Monitor - ISSN 1472-0221
The Newsletter for Data Acquisition and Control
Issue 52 November 2002
Welcome to November's issue of Monitor. This month how to connect NMEA 0183 instruments (GPS receivers, sonar, etc) to a PC.
Contents
* Interfacing NMEA Instruments
- NMEA sentence structure
- Connecting to a PC
- Software to Capture NMEA data
* Excel Corner
* Printing the Windmill Help Topics
Interfacing NMEA Instruments
Although each electronic measuring instrument may do a necessary job on its own, it is far more useful when its information is integrated with that from other pieces of equipment. Depth data from sonar and position data from a GPS receiver combined to plot a contour map for example. The NMEA interfacing standard ensures that conforming instruments all speak the same language. This language can be understood by PC software like the free version of Windmill.
The standard was developed by the US-based National Marine Electronics Association in the 1980s. The NMEA 0183 standard uses ASCII messages which can be sent over RS232 serial links. In 2000 a new standard was released - NMEA 2000. This uses CAN (Controller Area Networks). Today we're concentrating on NMEA 0183.
NMEA 0183 Sentence Structure
The standard specifies communication in coded sentences in ASCII (text) format. Each sentence begins with the dollar character, $, and ends with a carriage return and a line feed. Between the beginning and the end are identifier and data fields, separated by commas. The first two characters following the $ comprise the "talker" identifier, describing the type of instrument sending the data. For example $GP for GPS receiver or $ZA for atomic clock. Other talker identifiers include:
AP Magnetic Autopilot CV Communications - Radio-Telephone (VHF) DF Direction Finder EP Emergency Position Indicating Beacon (EPIRB) ER Engine Room Monitoring Systems GP Global Positioning System (GPS) HC Magnetic Compass HE North Seeking Gyro IN Integrated Navigation LC Loran C OM Omega receiver P Proprietary Code RA RADAR SD Depth Sounder SN Electronic Positioning System, other/general SS Sounder, Scanning TI Turn Rate Indicator VD Velocity Sensor, Doppler, other/general DM Velocity Sensor, Speed Log, Water, Magnetic VW Velocity Sensor, Speed Log, Water, Mechanical WI Weather Instruments YX Transducer ZA Timekeeper Atomic Clock ZQ Timekeeper Quartz
Following the talker ID is a three letter code identifying the type of sentence. For example:
DBT Depth below transducer GLL Geographic position, Latitude, Longitude MTW Water temperature MWD Wind direction MWV Wind speed and angle RMC Recommended minimum specific GPS/transit data RSD RADAR system data VBW Water referenced and ground referenced speed XDR Transducer measurements ZDA Time and date
Next are the comma separated data values.
Here is an example of a complete NMEA sentence.
$GPGLL,5330.25,N,00215.31,W,134531,A*2D
Following the $ marking the beginning of the code comes the 2-letter code identifying the instrument sending the data. In this case it is GP for GPS receiver. The 3-letter sentence identifier, GLL, tells us that the data that follows will give geographic position, latitude and longitude. The data shown is:
5330.12 - Latitude 53 degrees and 30.25 minutes,
ie 53 degrees, 30 minutes and 25 seconds.
N - North (or S for South).
00215.31 - Longitude 2 degrees and 15.33 minutes,
ie 2 degrees, 15 minutes and (around)
2 seconds.
W - West (or E for East).
124531 - UTC Time of 13:45 and 31 seconds (hhmmss).
UTC stands for Co-ordinated Universal Time.
This time scale is kept by time laboratories
around the world and is determined using
precise atomic clocks. It is equivalent
to Greenwich Mean Time (GMT).
A - Data valid (or V for data invalid).
*77 - The checksum. This is an optional calculation
resulting in 2 hexadecimal digits to check
that the sentence has been properly transmitted.
- Carriage return and line feed to mark the end
of the sentence. These are non-printing
characters that might not be seen but will be
transmitted as ASCII characters 013 and 010.
The maximum sentence length is 80 characters. If
data is unavailable two commas with nothing in
between are sent.
The standard allows a manufacturer to define its
own proprietary sentences. These allow more
functionality but are unique to each manufacturer.
These type of sentences start with "$P". This is
followed by a 3-letter manufacturer ID and a
1-letter sentence identifier.
Connecting to a PC
NMEA 0183 instruments have serial ports. With a
suitable cable they can generally by plugged into
the PC's RS232 serial connector. Software like Windmill
can then collect and interpret the data being sent
from the instrument to the PC. Some manufacturers
may use non-standard cables and plugs. It is often
easiest, therefore, to buy the cable from your
instrument supplier but you can make or modify your
own. The NMEA signal or output line needs to be
connected to the RS232 receiver or input line (line
2 on a 9-pin plug). The NMEA ground or earth needs
to be connected to the RS232 ground or earth (5 on
a 9-pin plug). Issue 42 of Monitor gave details on
RS232 connections.
The NMEA 0183 Interface Standard specifies the
communication settings as
Baud Rate: 4800
Data Bits: 8
Parity: None
Stop Bits: 1 (or more)
These should work for all instruments but, depending on
your device, you may be able to use a higher baud rate.
You will need to tell the data acquisition software
about the communication settings your instrument is
using.
Software to Capture NMEA data
After connecting your NMEA devices to the computer
you need some software to capture the data and save
it in a file. The free Windmill 4.3 software we
offer all subscribers does this - as does
the newer and easier to configure Windmill 6 with
COMIML software -
https://www.windmillsoft.com/daqshop/rs232-modbus.html.
Windmill also lets you chart data and export it to
other programs like mapping packages, spreadsheets
and databases.
When using Windmill you have to specify how to parse
the data sentence, storing each type of data in a
channel. For example, with our sentence
$GPGLL,5330.25,N,00215.31,W,134531,A*2D
to obtain a latitude reading you might
- search for "$GPGLL,"
- extract next 7 characters
and store the result in Channel 0.
Further Reading
For more on the National Marine Electronics
Association, or to purchase a copy of the standard,
see
http://www.nmea.org/
A fuller list of talker and sentence identifiers is
at
http://vancouver-webpages.com/peter/nmeatype.txt
For examples of how Windmill is capturing data from
NMEA 0183 instruments see
https://www.windmill.co.uk/fluorescence.html
https://www.windmill.co.uk/mapping.html
https://www.windmill.co.uk/train.html
For more on connecting GPS receivers to a PC see
https://www.windmill.co.uk/gps.html
For information on NMEA 2000 see
http://www.cankingdom.org/download/Download%20files/Basics/ME-a-Mar99-20001.doc
Excel Corner: Inserting a Line Break in a Cell
When using an Excel spreadsheet for analysis or display,
how do you control where the line break appears in a
cell with a long text entry? Pressing the Enter key just
moves you to the next cell. To force a line break
press the Alt key at the same time as the Enter key and
Excel will insert a carriage return.
For more tips on using Excel with Windmill see
https://www.windmill.co.uk/excel/
https://www.windmill.co.uk/excel/excel-charting.html
Printing the Windmill Help Topics
All the Windmill programs come with on-line Help. If you
wish to print the entire Help file you do not have to do
so topic by topic. Instead you can simply:
- press the Contents button and display the Contents
window
- then select a Heading and press the Print button.
All
topics under this heading will be printed.
To print the contents of a pop-up window:
- use your right mouse button to click the pop-up
- then select Print Topic.
You can download the latest version of each Help file
from https://www.windmill.co.uk/help.html
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