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

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Fri Feb 24 00:09:04 UTC 2012

David Woolley wrote:
> unruh wrote:
>> No, there is the RS232 spec, set up 20-30 years ago,

While RS232-C lasted the longest,
 and is likely the most referenced.
  RS232   (1962)? ~ 50 years,
  RS232-C (1969)? ~ 43 years
  RS232-D (1986)? ~ 26 years

>> but since the late 80s almost all rs232 receivers have
>>  used something like .5V and 1.5V as the transitions.
>>   In part to ease use of TTL voltages.
>> The more relevant question is not whether the rs232 are out of spec,
>>  but do you know of any rs232 receiver chipsets which do
>>  not work with TTL type signals.

That certainly is a good question,
 however its hard to maintain compatibility with
 things that break the standard they are designed after.

 What / where is the TTL compatible non-standard RS232
  implementation specification?  References?

I've used some current Maxium that switch at ~ 1.5 V;
I've also used some current TI (RS232-F speced),
 that don't switch if the voltage remains positive.

> The +/- 3volt specification is for the driver.
>  The receiver hysteresis is smaller and, and at least for
>   control signals the decision point is supposed to be biased
>   so that open circuit control signals default to off
>   (the data is also in the opposite logic polarity,
>    so that inputs default to marking,
>    which was a good thing for mechanical teletypes).

The current version is TIA/EIA-232-F (1997)
 Electrical Specification
 A logic 0 is represented by a driven voltage between 5 V and 15 V
  and a logic 1 of between –5 V and –15 V.
 At the receiving end, a voltage between 3 V and 15 V represents a 0
  and a voltage of between –3 V and –15 V represents a 1.
 Voltages between ±3 V are undefined and lie in the transition region.
  This effectively gives a 2-V minimum noise margin at the receiver.

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