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One place to find this document is at:
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ftp://sundae.triumf.ca/pub/peter/nmeafaq.txt
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|
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---------------------------------------------------------------------------
|
||||
|
||||
|
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The NMEA FAQ
|
||||
Version 6.1 Sept. 15, 1997
|
||||
(NMEA URL updated)
|
||||
|
||||
Additions, corrections, and comments should be emailed to the author,
|
||||
Peter Bennett bennett@triumf.ca
|
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|
||||
Contents:
|
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|
||||
1. What is NMEA?
|
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1.1 What is an NMEA Standard
|
||||
1.2 NMEA Address
|
||||
|
||||
2. Electrical Interface
|
||||
|
||||
3. NMEA-0180 and NMEA-0182
|
||||
3.1 Simple Format
|
||||
3.2 Complex Format
|
||||
|
||||
4. NMEA-0183
|
||||
|
||||
4.1 General Sentence Format
|
||||
4.2 Sentences sent by specific equipment
|
||||
4.3 Sample Sentences Dissected
|
||||
4.3.1 Standard Sentences
|
||||
4.3.2 Garmin Proprietary Sentences
|
||||
|
||||
5. RS-232 connections
|
||||
|
||||
6. Troubleshooting
|
||||
|
||||
7. About the author
|
||||
7.1 Acknowledgements
|
||||
|
||||
1. What is NMEA?
|
||||
|
||||
The National Marine Electronics Association is dedicated to the
|
||||
education and advancement of the marine electronics industry and
|
||||
the market which it serves.
|
||||
|
||||
It is a non-profit association composed of manufacturers,
|
||||
distributors, dealers, educational institutions, and others
|
||||
interested in peripheral marine electronics occupations
|
||||
(quoted from a promo in "NMEA News")
|
||||
|
||||
|
||||
1.1 What is an NMEA standard?
|
||||
|
||||
For the purposes of this article, an NMEA standard defines an
|
||||
electrical interface and data protocol for communications
|
||||
between marine instrumentation. (They may also have standards
|
||||
for other things.)
|
||||
|
||||
1.2 NMEA Address
|
||||
|
||||
P.O. Box 3435
|
||||
New Bern NC, 28564-3435
|
||||
U.S.A.
|
||||
Phone: 919-638-2626
|
||||
Fax: 919-638-4885
|
||||
email: nmea@coastalnet.com
|
||||
web page: http://www4.coastalnet.com/nmea/default.html
|
||||
|
||||
|
||||
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.
|
||||
|
||||
|
||||
|
||||
3. NMEA-0180 and NMEA 0182
|
||||
|
||||
NMEA-0180 and 0182 are very limited, and just deal with
|
||||
communcations from a Loran-C (or other navigation receiver,
|
||||
although the standards specifically mention Loran), and an
|
||||
autopilot.
|
||||
|
||||
From the information I have, it appears that 0180 and 0182 are
|
||||
identical. I suspect that equipment claiming to use NMEA-0180
|
||||
will use the "simple" format described below, while those using
|
||||
NMEA-0182 will use the "complex" format. (but this is really
|
||||
just a guess... corrections??)
|
||||
|
||||
3.1 "Simple" data format
|
||||
|
||||
The simple format consists of a single data byte transmitted at
|
||||
intervals of 0.8 to 5 seconds, at 1200 baud with odd parity.
|
||||
Bits 5 - 0 give the cross-track error in units of 0.1 uS or 0.01
|
||||
nautical mile. The error is given in offset binary, with a
|
||||
count of 1 representing full scale right error, 32 (hex 20) for
|
||||
on course, and 63 (hex 3f) full scale left error. Bit 6 is a 1
|
||||
if the data is valid, and bit 7 is 0 to indicate the simple
|
||||
data format.
|
||||
|
||||
3.2 "Complex" data format
|
||||
|
||||
The complex format consists of a data block of 37 bytes of
|
||||
(mostly) readable ASCII text giving cross-track error, bearing
|
||||
to waypoint, present Lat/Long, and a binary status byte. The
|
||||
data block shall be sent at intervals of 2 to 8 sec. All bytes
|
||||
in the complex format have bit 7 = 1 to distinguish them from
|
||||
the simple format. It is permissible for a sending device to
|
||||
send both simple and complex data, and even to send a "simple"
|
||||
data byte in the middle of a "complex" data block.
|
||||
|
||||
Byte Data
|
||||
1 $
|
||||
2 M | device
|
||||
3 P | address
|
||||
|
||||
4 K = kilometres | cross track
|
||||
N = nautical miles | error
|
||||
U = microseconds | units
|
||||
|
||||
5 - 8 0 - 9 or . cross track error value
|
||||
9 L or R cross track error position
|
||||
|
||||
10 T or M True or Magnetic bearing
|
||||
11 - 13 0 - 9 bearing to next waypoint
|
||||
|
||||
14 - 23 12D34'56"N or present latitude
|
||||
12D34.56'N
|
||||
24 - 34 123D45'56"W or present longitude
|
||||
123D45.67"W
|
||||
|
||||
35 non-ASCII status byte
|
||||
bit 0 = 1 for manual cycle lock
|
||||
1 = 1 low SNR
|
||||
2 = 1 cycle jump
|
||||
3 = 1 blink
|
||||
4 = 1 arrival alarm
|
||||
5 = 1 discontinuity of TDs
|
||||
6 = 1 always
|
||||
36 "NUL" character (hex 80)(reserved status byte)
|
||||
37 "ETX" character (hex 83)
|
||||
Any unavailable data is filled with "NUL" bytes.
|
||||
|
||||
|
||||
4. NMEA-0183
|
||||
|
||||
4.1 General Sentence Format
|
||||
|
||||
Under the NMEA-0183 standard, all characters used are printable
|
||||
ASCII text (plus carriage return and line feed). NMEA-0183 data
|
||||
is sent at 4800 baud.
|
||||
|
||||
The data is transmitted in the form of "sentences". Each
|
||||
sentence starts with a "$", a two letter "talker ID", a three
|
||||
letter "sentence ID", followed by a number of data fields
|
||||
separated by commas, and terminated by an optional checksum, and
|
||||
a carriage return/line feed. A sentence may contain up to 82
|
||||
characters including the "$" and CR/LF.
|
||||
|
||||
If data for a field is not available, the field is simply
|
||||
omitted, but the commas that would delimit it are still sent,
|
||||
with no space between them.
|
||||
|
||||
Since some fields are variable width, or may be omitted as
|
||||
above, the receiver should locate desired data fields by
|
||||
counting commas, rather than by character position within the
|
||||
sentence.
|
||||
|
||||
The optional checksum field consists of a "*" and two hex digits
|
||||
representing the exclusive OR of all characters between, but not
|
||||
including, the "$" and "*". A checksum is required on some
|
||||
sentences.
|
||||
|
||||
The standard allows individual manufacturers to define
|
||||
proprietary sentence formats. These sentences start with "$P",
|
||||
then a 3 letter manufacturer ID, followed by whatever data the
|
||||
manufacturer wishes, following the general format of the
|
||||
standard sentences.
|
||||
|
||||
Some common talker IDs are:
|
||||
GP Global Positioning System receiver
|
||||
LC Loran-C receiver
|
||||
OM Omega Navigation receiver
|
||||
II Integrated Instrumentation
|
||||
(eg. AutoHelm Seatalk system)
|
||||
|
||||
4.2 Sentences sent by specific equipment
|
||||
|
||||
This section lists the sentence types used by various equipment.
|
||||
The format and data included in each sentence type is given in
|
||||
section 4.3.
|
||||
|
||||
Eagle AccuNav
|
||||
Standard: RMB, RMC, GLL, APB
|
||||
Proprietary: PSLIB
|
||||
It also pretends it's a Loran, sending LCGLL, as well as GPGLL
|
||||
|
||||
Garmin GPS-38, NMEA-0183 V. 1.5 mode
|
||||
Standard: GLL, RMB, RMC, WPL, BOD, XTE, VTG, BWC
|
||||
Proprietary: PGRMM (map datum), PGRMZ (altitude), PSLIB (dgps ctrl)
|
||||
|
||||
Garmin GPS-38, NMEA-0183 V. 2.0 mode
|
||||
Standard: GLL, RMB, RMC, WPL, BOD, GSA, GSV, RTE, GGA
|
||||
Proprietary: PGRME (estimated error), PGRMM, PGRMZ, PSLIB
|
||||
|
||||
Garmin GPS-45 (and probably GPS-40 and GPS-90)
|
||||
Standard: BOD, GLL, RTE, RMB, RMC, GGA, GSA, GSV
|
||||
Proprietary: PGRME, PGRMM, PGRMZ
|
||||
|
||||
Garmin GPS-65 (and probably GPS-75)
|
||||
Standard: BWC, GLL, RMB, RMC, R00, WPL, XTE, VTG
|
||||
Proprietary: PGRMM, PGRMZ, PSLIB
|
||||
|
||||
Magellan Trailblazer
|
||||
Standard: APB, BWC, GGA, GLL, RMB, RMC, VTG
|
||||
Trimble Ensign XL
|
||||
Standard: APA, BWC, BWR, GGA, GLL, RMB
|
||||
|
||||
Trimble Flightmate Pro and Scoutmaster
|
||||
Standard: APA, APB, BWC, GGA, GLL, GSA, GSV, RMB, RMC,
|
||||
VTG, WCV, XTE, ZTC
|
||||
|
||||
Autohelm Seatalk
|
||||
Autohelm Seatalk is a proprietary bus for communications
|
||||
between various intruments. Some of the instruments can act
|
||||
as NMEA-0183 talkers or listeners. Data received from an
|
||||
external NMEA-0183 device will, if Seatalk understands the
|
||||
sentence, be re-transmitted, but not necessarily in the same
|
||||
sentence type.
|
||||
|
||||
The specific sentences sent will depend on the data
|
||||
available on the Seatalk bus (i.e. sentences containing wind
|
||||
speed and direction will only be sent if the system includes
|
||||
a wind instrument)
|
||||
|
||||
Seatalk output:
|
||||
Standard: APB, BPI, BWC, VWR, VHW, DBT, GLL, HDM, HDT, HCS,
|
||||
MTW, VTG
|
||||
|
||||
Seatalk input:
|
||||
Standard: APA, APB, RMB, XTE, XTR, BPI, BWR, BWC, BER,
|
||||
BEC,WDR, WDC, BOD, WCV, VHW, VWR, DBT
|
||||
|
||||
|
||||
4.3 Sample Sentences Dissected
|
||||
4.3.1 Standard Sentences
|
||||
|
||||
A talker typically sends a group of sentences at intervals
|
||||
determined by the unit's update rate, but generally not more
|
||||
often than once per second.
|
||||
|
||||
Characters following the "*" are a checksum. Checksums are
|
||||
optional for most sentences, according to the standard.
|
||||
|
||||
APB - Autopilot format B
|
||||
APB,A,A,0.10,R,N,V,V,011,M,DEST,011,M,011,M
|
||||
A Loran-C blink/SNR warning
|
||||
A Loran-C cycle warning
|
||||
0.10 cross-track error distance
|
||||
R steer Right to correct (or L for Left)
|
||||
N cross-track error units - nautical miles
|
||||
V arrival alarm - circle
|
||||
V arrival alarm - perpendicular
|
||||
011,M magnetic bearing, origin to destination
|
||||
DEST destination waypoint ID
|
||||
011,M magnetic bearing, present position to destination
|
||||
011,M magnetic heading to steer
|
||||
(bearings could be given in True as 033,T)
|
||||
(note: some pilots, Roberston in particular, misinterpret "bearing
|
||||
from origin to destination" as "bearing from present position to
|
||||
destination". This apparently results in poor performance if the
|
||||
boat is sufficiently off-course that the two bearings are
|
||||
different.)
|
||||
|
||||
BOD - Bearing - origin to destination waypoint
|
||||
BOD,045.,T,023.,M,DEST,START
|
||||
045.,T bearing 045 True from "START" to "DEST"
|
||||
023.,M breaing 023 Magnetic from "START" to "DEST"
|
||||
DEST destination waypoint ID
|
||||
START origin waypoint ID
|
||||
|
||||
BWC - Bearing and distance to waypoint - great circle
|
||||
BWC,225444,4917.24,N,12309.57,W,051.9,T,031.6,M,001.3,N,004*29
|
||||
225444 UTC time of fix 22:54:44
|
||||
4917.24,N Latitude of waypoint
|
||||
12309.57,W Longitude of waypoint
|
||||
051.9,T Bearing to waypoint, degrees true
|
||||
031.6,M Bearing to waypoint, degrees magnetic
|
||||
001.3,N Distance to waypoint, Nautical miles
|
||||
004 Waypoint ID
|
||||
|
||||
BWR - Bearing and distance to waypoint - rhumb line
|
||||
(format same as BWC)
|
||||
|
||||
DBT - Depth below transducer
|
||||
DBT,0017.6,f,0005.4,M
|
||||
0017.6,f 17.6 feet
|
||||
0005.4,M 5.4 Metres
|
||||
|
||||
GGA - Global Positioning System Fix Data
|
||||
GGA,123519,4807.038,N,01131.324,E,1,08,0.9,545.4,M,46.9,M, , *42
|
||||
123519 Fix taken at 12:35:19 UTC
|
||||
4807.038,N Latitude 48 deg 07.038' N
|
||||
01131.324,E Longitude 11 deg 31.324' E
|
||||
1 Fix quality: 0 = invalid
|
||||
1 = GPS fix
|
||||
2 = DGPS fix
|
||||
08 Number of satellites being tracked
|
||||
0.9 Horizontal dilution of position
|
||||
545.4,M Altitude, Metres, above mean sea level
|
||||
46.9,M Height of geoid (mean sea level) above WGS84
|
||||
ellipsoid
|
||||
(empty field) time in seconds since last DGPS update
|
||||
(empty field) DGPS station ID number
|
||||
|
||||
GLL - Geographic position, Latitude and Longitude
|
||||
GLL,4916.45,N,12311.12,W,225444,A
|
||||
4916.46,N Latitude 49 deg. 16.45 min. North
|
||||
12311.12,W Longitude 123 deg. 11.12 min. West
|
||||
225444 Fix taken at 22:54:44 UTC
|
||||
A Data valid
|
||||
(Garmin 65 does not include time and status)
|
||||
|
||||
GSA - GPS DOP and active satellites
|
||||
GSA,A,3,04,05,,09,12,,,24,,,,,2.5,1.3,2.1*39
|
||||
A Auto selection of 2D or 3D fix (M = manual)
|
||||
3 3D fix
|
||||
04,05... PRNs of satellites used for fix (space for 12)
|
||||
2.5 PDOP (dilution of precision)
|
||||
1.3 Horizontal dilution of precision (HDOP)
|
||||
2.1 Vertical dilution of precision (VDOP)
|
||||
DOP is an indication of the effect of satellite geometry on
|
||||
the accuracy of the fix.
|
||||
|
||||
GSV - Satellites in view
|
||||
GSV,2,1,08,01,40,083,46,02,17,308,41,12,07,344,39,14,22,228,45*75
|
||||
2 Number of sentences for full data
|
||||
1 sentence 1 of 2
|
||||
08 Number of satellites in view
|
||||
01 Satellite PRN number
|
||||
40 Elevation, degrees
|
||||
083 Azimuth, degrees
|
||||
46 Signal strength - higher is better
|
||||
<repeat for up to 4 satellites per sentence>
|
||||
There my be up to three GSV sentences in a data packet
|
||||
|
||||
HDM - Heading, Magnetic
|
||||
HDM,235.,M
|
||||
HDM Heading, Magnetic
|
||||
235.,M Heading 235 deg. Magnetic
|
||||
(HDG, which includes deviation and variation, is recommended
|
||||
instead)
|
||||
|
||||
HSC - Command heading to steer
|
||||
HSC,258.,T,236.,M
|
||||
258.,T 258 deg. True
|
||||
236.,M 136 deg. Magnetic
|
||||
|
||||
MTW - Water temperature, Celcius
|
||||
MTW,11.,C
|
||||
11.,C 11 deg. C
|
||||
|
||||
R00 - List of waypoint IDs in currently active route
|
||||
R00,MINST,CHATN,CHAT1,CHATW,CHATM,CHATE,003,004,005,006,007,,,*05
|
||||
(This sentence is produced by a Garmin 65, but is not listed
|
||||
in Version 2.0 of the standard. The standard lists RTE for
|
||||
this purpose.)
|
||||
|
||||
RMB - Recommended minimum navigation information (sent by nav.
|
||||
receiver when a destination waypoint is active)
|
||||
RMB,A,0.66,L,003,004,4917.24,N,12309.57,W,001.3,052.5,000.5,V*0B
|
||||
A Data status A = OK, V = warning
|
||||
0.66,L Cross-track error (nautical miles, 9.9 max.),
|
||||
steer Left to correct (or R = right)
|
||||
003 Origin waypoint ID
|
||||
004 Destination waypoint ID
|
||||
4917.24,N Destination waypoint latitude 49 deg. 17.24 min. N
|
||||
12309.57,W Destination waypoint longitude 123 deg. 09.57 min. W
|
||||
001.3 Range to destination, nautical miles
|
||||
052.5 True bearing to destination
|
||||
000.5 Velocity towards destination, knots
|
||||
V Arrival alarm A = arrived, V = not arrived
|
||||
*0B mandatory checksum
|
||||
|
||||
RMC - Recommended minimum specific GPS/Transit data
|
||||
RMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*68
|
||||
225446 Time of fix 22:54:46 UTC
|
||||
A Navigation receiver warning A = OK, V = warning
|
||||
4916.45,N Latitude 49 deg. 16.45 min North
|
||||
12311.12,W Longitude 123 deg. 11.12 min West
|
||||
000.5 Speed over ground, Knots
|
||||
054.7 Course Made Good, True
|
||||
191194 Date of fix 19 November 1994
|
||||
020.3,E Magnetic variation 20.3 deg East
|
||||
*68 mandatory checksum
|
||||
|
||||
RTE - Waypoints in active route
|
||||
RTE,2,1,c,0,W3IWI,DRIVWY,32CEDR,32-29,32BKLD,32-I95,32-US1,BW-32,BW-198*69
|
||||
2 two sentences for full data
|
||||
1 this is sentence 1 of 2
|
||||
c c = complete list of waypoints in this route
|
||||
w = first listed waypoint is start of current leg
|
||||
0 Route identifier
|
||||
W3IWI... Waypoint identifiers
|
||||
|
||||
VHW - Water speed and heading
|
||||
VHW,259.,T,237.,M,05.00,N,09.26,K
|
||||
259.,T Heading 259 deg. True
|
||||
237.,M Heading 237 deg. Magnetic
|
||||
05.00,N Speed 5 knots through the water
|
||||
09.26,K Speed 9.26 KPH
|
||||
|
||||
VWR - Relative wind direction and speed
|
||||
VWR,148.,L,02.4,N,01.2,M,04.4,K
|
||||
148.,L Wind from 148 deg Left of bow
|
||||
02.4,N Speed 2.4 Knots
|
||||
01.2,M 1.2 Metres/Sec
|
||||
04.4,K Speed 4.4 Kilometers/Hr
|
||||
|
||||
VTG - Track made good and ground speed
|
||||
VTG,054.7,T,034.4,M,005.5,N,010.2,K
|
||||
054.7,T True track made good
|
||||
034.4,M Magnetic track made good
|
||||
005.5,N Ground speed, knots
|
||||
010.2,K Ground speed, Kilometers per hour
|
||||
|
||||
WCV - Waypoint Closure Velocity
|
||||
WDC - Distance to Waypoint
|
||||
WDR - Waypoint Distance, Rhumb Line
|
||||
|
||||
WPL - waypoint location
|
||||
WPL,4917.16,N,12310.64,W,003*65
|
||||
4917.16,N Latitude of waypoint
|
||||
12310.64,W Longitude of waypoint
|
||||
003 Waypoint ID
|
||||
When a route is active, this sentence is sent once for each
|
||||
waypoint in the route, in sequence. When all waypoints have
|
||||
been reported, GPR00 is sent in the next data set. In any
|
||||
group of sentences, only one WPL sentence, or an R00
|
||||
sentence, will be sent.
|
||||
|
||||
XTE - Cross track error, measured
|
||||
XTE,A,A,0.67,L,N
|
||||
A General warning flag V = warning
|
||||
(Loran-C Blink or SNR warning)
|
||||
A Not used for GPS (Loran-C cycle lock flag)
|
||||
0.67 cross track error distance
|
||||
L Steer left to correct error (or R for right)
|
||||
N Distance units - Nautical miles
|
||||
|
||||
XTR - Cross-Track Error - Dead Reckoning
|
||||
XTR,0.67,L,N
|
||||
0.67 cross track error distance
|
||||
L Steer left to correct error (or R for right)
|
||||
N Distance units - Nautical miles
|
||||
|
||||
|
||||
4.3.2 Proprietary Sentences
|
||||
|
||||
The following are Garmin proprietary sentences. "P" denotes
|
||||
proprietary, "GRM" is Garmin's manufacturer code, and "M" or "Z"
|
||||
indicates the specific sentence type.
|
||||
|
||||
$PGRME,15.0,M,45.0,M,25.0,M*22
|
||||
15.0,M Estimated horizontal position error in metres (HPE)
|
||||
45.0,M Estimated vertical error (VPE) in metres
|
||||
25.0,M Overall spherical equivalent position error
|
||||
|
||||
$PGRMZ,93,f,3*21
|
||||
93,f Altitude in feet
|
||||
3 Position fix dimensions 2 = user altitude
|
||||
3 = GPS altitude
|
||||
This sentence shows in feet, regardless of units shown on the display.
|
||||
|
||||
$PGRMM,NAD27 Canada*2F
|
||||
Currently active horizontal datum
|
||||
|
||||
Proprietary sentences to control a Starlink differential beacon
|
||||
receiver. (I assume Garmin's DBR is made by Starlink)
|
||||
$PSLIB,,,J*22
|
||||
$PSLIB,,,K*23
|
||||
These two sentences are normally sent together in each group
|
||||
of sentences from the GPS.
|
||||
The three fields are: Frequency, bit Rate, Request Type. The
|
||||
value in the third field may be:
|
||||
J = status request
|
||||
K = configuration request
|
||||
blank = tuning message
|
||||
|
||||
When the GPS receiver is set to change the DBR frequency or
|
||||
baud rate, the "J" sentence is replaced (just once) by (for
|
||||
example): $PSLIB,320.0,200*59 to set the DBR to 320 KHz, 200
|
||||
baud.
|
||||
|
||||
5. RS-232 connections
|
||||
|
||||
Although this is not really related to NMEA, many people want to
|
||||
connect a GPS to a computer, so need to know about the RS-232
|
||||
serial ports on a computer.
|
||||
|
||||
The RS-232 standard defines two classes of devices that may
|
||||
communicate using RS-232 serial data - Data Terminal Equipment
|
||||
(DTE), and Data Communication Equipment (DCE). Computers and
|
||||
terminals are considered DTE, while modems are DCE. The
|
||||
standard defines pinouts for DTE and DCE such that a "straight
|
||||
through" cable (pin 2 to pin 2, 3 to 3, etc) can be used between
|
||||
a DTE and DCE. To connect two DTEs together, you need a "null
|
||||
modem" cable, that swaps pins between the two ends (eg. pin 2 to
|
||||
3, 3 to 2). Unfortunately, there is sometimes disagreement
|
||||
whether a certain device is DTE or DCE, hence my standard RS-232
|
||||
disclaimer:
|
||||
if it doesn't work, swap pins 2 and 3!
|
||||
|
||||
The standard RS-232 connector is a 25 conductor DB-25, although
|
||||
many PCs (and some other equipment) now use a 9 pin DE-9 (often
|
||||
incorrectly called DB-9)
|
||||
|
||||
Serial Port Connections
|
||||
Computer (DTE) Modem
|
||||
DB-25 DE-9 Signal Direction DB-25
|
||||
2 3 Tx Data -> 2
|
||||
3 2 Rx Data <- 3
|
||||
4 7 Request to send -> 4
|
||||
5 8 Clear to send <- 5
|
||||
6 6 Data Set Ready <- 6
|
||||
7 5 signal ground 7
|
||||
8 1 Data CarrierDetect <- 8
|
||||
20 4 Data Terminal Ready -> 20
|
||||
22 9 Ring Indicator <- 22
|
||||
|
||||
For NMEA-0183 interfacing, we are only concerned with Rx Data,
|
||||
signal ground (and possibly Tx Data, if we want the computer to
|
||||
talk to the GPS)
|
||||
|
||||
NMEA-0183 data is sent at 4800 baud.
|
||||
|
||||
6. Troubleshooting
|
||||
|
||||
First check that the talker (usually GPS or Loran) can send
|
||||
NMEA-0183, and determine what sentences it sends. Also, verify
|
||||
that the listener understands NMEA-0183, and that it understands
|
||||
the sentences the talker is sending. In some cases the same
|
||||
information may be sent in two or more different sentences. If
|
||||
the talker and listener don't both use the same sentences, there
|
||||
will be no communication. It may be possible to change the
|
||||
sentences sent by the talker, to match those understood by the
|
||||
listener.
|
||||
|
||||
Next, check that the talker is indeed set to send NMEA-0183
|
||||
data. Some talkers may have provision to send NMEA-0180 or
|
||||
0182, or some proprietary format.
|
||||
|
||||
A computer, using any convenient terminal program (Telix,
|
||||
Procomm, Windows Terminal, etc.) set to 4800 baud, can be used
|
||||
to monitor the NMEA data, and confirm what sentences are sent,
|
||||
and that the data is in the correct format.
|
||||
Verify that the wiring is correct - that the talker data output
|
||||
is connected to the listener data input, and that a signal
|
||||
ground line is connected between the two pieces of equipment.
|
||||
|
||||
If you have multiple listeners connected to a single talker, you
|
||||
may be overloading the talker port. Try connecting only one
|
||||
listener at a time.
|
||||
|
||||
On any NMEA-0183 circuit, there can _only_ be one talker. If
|
||||
you must have more than one talker, and one of the talker
|
||||
devices can also act as a listener, you may be able to connect
|
||||
things "in series", so a talker-only output is connected to a
|
||||
listener/talker input, and the listener/talker output is
|
||||
connected to other listeners. However, some listener/talker
|
||||
devices may reformat the data, or only pass data they
|
||||
understand. (The Autohelm Seatalk system does this, and claims
|
||||
the data as it's own, starting all output sentences with "$II".)
|
||||
|
||||
Particularly with older equipment, the equipment may claim to
|
||||
comply with NMEA-0183, but in fact have an error in the data
|
||||
format. (My Kings 8001 Loran-C claims to send an APB sentence,
|
||||
but gets some of the fields in the wrong order, so my autopilot
|
||||
can't understand it.) This sort of problem can be verified by
|
||||
capturing the NMEA-0183 data on a computer, and comparing the
|
||||
data formats with those given above.
|
||||
|
||||
|
||||
7. About the author
|
||||
|
||||
This FAQ was written by:
|
||||
Peter Bennett
|
||||
bennett@triumf.ca
|
||||
|
||||
I have an FTP site containing this file, a GPS FAQ, and other
|
||||
NMEA information files and PC programs for capturing and
|
||||
displaying NMEA data, and related things:
|
||||
|
||||
ftp://sundae.triumf.ca/pub/peter/index.html
|
||||
This site is mirrored in Germany at:
|
||||
ftp://ftp-i2.informatik.rwth-aachen.de/pub/arnd/GPS/peter/index.html
|
||||
|
||||
7.1 Acknowlegments
|
||||
|
||||
I would like to thank the following for their contributions
|
||||
or corrections to this document:
|
||||
Tom Clark, clark@tomcat.gsfc.nasa.gov
|
||||
Bob Shearer, t.shearer@unixa.nerc-wallingford.ac.uk
|
||||
David Steckler, davidst@nobeltec.com
|
||||
Karl Olmstead, olmstead@ridgecrest.ca.us
|
||||
Dave Wells, KD6TO, davew@cruzio.com
|
||||
Mike Morrow, caveman@castles.com
|
||||
|
||||
|
||||
|
||||
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Reference in a new issue