[ntp:questions] Oddities in termination of cable from gps18.

Chris Albertson albertson.chris at gmail.com
Thu Feb 23 23:04:46 UTC 2012

On Thu, Feb 23, 2012 at 2:55 PM, Chris Albertson
<albertson.chris at gmail.com> wrote:
>> 80s almost all rs232 receivers have used something like .5V and 1.5V as
>> the transitions. In part to ease use of TTL voltages.

 You are right.  I notice you used the important qualifiers too
"almost all" and "something like".   That is a good description of
current state.  .     Yes most do try to accept TTL now days.

 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 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.

 People forget that TTL does not source much current.  It can sink
tens of milliamps but only can source single digits of mA.  So a
terminator pulls TTL  down below the 3.5 volt threshold.

 Other RS232 chips might be different but the MAX232 is common

 Chris Albertson
 Redondo Beach, California

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