Re: In search of a better line driver
From: Chris Carlen (crobc_at_BOGUS_FIELD.earthlink.net)
Date: 06/04/04
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Date: Fri, 04 Jun 2004 10:46:32 -0700
Stefan Heinzmann wrote:
> Do you *need* to drive open (unterminated) lines? If so, you can hardly
> avoid reflections, which will inevitably disturb the signal shape. The
> only thing you can do then is to make the edges as slow as possible
> without violating your timing requirements.
John Larkin responded adequately to this below.
> There are other ways to solder such parts. It involves using liquid
> flux, wetting your soldering iron (with a broad tip) with a bit of
> solder, and swiping it across the pins. The flux should prevent the
> solder from bridging the pins.
Yeah, I should really try some different tips, or maybe get one of the
Pace "mini-wave" tips. I also need a proper illuminated stereo magnifier.
I have used the manner you describe for 100-pin TQFP type packages, and
had little trouble. The pins were closer. Actually, I let some bridge,
and just wicked a little off later. This little 6-pin package however
has a relatively wide lead spacing.
> There's a little bit of extra information needed first. What levels do
> you want on the cable? Do you want to be able to receive the signal on
> the other end with an ordinary TTL input (that is 0..0.8V means 0 and
> 2.0..5V means 1)? What signal rise and fall times do you need? What are
> the cable lengths you need to support, and at what loss? Is it necessary
> to support unterminated cables?
Yes, TTL input on the other end. So I want to get at least 2.4V into a
50-ohm terminator, and at least 4.5V into an unterminated end. I need
to be able to handle terminated and unterminated ends. The reasons are
complex, because this driver design must serve many applications in a
lab environment where all sorts of things may be at the receiving end.
Most can deal with the TTL levels, but some are quirky and need more
than 2.4V. I think I have some stuff that is incorrectly labelled "TTL"
compatible but really has CMOS 2.5V thresholds. Thus, I need clean
transitions even if the line is unterminated to get the full close to 5V
swing. Hence the back terminated design. There is no need in a case
like this to have clean edges at intermediate points in the cable. Of
course the reflections will be visible in the middle of the cable. So
mostly these are point-to-point connections. In cases where there are
daisy-chained connections, I have instructed my users to terminate the
lines, and use short stubs or only T fittings to tap the lines.
No matter what I say, they still wire things up all wrong!
Anyway, for the rest of the specs, I'd like to have about <10ns rise and
fall times for 50 ft. or less cables, and the driver should have <40ns
of propagation delay, with as symmetric as possible propagation delays.
Ok, thanks for the input Stefan.
Have a good day!
-- _____________________ Christopher R. Carlen crobc@earthlink.net Suse 8.1 Linux 2.4.19
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