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

[Jim Lux]

On 8/20/13 8:20 AM, Brian Inglis wrote:
> 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,

Not in the sense of what the spec calls for nor in terms of a "matched 
transmission line".

The RS-232 standard says the transmitter has to tolerate shorts to any 
voltage -25V to 25V.  Then, there's also a "shall source 30mA or sink 
30mA" (or maybe it's 20 mA)

max slew rate is 30V/microsecond, which folks regularly bust to run 
faster than the notional 20kbps max for RS232.

The standard capacitance of the load is 2500pF.


Driving 12 or 15V at 20 mA is 600 ohms..