"Dave" wrote in message
...
On Tue, 10 Oct 2006 14:35:19 -0600, "Gilligan"
said:

Danger Will Robinson! Danger! That terminal block connection may
introduce
noise. Don't be surprised if the errors go up.

Check the RS232 levels out of the computer and what the serial input on
the
GPS requires. Some of those notebooks are not up to RS232 standards (they
are +/-5V rather than +/- 12V) or even pseudo RS232 single ended 0-5 Volt.

Sorry, but you've got this a bit mixed up. The signals I have to take off
for the VHF are not the output from the 'puter, but the output from the
GPS.
The lines carrying those signals are currently going to the RS232 port as
input. The signal has to be routed to the radio input lines at the same
time.

I'm sorry, I thought you said:

Well, I finally got my el cheapo nav system up and running properly. Works
like a charm. It consists of an ancient Toshiba Libretto I had sitting
around the house, hooked up to a Garmin GPS via the serial port, and running
the totally free SeaClear II software. The system uses an off-the shelf
interface that plugs into a cigarette lighter plug. SeaClear uses the free
NOAA charts, and if set on automatic will load each chart as needed when you
come to the end of the currently loaded chart. My only real issue with it is
that it insists on showing courses in true rather than magnetic, so you've
got to remember to add in the variation to set a compass course.


The specs for RS232 can be found he

http://www.rs485.com/rs485spec.html

As you can see, the RS232 standard is +/-5 volts, minimum. Many laptop
computers are capable of only +5V/0V outputs on the RS232 port. 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.

Here's just one example of the converter for true RS232 to RS232C (5Volt)

http://www.3d.curtin.edu.au/pce500/rs232-12.html

Here's what can happen:

http://www.digitalworldtokyo.com/upl...macsonybat.jpg

http://www.voanews.com/specialenglis...g06_150_se.jpg





Sometimes the problems can be very intermittent and will drive you crazy
in
trying to find them.

Lame-O batteries make the RS232 even worse.

Er...I'm talking about the CMOS battery that's bad, not the computer
battery
itself. That's effectively totally dead also, but irrelevant to the issue.

CMOS or BIOS? All the circuitry in the notebook computer is CMOS. The
lithium battery you are referring to is for the clock and some memory,
usually called the BIOS battery.

Power that replaces the regular battery is coming from the house 12V
battery
through an adapter.

I'm just giving you this advice in case you start seeing problems or the
whole thing just quits one day.