[time-nuts] NTP/1-PPS/RS232 question

[Brian Inglis]

On 2013-08-20 02:45, "Björn" wrote:
>
>> bg at lysator.liu.se said:
>>> The PTTI 1PPS is defined in
>>>      http://www.navcen.uscg.gov/pdf/gps/ICD-GPS-060B.pdf
>>> It is 20us long and common in some applications. However the voltage
>>> levels
>>> are a bit high...
>>
>> The section I saw said 10 V nominal, +1, -2.  That's 8-11 V.
>>
>> I think that would work fine with a RS-232 receiver.
>
> Yes, but driving 10V into _50ohms_ is a lot of power for a modern receiver...
>
> /Björn

RS-232 transmitter output impedance is 50 ohms, and that may be what the PPS 
timing info diagram shows the PPS signal being driven into. That PTTI spec 
Electrical Characteristics states receiver input impedance is 5kohms, so the 
diagram could also have a typo.
Those levels are compatible with RS-232 3k-7k receiver input impedance range, 
*BUT* the transmitter output should be able to drive 10 receiver inputs, which 
may be possible with some RS-232 transmitters, but is not required with RS-232.