26 February 2011

Chetco SeaSmart

Panbo has an item on the new Chetco SeaSmart Serial, Wifi and Ethernet devices.

These seem to be nicely wrapped-up versions of the N2K to Ethernet bridge that has been talked about and that I've been writing myself for my own use.

At the moment the specifications on the protocol are still quite vague, and contain strange terminology (as Jeff Siegel pointed out on Panbo) where they talk about "HTTP Header" that looks like a reference to a NMEA 0183 style message ($Pxxxxx).

Whatever their protocol is going to be, it looks interesting enough to add such a protocol to my NMEA0183 forwarder and integrate it with my NMEA2000 packetlogger application.

25 February 2011

SRT proprietary AIS commands

My Navico NAIS-300, like most other commercial AIS class B products that have been available for a while, uses the SRT AIS class B OEM board. I know the same board is used in the Raymarine 500, True Heading AIS-CTRX, Transas, Digital Yacht AIT250/1000, and many more. The tell-tale seems to be that the AIS transponder comes with Pro AIS software. I have hooked up a serial data logger so that I could find out what NMEA-0183 commands the Pro AIS software sends to the transponder. Unfortunately neither SRT nor any of the vendors publish the protocol.

My research into this started because I wanted to control my AIS transponder's transmit setting using software. For some reason there are many functions that you can assign to the remote button input, but not a reliable level-style on/off of the transmit functionality. This has been added to recent firmware versions, but mine contains older firmware. However, an hour's worth of analysis shows that controlling the transmitter is actually quite easy. In fact, almost everything that you can set up using Pro AIS is easy as pie to implement.


The SRT product, logically, uses the 'P' prefix to indicate Proprietary, followed by SRT. Here is an example of a PSRT message:
Just like all other sentences I document on this page the shown message format is inclusive of the NMEA style checksum bytes at the end of the line, e.g. *49 in the message above.

For some reason some sentences start with a prefix $DUAIQ. I have no theory as to why this is so, maybe the data is intercepted by a different microcontroller.


Some sentences require the passing of a password. Luckily, the protocol to do this is very simple -- just send the following sentence before every authorized message:

Silent mode

To make the AIS silent (not transmit its own position) send the following authorized (prefix with the command above) sentence:

To make the AIS transmit its own position send the following authorized sentence:

Alarm mode

To make the AIS output all alarms every minute send the following authorized sentence:

To make the AIS output only the active alarms send the following authorized sentence:

GPS update speed

To make the AIS output GPS data every second send the following authorized sentence:

To make the AIS output GPS data every four seconds send the following authorized sentence:

GPS data

The SRT board has a complete GPS on-board. For some reason it only sends out two GPS sentences: RMC and GBS. See the GPSD source for more information on these sentences (as well as those below.)

To get the GPS to send out more GPS sentences send the following authorized sentence:
This will cause the board to send out VTG, GGA, GSV, GLL and ZDA sentences as well as RMC and GBS. My particular board has a small bug in that it also starts sending out two copies each of the RMC and the GBS commands.

To get it to stop sending the additional GPS messages, send the following authorized command:

Interrogating the board

There is a whole stack of sentences that can be used to read out system information. These are

LED status
Send: $DUAIQ,LED*29
Recv: $PSRT,LED,a*hh
a bit 1: Power On
a bit 2: TX timeout
a bit 3: Error
a bit 4: SRM status
hh: checksum

Internal data
Send: $DUAIQ,ADC*22
Recv: $PSRT,ADC,a,b,c,d,e,f,g*hh
a: Tx forward power
b: Tx reverse power
c: RSSI Rx 1
d: RSSI Rx 2
e: Internal 3V3 supply
f: Internal 6V supply
g: Supply voltage
hh: Checksum

Station Static Data (AISSD)
Send: $DUAIQ,SSD*20
Recv: $AISSD,a,b,c,d,e,f,g,h*hh
a: Callsign, 8 bytes fixed length; @ for unused bytes
b: Ship's name,20 bytes fixed length; @ for unused bytes
c: GPS antenna distance from bow, in m
d: GPS antenna distance from stern, in m
e: GPS antenna distance from port side, in m
f: GPS antenna distance from SB side, in m
g: DTE
h: Source Identifier
hh: Checksum

Send: $DUAIQ,010*55
Recv: $PSRT,010,,,c*hh
a: ?
b: ?

OEM name
Send: $DUAIQ,SRM*28
Recv: $PSRT,SRM,a,b,c*hh
a: ?
b: ?
c: OEM name used in AIS messages, 7 bytes fixed length; unused bytes are filled with @.
hh: Checksum

Vessel Static Data (VSD)
Send: $DUAIQ,VSD*25
Recv: $AIVSD,a,b,c,d,e,f,g,h*hh
a: Type of ship and cargo. For recreational use: 36 = Sailing vessel, 37 = Pleasure craft
b: Maximum present draught, always 00.0
c: Persons on board, always 0000
d: Destination, always @@@@@@@@@@@@@@@@@@@
e: Est. UTC of arrival, always 000000
f: Est. day of arrival, always 00
g: Est. month of arrival, always 00
h: Navigational status, always 00
i: Regional application flags, always 00
hh: Checksum

Software version
Send: $DUAIQ,SWF*26
Recv: $PSRT,SWF,a,b*hh
a: AIS software version
b: FPGA version
hh: Checksum

Transponder Serial Number
Send: $DUAIQ,SNO*36
Recv: $PSRT,SNO,a*hh
a: Serial number in ASCII, 10 digits. Usually all 0.

Reset Data Programming

The following command can be used to reset the AIS back to its factory settings, inclusive of the MMSI number, so that it can be reprogrammed, for instance when you want to sell your AIS transceiver.


I have received numerous reports from people who tell me that this works fine, over a range of devices. Just make sure your AIS came with ProAIS and you should be OK.

19 February 2011

NMEA 2000 connector pinout - Smart!

Today my attention fell to this diagram in a NMEA 2000 device manual from Maretron:

As you can see the pin-out is:

Pin 1Shield-Shield
Pin 2RedNET-S+12V
Pin 3BlackNET-C0V
Pin 4WhiteNET-HCAN-H (data high)
Pin 5BlueNET-LCAN-L (data low)

I noticed that the center pin is not carrying ground or even shield, which I naively thought would be the case. In fact the designers of this pin-out were a lot smarter! As this is a circular connector, a dumb user could theoretically try to push two connectors together whilst forcing a wrong orientation. That would force power down the wrong wires.

Let's see what happens if you rotate the connectors:
FemaleMale 90° Male 180° Male 270°
Shield+12V 0V CAN-H>
+12V 0V CAN-H> Shield
0V CAN-H Shield +12V
CAN-H Shield +12V 0V

The chosen layout ensures that there is no case where both +12V and 0V are connected at the same time to the two data connections, so if you are really obnoxious and try to ram a connector the wrong way into the T socket the worst that will happen is that the fuse will blow. Once you remedy the situation and replace or reset the fuse the entire system should come back up unharmed. That is Really Cool!

17 February 2011

Simnet Noise Filter

Recently I became aware of the fact that (a) Simrad has switched to a new specification for their Simnet cabling and (b) that there is a "Simrad Noise Filter" that they recommend for installation in sailing boats.

As you can see here in an report from Ben @ Panbo on N2K cable mixing, Simnet cables used to contain a yellow/blue pair for the data wires. Apparently this type of cable caused issues as the impedance of a long stretch was not according to NMEA specifications. This naturally ends up giving problems when you have a IS 20 wind instrument at the other end of a 20m cable. In my case it's even worse as I had to order a 30m cable as my mast is 25m tall.

I've been told that Simrad has switched to a different cable that has white/blue data wires which has better characteristics. To 'fix' existing installations Simrad has also produced a "Simnet Noise Filter", P/N 24006934. The Simrad NSE documentation states on PDF page 32:

For a large systems or sailboats with a mast head wind sensor and long mast cable, it is recommended to use a SimNet noise filter (24006934). On sail boats the filter should be inserted at the mast junction. On larger systems without a mast head unit, the filter should be inserted centrally along the backbone.

The original 2006 era SimNet installation manual contains no reference to this. Probably these issues have become more prevalent with larger networks being deployed.

I've ordered such a noise filter. I've been told it contains a 120 Ohm termination resistor as well as a 220 μF capacitor to damp out supply issues. Once it gets delivered I'll report on what it contains and the effects on my network.