David Woolley <[email protected]> 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. _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
