"Dave" wrote in message
...

Next step is to hook the serial connection up through a terminal block so
I
can also pull off the signal for routing to the DSC function of the VHF,
and
then create a decent mounting to keep it from moving around in a seaway.



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.

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.

"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.

"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.

Page 5 of this manual also discusses the NMEA-0183 to PC interface

http://www.autonnic.com/autonnic/Autonnic.nsf/0/2859F4421DBFF9A18025713A004B1A38/$file/A5020%20manual%2003.pdf#search=%22nmea%200183%20rs 232%20converter%22

Here's a level converter for NMEA-0183 to RS232:

http://www.cruzpro.com/nc20.html

Here's another interesting link about NMEA-0183/PC:

http://www.marinelektronik.se/faq.htm

They say:

NMEA is not the same as RS232. The voltage levels are different, which can
cause data transmitted by
the computer's serial port not to be understood by the receiving equipment,
and also for the PC either
not to understand the incoming NMEA sentences, or in some cases for the PC's
serial port to be damaged.
In addition, NMEA 0183 is designed to protect devices and to minimize
interference problems
by having the data input opto-isolated (further interference protection is
provided by specifying the use
of shielded twisted pair cabling). For this reason, we strongly recommend
the use of an NMEA/RS232 converter.

NMEA 0183 compatability does not guarantee that systems can talk to each
other.
There are many sentences available, so the same data can be transmitted in
many different ways.
Also, within a sentence, not all of the data fields need to be filled in, so
you cannot always check
compatability by looking at the lists of sentences transmitted/received.
In some cases a NMEA 0183 sentence filter is necessary to establish
compatability.

Because of the difference in signal levels between NMEA and RS232C we
recommend
the use of a NMEA/RS232C interface. If the computer is powered by the ships
AC
outlet you will need an isolated type of interface to avoid ground voltages.

More he

http://www.caracolyachts.com/t_pcintfc.htm

When wiring an NMEA device to an RS-232 device, be aware that you are wiring
together devices which use a different electrical specification. So if on a
device you have a choice of an NMEA port or a RS-232 port, choose to match
if possible NMEA to NMEA or RS-232 with RS-232.

However almost invariably an RS-232 device can be connected to an NMEA
device and vice versa. There are two potential problems, firstly the RS-232
and NMEA specifications use different voltage levels. This is seldom a
problem, except on very old devices.

The biggest potential problem is that NMEA is a differential signal, with
the Transmit and Receive pairs, and RS-232 signals are referenced to a
common ground.

When wiring an NMEA device to provide input to an RS-232 device, connect the
Transmit + of the NMEA device to the Receive of the RS-232 device, and the
Transmit - of the NMEA device to the RS232 signal ground.

When wiring an NMEA device to take input from an RS-232 device, connect the
Receive + of the NMEA device to the Transmit of the RS-232 device, and the
Receive - of the NMEA device to the RS232 signal ground.

When wiring an NMEA device to both provide input to and take input from an
RS-232 device, connect the Transmit + of the NMEA device to the Receive of
the RS-232 device, connect the Receive + of the NMEA device to the Transmit
of the RS-232 device, and the Transmit - of the NMEA device AND the
Receive - of the NMEA device to the RS232 signal ground.

In some instances tying the Transmit- and Receive - of the NMEA port
together can cause problems. In this case you need to use an NMEA to RS-232
converter between the two devices. We find this to be a requirement in less
than 1% of cases, but should be borne in mind if you are experiencing
problems.

Modern Furuno radars almost always require this type of converter.

http://www.euronav.co.uk/Products/Ha...EAProducts.htm

Most personal computers communicate with the outside world via a protocol
called RS232. NMEA and RS232 operate on slightly differing signal voltage
levels and with some older equipment there is a danger of unreliable
communication and possible damage to the computer serial port unless a
suitable interface cable is used. Some GPS manufacturers supply suitable
cables with their devices - if you have one of these, do check that it is
fully opto-isolated and that it protects against reverse polarity before
using it.

http://web.inter.nl.net/hcc/van.het....center/pcc.htm

It is possible to use a simple home-made connection cable to connect the
instrument's NMEA port directly to the PC's RS-232 com port, but it is
strongly recommended to use a NMEA to RS 232 converter instead. You will
find more information regarding this is in the manual file (MANUAL.TXT) of
the program.


http://www.actisense.com/HTML/Produc...r%202/FAQs.htm

This adapter cable also converts the RS422 / RS485 (NMEA) voltage levels to
RS232 (PC) voltage levels, and vice-versa.


http://www.epa.gov/glnpo/image/vbig/221.jpg

http://mishami.image.pbase.com/u11/l...._STA50032.jpg

http://www.frenchcreekboatsales.com/...ccaneer_01.jpg

"Dave" wrote in message
...
On Tue, 10 Oct 2006 19:31:00 -0600, "Gilligan"
said:

It's
extremely unlikely that taking a second set of outputs from the GPS to the
radio, duplicating the connection that was there when the GPS was
installed
initially, is gonna be problematic.

That's a 25 watt VHF radio transmitter? Is the RF ground at the SO-239
antenna connector the same as the signal ground for the NMEA-0183? 25 watts
into a 50 ohm antenna is 50 volts RMS, let's say the VSWR is 2:1 on the band
edge you'll have 280 volts peak to peak on the coax. Let me know how well it
works during and after you transmit a few times.

Does your radio ground go to a ground plate? Is it tied to the other grounds
on your boat? Do the ground currents between the radio and GPS flow through
the notebook or is it a floating node? Is the 12V power adaptor ground the
same as the RS232 ground? Are the units individually fused or do they share
the same power wire/ckt?

Since you are a lawyer what does "extremely unlikely" mean under Murphy's
Law?

http://na.amarc.org/gallery/albums/u...2006-02-05.jpg

http://groups.google.com/group/rec.b...627d02f6933131

http://groups.google.com/group/sci.g...cb0a19adbddf3d

"Dave" wrote in message
...
On Wed, 11 Oct 2006 09:41:12 -0600, "Gilligan"
said:

Since you are a lawyer what does "extremely unlikely" mean under Murphy's
Law?

In your case, it means get lost.

If you (or anyone) can refute one thing I've said or show how any of it was
misleading in the least I'm all ears. I've even provided links to people who
have had the "extremely unlikely" problems happen to them as well as
technical standards, industry recommendations, hardware solutions and
practical caveats. All I offer is some helpful advice, handy tips and all
you can say is "get lost". Bad Karma. I shall no longer make my helpful
services available to you.

"Dave" wrote in message
...
On Wed, 11 Oct 2006 13:19:29 -0600, "Gilligan"
said:

I shall no longer make my helpful
services available to you.

Oh, the horror!

That's right! You'll spend the rest of your life in misery!