Reply-To: "George Gonzalez"
From: "George Gonzalez"
Subject: BrainFlash-- is this doable?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
NNTP-Posting-Date: Mon, 14 Oct 2002 12:00:56 GMT
Organization: AT&T Broadband
Date: Mon, 14 Oct 2002 12:00:56 GMT
I have a sudden requirement at work to generate short bursts of a 2.5Mhz
sine wave, phase modulated, from a older (50 MHz, 486), PC.
Now I can do this with a couple chips hooked to the parallel port, not too
difficult, but it would be neat to have a software-only solution.
First I thought of making a crude D/A by hooking some resistors on the
parallel port But with a 50MHz 486, you can't pump out the data fast enough
to make a credible 2.5MHz signal. Even a fast D/A card with DMA is going to
have a hard time of it.
So I put the D/A idea on the mental back burner. Wouldnt you know it, just
as I was falling asleep, my subconcious came out with a ingenious solution:
USE THE VIDEO CARD!
A PC video card, looked at from a unusual angle, can be considered as a fast
DMA-only 3-channel, 8-bit D/A converter. Probably with questionable
monotonicity and stability, but just maybe good enough for undemanding
Big question: can one program the card to have zero-length H and V retrace
IIRC there are several register that let you set the retrace durations, but
it's quite possible the designers never considered "zero" as a possible
Anybody KNOW if this is possible, before I drag the scope over to the PC and
see if it works?