Thanks, Joel! This is all very good information. It does bring up another
point, though, related to my original question about interfacing the GPS to
the TinyTrak3.

The GPS puts out +/- 5 V serial, but the Tiny Trak's serial input is a
TTL-level input to the PIC microprocessor. The PIC runs off of a single +5V
supply. I guess it can withstand the negative voltage swing without failing?
And... if I looked at the serial output from the GPS into an open circuit,
there was no signal. It was as if the GPS shut off its serial if it detected
an open circuit. Is this normal? Even after connecting it to the TinyTrak,
many times it wouldn't 'wake up' and start sending serial data. This was
very confusing.

Thanks

Al W6LX





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