Understanding NMEA 0183

[Graham_Wright]

As a start, I need a
Pardon our interruption....
At £45.99 for piece of USB cable?!!!!!!
I don't think so!

 

[AngusMcDoon]

It's not a cable, it's a converter. USB and NMEA0183 are very different. You can get away with a cheapo USB to RS232 converter, but they are not NMEA0183 compliant. The NMEA0183 standard requires opto-isolation of the inputs and RS422 voltage level outputs which a cheapo USB to RS232 converter won't have. Hopefully this one does.

 

[S04]

I have a couple of those cables which I have used to connect a Digital Yacht AIS into a plotter. and from the PC based nmea simulator on my PC to my Openplotter RPi. One I bought from RS Components the other from Aliexpress (or similar).

The spec you want is USB-RS232-WE-1800-BT_0.0

USB-RS232 - self explanatory
WE - wire ended
1800 - length in mm
BT - Black cable, Transparent plug (some come with a black plug but the advantage of the transparent one is that there are LEDs to show Tx/Rx activity).
0.0 - 0V on the red wire (this is important, you can get this cable configured with 0V, 3.3V or 5V on the red wire, it MUST be 0V)

I'd be happy to lend you one if you like.

 

[Graham_Wright]

It's not a cable, it's a converter. USB and NMEA0183 are very different. You can get away with a cheapo USB to RS232 converter, but they are not NMEA0183 compliant. The NMEA0183 standard requires opto-isolation of the inputs and RS422 voltage level outputs which a cheapo USB to RS232 converter won't have. Hopefully this one does.

I hadn't spotted the components in the plug. Still seems a lot to achieve photo isolation and a level change.

 

[AngusMcDoon]

I hadn't spotted the components in the plug. Still seems a lot to achieve photo isolation and a level change.

There's a lot more to USB to NMEA0183 conversion than just opto-isolation and level change. The data formats and hardware are completely different. NMEA0183 at hardware level is just RS422 - a byte at a time with some control bits on each one. USB has speed negotiations, power negotiations, host/device negotiations, device class type, manufacturer and device identification, control transfers, real time transfers, bulk transfers, serial or 4-bit parallel data transfer negotiations - the complexity goes on and on, but fortunately companies like Prolific make cheap interface chips that do the bulk of the work for you, but there will still be a microcontroller in their driving the interface chip. The opto-isolation and RS422 signal levels, if there, is the final part at the NMEA0183 end.

Looking at the other similar items linked in later posts your linked device may not be fully NMEA0183 compliant as it looks too similar to the cheapies, which suggests it has RS232 style output, not isolated RS422. If that's the case, you are right. It's a rip off.

If it is a different design that is compliant, it's not the extra components that you are paying for - they cost pennies. It's the cost of design, testing, manufacturing setup, compliance and certification that is shared by only low sales volume. This explains the higher cost of marine electronics versus consumer electronics. When I worked for B&G the projected lifetime sales volume of some marine electronics devices was in the thousands, while when I was at Samsung the lifetime sales of a Galaxy phone was in the tens of millions.

 

[S04]

The adapter cable the OP links to is sold by Digital Yacht as their NMEA0183 to USB adapter but is not made by them. It is a standard component made by FTDI, Digital Yacht just rebadge it.

I have found that the FTDI chips are more reliable than say Prolific or Silicon Labs and I have never had issues with drivers (Several flavours of Linux, Windows and Raspberry Pi OS).

I've attached the guide for the Digital Yacht cable which clearly states that it does not provide opto-isolation.

This cable is designed to connect conventional NMEA 0183 or RS232 serial ports to a PC via a USB port. It does not
provide any optical isolation and should full opto-isolation be required, then this cable should not be used. However, in
most pleasure boat marine installations, opto-isolation is not necessary and this cable will allow good electrical interfacing
between the PC and Marine Navigation Equipment.

All I can say is that I have used these for close to 15 years now ashore and afloat without a single issue. I've used it with Digital Yacht electronics (AIT2000) and elsewhere where I need to transfer nmea0183 data over serial/USB.

If you buy the cable from Digital Yacht you will pay a premium. If you search for what the cable actually is, an FTDI USB-RS232-WE-1800-BT_0.0 you will find it cheaper. It's the same cable that DY sell. It is important that you make sure the serial chip is indeed an FTDI FT232R. Otherwise it is not the same as the one DY sell and recommend.

As I stated before, I'd be happy to lend the OP one if required. I have a few of them.

 

[Tintin]

What is opto-isolation?

 

[AngusMcDoon]

What is opto-isolation?

A means of transferring a data signal from one place to another with no electrical connection. The data goes over an optical link via a pulses of light. It means that if there is an over-voltage fault on the communications line then devices connected to that line won't get their electronics fried. Boats, like cars, are electrically noisy places. When a start motor stops a voltage spike can happen on the boat's power lines which may be transferred to communications links like NMEA0183, NMEA2000 or Seatalk etc. That may cause them to cease to be, but if it happens then you want to limit the extent of the damage by isolating their input circuits from the spike.

NMEA0183 requires opto-isolation on inputs as a mandatory feature. If a device does not have it then it is not compliant with NMEA0183.

There are of course limits to the protection. A 20 million volt lightening strike or a nuclear weapon's EMP is going to frazzle everything. Static from walking across a polyester carpet on a dry day can be surprisingly damaging as well, but electronics has improved over the decades to cope better with this.

 

[Graham_Wright]

The adapter cable the OP links to is sold by Digital Yacht as their NMEA0183 to USB adapter but is not made by them. It is a standard component made by FTDI, Digital Yacht just rebadge it.

I have found that the FTDI chips are more reliable than say Prolific or Silicon Labs and I have never had issues with drivers (Several flavours of Linux, Windows and Raspberry Pi OS).

I've attached the guide for the Digital Yacht cable which clearly states that it does not provide opto-isolation.

This cable is designed to connect conventional NMEA 0183 or RS232 serial ports to a PC via a USB port. It does not
provide any optical isolation and should full opto-isolation be required, then this cable should not be used. However, in
most pleasure boat marine installations, opto-isolation is not necessary and this cable will allow good electrical interfacing
between the PC and Marine Navigation Equipment.

All I can say is that I have used these for close to 15 years now ashore and afloat without a single issue. I've used it with Digital Yacht electronics (AIT2000) and elsewhere where I need to transfer nmea0183 data over serial/USB.

If you buy the cable from Digital Yacht you will pay a premium. If you search for what the cable actually is, an FTDI USB-RS232-WE-1800-BT_0.0 you will find it cheaper. It's the same cable that DY sell. It is important that you make sure the serial chip is indeed an FTDI FT232R. Otherwise it is not the same as the one DY sell and recommend.

As I stated before, I'd be happy to lend the OP one if required. I have a few of them.

As you say - cheaper.
Pardon our interruption...
£8.20.
I have a problem in that I like to understand things. With a background in main frame computer design, I find it difficult to accept my limitations.
I have been trying to interface GPS which outputs as SeaTalk or NMEA0183 at 4800 baud (but not both).
A VHF radio that requires GPS position as NMEA 4800.
A Matsutek AIS engine that outputs at 38400 baud (and also GPS with no details explained).
A Raymarine E80 that will accept SeaTalk and NMEA at 4800 or 38400 baud (but not both).
My solution was to input GPS Seatalk to a Quark multiplexer to convert to NMEA4800 for the radio and set the E80 to accept AIS at 38400 baud. GPS as SeaTalk will be accepted by the E80.
So far, this has not worked. (I have requested help from Quark and await their response).
I have acquired a 'scope with memory which displays expected signals but, of course, does not decode them.
Next step was to interface the various signals with my laptop which is a Mac so probably that presents another hurdle!

 

[S04]

That is not the correct cable. It has 5V on the red wire. You need one with 0V.
Also that one is RS485, you need RS232.

Please see my comments in post #3.

 

[Graham_Wright]

Th

That is not the correct cable. It has 5V on the red wire. You need one with 0V.
Also that one is RS485, you need RS232.

Please see my comments in post #3.

Thanks for the offer of a loan but I will respectfully decline! I am always fearful of breaking loaned items and have found items I have loaned have been returned damaged which causes embarrassment.
"Never a borrower or lender be!"