at the transmitter. This is definitely the way to go. Peter Alfke

Reply to
Peter Alfke

I would definitely put some protection diodes on the lines. Multiple years ago, a company I worked at had problems with video RamDacs blowing out randomly. We finally realized it only happened when we plugged a monitor into the card. There would somtimes be enough voltage on the lines that the parts were zapped even though voltage levels were fine after things had been plugged together.

John P

Reply to
johnp

Thanks to you all for some excellent advice. I now know exactly what I'm doing!!

Rog.

Reply to
Roger

Oops. One correction. The reflection back toward the source is V/2, not -V/2. The polarity of the reflection only inverts if the wave encounters an impedance lower than its path. In this case, it's higher than the impedance of the transmission line.

Bob

Reply to
Bob

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