Interesting STng to ST1 conversion info

Jun 1, 2009
1,735
Hunter 49 toronto
So, something has puzzled me, (and a lot of other people), for awhile, so I thought I’d dig into it.
As everyone is aware, STng is basically Nmea 2000 with proprietary Raymarine connectors at the cable ends.
Now, here is where the confusion starts:
There are 2 types of STng connectors;
Backbone & Spur
The Backbone is the data trunk, and the Spur connects to devices.
Where this differs from Nmea 2000 is that the backbone & Spur connectors have the same pinout, and number of pins.
Not so on STng
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The Spur has a 6th pin, which is dedicated to Seatalk 1 data.
What makes this more confusing is that Raymarine instruments used to come with a STng to Seatalk 1 cable
A918FAD0-9998-4F8C-A26F-88BC25D31FF4.jpeg


This led people to believe that you could plug this into your new STng instrument, and then plug the other end into a Seatalk 1 head, and everything would work.
Well, it doesn’t work that way.
One would infer that because you received this cable with your new instrument, it was somehow taking its data, and converting it so that your older heads could be used as repeaters, or that you could get the STng data onto your existing Seatalk 1 bus.
The reason this confused me was the lack of a 6th pin on the backbone. This essentially means that any Seatalk1 data cannot be transmitted outside its own T connector environment.
So, here’s how it actually works, and the purpose of the adapter cable shipped with some of the Raymarine gear awhile ago (now you have to buy it separately)
A STng head, such as an I70 does indeed have a STng to Seatalk conversion built into it, but in receive mode only.
In other words, it will accept Seatalk 1 data, propagate it down its 6th spur wire to other instrument heads connected in daisy chain, but it does not convert any of its STng data back to Seatalk 1.
So, what is the purpose of this cable?
Let’s assume you buy a I70 general purpose data display which will repeat anything.
If you have a ST60+ wind display which is connected to the masthead transducer, then by connecting this to the I70 will permit the I70 to display wind.
But, if you get an ITC-5 transducer interface and wire the masthead unit to it, the I70 will display wind, but the ST60+ that you plug into it, (to act as an analog repeater), won’t .

Hope this is helpful to people who are upgrading their instruments !!
 

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Sep 11, 2015
147
Hunter 31 Marina del Rey
Yep, the entire Seatalk 1 to Seatalk NG is a mess. When you connect Seatalk 1 instruments in this way, you should not connect the red 12V Seatalk 1 wire because Seatalk 1 can draw a lot more power and will trip your Seatalk NG connection.
 
Jun 1, 2009
1,735
Hunter 49 toronto
Yep, the entire Seatalk 1 to Seatalk NG is a mess. When you connect Seatalk 1 instruments in this way, you should not connect the red 12V Seatalk 1 wire because Seatalk 1 can draw a lot more power and will trip your Seatalk NG connection.
You bring up an interesting point, but allow me to elaborate a bit.
It is absolutely necessary to have only 1 voltage feed on the STng bus. On new installations, this is often fed in through the autopilot..
Now, if you connect a Seatalk 1 instrument directly through to a STng head which is already on the STng bus, there will definitely be a slight voltage potential difference between the Seatalk 1 red wire output, and the voltage coming down the STng bus. This is due to bus loading (call LEN), which will create voltage drops on the bus.
What happens is you will have two voltages “meeting”, which will only be a fraction of a volt apart, but with very low resistance.
This is why it seems that the Seatalk 1 instruments are sucking a lot of current.
It’s not the instruments consuming power, but the bridging of 2 voltages across a low resistance.
Nevertheless, your point is well taken, that you must be mindful of this when using this adapter cable.
Of course, the correct answer is to pony up $100 and use a STng to Seatalk 1 converter