[ntp:questions] Performance?

Unruh unruh-spam at physics.ubc.ca
Wed Dec 31 05:19:55 UTC 2008


David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:

>dhavey wrote:

>> "Voltage levels
>> Diagrammatic oscilloscope trace of voltage levels for an uppercase
>> ASCII "K" character (0x4b) with 1 start bit, 8 data bits, 1 stop bit
>> 
>> The RS-232 standard defines the voltage levels that correspond to
>> logical one and logical zero levels. Valid signals are plus or minus 3
>> to 15 volts. The range near zero volts is not a valid RS-232 level; "
>> 
>> Wikipedia.

>Never use Wikipedia as a source.  A correctly constructed Wikipedia 
>article will always identify an original source for every significant 
>fact, and you should only use Wikipedia to trace that source.  In this 
>case, I think they are over-simplifying because the author is going by 
>popular understanding, rather than the actual spec.

>Paragraph 2.5 of RS232-C requires that RTS, DSR, DTR, SecRTS (CA, CC, 
>CD, and SCA) interpret any voltage between -2 and +2 volts, with a 
>source impedance of at least 300 ohms as being OFF, and any interface 
>point voltage of more than +3V be treated as on.  As the input 
>resistance can be as high as 7k, this seems to imply that the zero going 
>threshold must be at least +1.92V, at the interface point.

>Specifically, the last paragraph of the Voltage Levels section of edit 
>260896780 of the Wikipedia article is misleading, as it omits the power 
>off detection case.  Also the oscilloscope diagram (as of 
>2007-12-04T03:01:53) is incorrect, because it takes more than 4% of the 
>signalling unit to cross the transition region.

>Driver open circuit voltage can be as high as 25 volts, but in a steady 
>state must be in the range 5 to 15V when driving a resistance between 3k 
>and 7k.

>I practice, though, any RS232 interface will be emulating the 1489A line 
>receiver chip, which doesn't quite meet the 2 volt power down source 
>voltage requirement.  It has a configurable threshold, but the 
>unconfigured negative going threshold is at least +0.75V and the 
>unconfigured positive going threshold is at most +2.25V, which seem to 
>me to be very TTL compatible.

>The receiver has Schmidt type characteristics.  Although it isn't 
>formally characterised, it looks like the typical hysteresis ia just 
>over 1V.

>There is an alternative version, with a smaller typical hysteresis and a 
>maximum positive going threshold of 1.5V.

>Chip data from a, rather old, Fairchild Linear Interface Data Book.
>> 
>> I thought it was plus or minus 3-12 volts with the range between -3
>> and +3 volts undefined.

>Upper limit is 15 volts, although the receiver must tolerate 25V.  The 
>driver should produce at least 5 V at the interface point, in the steady 

OK, I will believe you. However since the driving voltage of the garmin 18
is usually taken from the USB and is 5 V, the output PPS voltage will be
less than 5V. ( maybe 4.5) Does that make a difference?

>state.




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