Quick nquestion on NMEA sentence

[Durcott]

Hi Folks,

This is a follow up to the NMEA interface question last week.

Thanks to that thread, I found the diagnostic display on the Simrad RADAR - it shows that the Autocomp compass is sending the following sample sentence:

HCHDM,348.5,M*22

That prefix looks odd to me, it's certainly not in the set of codes according to the Simrad manual.

Anyone got a clue while I'm still on the boat?

Jeff

 

[rickp]

According to the NMEA spec:

HDM Heading – Magnetic
$--HDM,x.x,M*hh
1) Heading Degrees, magnetic
2) M = magnetic
3) Checksum

HC is "Heading - Magnetic Compass"

Rick

 

[Durcott]

Hi Rick,

thanks for that, myt question really concerns the 5 character prefix code 'HCHDM'. I was expecting three characters, and I'm guessing this is why the Simrad is ignoring the string.

Any thoughts?

Jeff

 

[Durcott]

Ah - sorry - being thick - I understand now /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif

 

[rickp]

Hmm, I thought it was 5 as you get, but I'm not sat in front of my simrad kit at the moment so can't check. What simrad unit it is?

Tome is the expert on this stuff, I think. Perhaps PM him?

Rick

 

[tome]

Jeff

$HCHDM,348.5,M*22 surely!

The dollar sign identifies the start of an NMEA message

The next 2 characters identify the type of talker, HC is a heading compass (GP would be a GPS receiver, II would be your instruments)

the next 3 (HDM) identify the sentence which is Heading, magnetic - this is prob what is listed in the manual

NMEA fields are 'comma delimited' which means separated by comma, so the next field shows the compass reading 348.5 degrees

Next field shows M which means the 348.5 degrees is magnetic

the asterisk marks the end of the data, and the 2 characters following are a checksum to make sure the sentence was received ok

On the end of it all are a carriage return and linefeed, but you won't see this

So 19 characters in all and it looks good to me

Cheers
Tom

 

[tome]

No probs, glad you found it useful

 

[hightech]

The radar should ignore the HC header and look for the HDM bit only. Ho into the NMEA input pge and bring the HDT message header to the fron of the list to prioritise it. You should then be able to use the data.