[ntp:questions] Performance?
David Woolley
david at ex.djwhome.demon.co.uk.invalid
Wed Dec 31 00:15:03 UTC 2008
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
state.
More information about the questions
mailing list