Phil,
Repeaters that can be programmed to display any NMEA data (ASCII
characters), will display whatever is in the depth data field in the DBT
sentence, including the minus sign (put there by the multiplexer, as
explained) The Cruzpro repeater instrument would be a good candidate.
If you already have the ST60 Multi, you can try if it will display a
negative depth by creating a DBT sentence on your laptop with Notepad. Put
e.g. -5.5 in the depth field. Save the sentence in a file. Connect the COM
port of your laptop to NMEA IN of the ST60 instrument (pin 3 to NMEA IN +,
Gnd to NMEA -) and send the .txt file containing the sentence to the
instrument with Hyperterminal (@4800bps, no parity, no flow control). Repeat
this is couple of times and check what the instrument displays. First
configure the instrument for NMEA input, of course.
Wout






"Phil Stanton" wrote in message
...
Hi Wout

Thanks for the information. I will definitely investigate this further, as
in addition to the depth problem, I am having problems getting my Laptop
to
talk to the Autopilot. May be a wiring fault, but apparently the ST60
multi
instrument has NMEA In and Out.
Back to the depth problem. Have you any idea which display instruments
will
display a -(minus) sign.
Thanks again for starting me off in the right direction

Phil



"Wout B" wrote in message
...

"Phil Stanton" wrote in message
...
Does anyone know if there is a depth sounder which shows a negative
depth.
OK sounds like a loony question.
I have a lift keel boat and want to set the offset when there is no
water
under the keel with the keel down. Same as all sensible people with
fixed
keel boats. Easy to do with my Raymarine ST60 Tridata echo sounder. Now
if
I
raise the keel to go into shallower waters, there isn't 7 ft (2.2m) of
water
in a lot of places on the East Coast of England, I still want to know
how
much water there is underneath me. Hence I need to find an echo sounder
that
shows 0.0m with no water under the keel when down, an say -0.7m when
the
depth is 0.7m less than with the keel fully down.
Hence my question. I should have thought with the number of lift keel
boats
there would be a reasonable demand, and lets face it, it isn't rocket
science to make a minus sign show up on a LCD display



Hi Phil,
If that's what you like, it can be done with a Brookhouse NMEA
multiplexer.
They do much more than NMEA combining/multiplexing alone. Maybe we
should
rename them "NMEA controllers" or something.
One of the functions is NMEA sentence editing "on the fly", i.e.
real-time
editing of NMEA parameters, as the sentences pass through the
multiplexer.
Editing directives can be uploaded to the multiplexer in setup mode.
http://brookhouseonline.com/pdf%20fi...%20Editing.pdf
We have recently added a new editing directive, for making adjustments
to
fields in NMEA sentences, which is ideal for depth offsets, water-speed
correction etc.
The *A directive allows you to add or subtract a signed value from any
given
NMEA parameter field of any specified sentence and if the result is
negative, it places a minus-sign in front of it. Of course it
re-calculates
the sentence-checksum.
If you connect a repeater instrument to the multiplexer RS422 output and
program it for depth, you'll get exactly what you want, provided you use
a
repeater that displays the NMEA field exactly as found in the sentence.
You
won't need a new depthsounder. Your ST60 instrument will be suitable as
there is an option for Seatalk to NMEA conversion. Adjust the offset of
the
ST60 to display actual water depth and set the "correction" in the
multiplexer to -7.0 and you will get negative depths on the repeater for
any
depths under 7 feet.
Wout