[ntp:questions] Oddities in termination of cable from gps18.
david at ex.djwhome.demon.invalid
Fri Feb 24 00:34:05 UTC 2012
Chris Albertson wrote:
> I just looked up the most common RS232 chip, the MAX232. The
> receiver section of the MAX232 sees anything over 3.5 volts as "high"
> and anything under 0.4 volts as low
The common chips from the seventies, 1489 and 1489A, had adjustable
thresholds, but the default threshold region started at about +0.8V.
The 1489 hysteresis region extended to about 1 volt, and the 1489A to
just under 2 volts. (The lowest specified negative going switching
point for both was 0.75V and the highest specified positive going one
was 1.5V for the 1489 and 2.25V for the 1489A.)
They have a lot of hysteresis; they are basically Schmidt triggers.
There is no input rate of rise constraint, so I suspect that they had a
very small meta-stable region.
As I noted elsewhere, the default transition region is deliberately all
above zero volts.
> The driver section sends at least a +/-5 volts, +/-7 is typical.
> I think the problem is that with TTL you typically have a high value
> pull-up resister then we find someone trying to "terminate" the line
> with a 200R or less resistor. All the "terminator" does is form
> voltage divider with the pull up resistor.
Typically you have CMOS masquerading as TTL, these days!
Incidentally, for non-line driver, real, TTL devices, I seem to remember
the maximum recommended line length was about 10 inches.
> Other RS232 chips might be different but the MAX232 is common
The MAX232 should only be common in single rail environments. It used
to be a lot more expensive than the 1488/9 chips, which need dual rails
for the driver, although the receiver uses a single, 5V, rail.
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