"Dave" wrote in message
...
On Tue, 10 Oct 2006 16:39:47 -0600, "Gilligan"
said:

If whatever
the computer is communicating with is true RS232, the 0 volt level is
indeterminate. For signals coming into your computer, suppose it is true
+/-25V. This 25 volts is greater than the 5Volt of the notebook and will
forward bias the RS232 input protection diodes and apply that 25 volts to
the positive rail of the computer. Most of the CMOS circuitry in the old
notebooks are rated 5.0 +/- 0.5V for supply voltages.

Do you suppose Gamin might have taken that into account in designing the
GPS
unit to interface to a laptop?

Is it a Garmin interface unit that you are using?

The information you supplied was:

"The system uses an off-the shelf
interface that plugs into a cigarette lighter plug."

The Garmin interface on the Garmin website is simply a cable of various
configurations. There are no level converters in the cable. If your GPS
(unknown model #) is NMEA 0183 Format it is recommended for RS422 rather
than RS232:

http://vancouver-webpages.com/peter/nmeafaq.txt

Which has:

2. Electrical Interface

These standards allow a single "talker", and several "listeners"
on one circuit. The recommended interconnect wiring is a
shielded twisted pair, with the shield grounded only at the
talker. The standards do not specify the use of any particular
connector.


The NMEA-0180 and 0182 standards say that the talker output may
be RS-232, or from a TTL buffer, capable of delivering 10 mA at
4 V. A sample circuit shows an open collector TTL buffer with a
680 ohm resistor to +12 V, and a diode to prevent the output
voltage from rising above +5.7 V.

NMEA-0183 accepts this, but recommends that the talker output
comply with EIA-422. This is a differential system, having two
signal lines, A and B. The voltages on the "A" line correspond
to those on the older TTL single wire, while the "B" voltages
are reversed (while "A" is at +5, "B" is at ground, and vice
versa)

In either case, the recommended receive circuit uses an
opto-isolator with suitable protection circuitry. The input
should be isolated from the receiver's ground.

In practice, the single wire, or the EIA-422 "A" wire may be
directly connected to a computer's RS-232 input.

There is the possibility of burning out your laptop's serial port. Simply
check the voltage levels coming out of the GPS with an oscilloscope, making
sure the lines are loaded with a 1K resistor while disconnected from the
notebook. The radio repair guy at the boatyard will probably do this for
free. As it says, in practice the NMEA-0183/EIA-422 talker "A" wire may be
connected directly to the notebook. Let the "B" wire float, connect the
signal ground and let the shield float at the notebook. Try to maintain this
connectivity with the strip connector and remember you will be breaking the
shield at the strip or introducing unshielded lines into the circuit. You
can still shield the extra lines on the strip but remember to float the
shield on the end. DO NOT use the shield as a ground.

You can ignore all this, the thing may work, even well. If there are
problems, check this stuff first.If you have a gasoline a powered engine,
run it (over the full RPM range), it is a good test. I know of a $30 Million
piece of space junk in orbit because someone ignored the differences between
a RS422 and a RS232 . Hopefully you have no plans of sailing beyond the
terrasphere.