[SI-LIST] Re: VGA/VIDEO: IMPEDANCE & LENGTH
- From: Chandan <chandan_career@xxxxxxxxx>
- To: lmiller@xxxxxxxxxxxxxxxxxxxx, SI FORUM <si-list@xxxxxxxxxxxxx>
- Date: Wed, 1 Aug 2001 20:09:48 -0700 (PDT)
Hi Larry,
I think you are referring to cable lengths. My
question concerns length & characteristic impedance
of traces on the motherboard. Specifically, the
lengths of the traces from the VGA controller
driver/IC to the 15-pin D-sub receptacle on the
motherboard.
Thanks,
Chandan
--- Larry Miller <lmiller@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> Standard video is 75 ohms, 1V p-p. The video and
> sync signals should be
> length matched to an inch or two. I have run VGA
> signals 40 to 80 feet, but
> that is about the limit; you start to get
> misregistration due to cable
> dispersion (caused, I expect, by mismatch in cable
> propagation velocities).
> We had an application where we needed to run 100
> meters; we had to put in an
> adapter that made it composite video TV-station
> style and then split it at
> the far end.
>
> Larry Miller
>
> -----Original Message-----
> From: Chandan [mailto:chandan_career@xxxxxxxxx]
> Sent: Wednesday, August 01, 2001 9:24 AM
> To: SI FORUM
> Subject: [SI-LIST] VGA/VIDEO: IMPEDANCE & LENGTH
>
>
>
> Hi everybody,
>
> The video interface between a motherboard & a
> monitor
> typically consists of the following signals:
> 1. RED, GREEN & BLUE
> 2. HSYNC, VSYNC etc.
>
> My questions:
> 1. Are there specifications for the characteristic
> impedance of these signals? For e.g., the USB
> specification defines the characteristic impedance
> with which the USB signals must be routed with.
> I'm not too sure, but I think that the RED, GREEN &
> BLUE lines must be routed with a characteristic
> impedance of 37.5 ohms, whereas the HSYNC & VSYNC
> signals must be routed with a trace of
> characteristic
> impedance of 40 ohms. Could someone clarify please.
>
> 2. Are there specifications for the maximum length
> of
> these signals?
> As of now, I think that the length on the
> motherboard
> really does not matter. This is because, after the
> signals leave the motherboard, the cable that
> connects
> to the monitor is at least a foot in length. PLease
> comment.
>
> Thanks,
> Chandan
>
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