Hi Al,

"Anonymous" wrote in message
...
Actually, RS-232 has always baffled me, and from some measurements I made it
doesn't seem to be a very strict standard.

There really is an RS-232 standard, it's just commonly abused. Here's what I
have from a "cheat sheet" I have at hand:

-- Transition levels are between +/-3V (this is what you have to detect)
-- Maximum open circuit voltage is 25V (!)
-- Maximum load capacitance is 2.5nF
-- Load resistance is 3-7kohm (this is ignored all the times -- often the load
is much, much higher than this)
-- Maximum slew rate is 30V/us (probably violated occasionally but no one
checks :-) )
-- Maximum transition time is 4% of bit time
-- Maximum data rate is 20,000 bits/second (obviously ignored with good
reason...)

The above is supposedly the official EIA-232 standard, "revision D," approved
in November, 1986.

As you can see, both +/-5V (open circuit) and +/-12V drivers can readily meet
the spec so long as they still swing +/-3V into 3k loads.

Often times the receivers are designed so that the "logic 1" input is anything
below about a voltage or two and anything above that is a "logic 0". This is
done (rather than having the threshold at 0V) so that you can get away with
hooking a TTL or CMOS logic output directly to the RS-232 input and still
successfully receive characters. You'll occasionally see people ask whether
or not some given serial card or USB to serial adapter will support such
shenanigans. :-)

---Joel