As far as I remember, the Raymarine plug can comminucate on SeaTalk or NMEA. At least on the st1000 tiller pilots.

So far what you have described is absolutely standard RS232 (async serial) communications with 9 data bits (you don't count the start and stop bit).

And from the Atmega spec on page 180 it says:

A serial frame is defined to be one character of data bits with synchronization bits (start and stop bits),
and optionally a parity bit for error checking.

The USART accepts all 30 combinations of the following as valid frame formats:

• 1 start bit
• 5, 6, 7, 8, or 9 data bits
• no, even or odd parity bit
• 1 or 2 stop bits

So, you want to configure it for 9 data bits, no parity, and 1 stop bit (and of course the right baud rate).

I don't have a SeaTalk to test it with, so how about showing your test code and we'll take it from there?

Ah I see you have a Mega. I was initially confused because they are all Atmega chips, so this is the Mega Atmega, right?

Well, I'm not filled with confidence, but this works converting my Uno to output 9 bits (on normal Serial, not Serial3):

Serial.begin(9600);   
UCSR0B = 0;  // turn off
UCSR0B = 0b10011100;  // turn on again in 9-bit mode


And this might possibly work for Serial3:

Serial3.begin(4800);
UCSR3B = 0;
UCSR3B = 0b10011100;


I'm a little troubled about where that 9th bit is going to come from (after all, normally a byte is 8 bits).

You could try that in reading mode anyway to see if you get reasonable data from the gadget.

Also this page seems relevant:

http://sailboatinstruments.blogspot.com/2011/02/seatalk-interface.html

I looked in the AVR 328 datasheet under the USART section. And indeed the hardware USART can be made to handle 9 data bit sending and receiving data transactions. This is not related to enabling the parity bit, as it appears if you want to utilize the 9 bit mode you can't also have parity enabled, one or the other or neither option are your choices.  At least that's my read on the subject. It is however a little complex and requires extra steps to set up the 9th bit before sending the character out, or reading the 9th bit before reading the 8 bit character on received characters. This is certainly not something you can configure with the standard arduino serial commands. You would have to drop down to direct USART register controls and write your own serial library to accomplish 9 bit transactions. Like I said, a little complex and obscure but it is covered in the 328 datasheet in the USART section.

Lefty

My solutions was to adapt the softserial library to get a 9th data bit. It was quite easy in the recv function. afther the loop, add an extra delay and receive the parity bit.

You have to juggle a bit with data types to get the 9th bit in the recv buffer. I cahnged the type from byte to int and shift the 9th bit completely to the left.

Teeman,

Add me to the list of people interested in seeing your mods.  I'm trying to talk to a raymarine display (no nmea) and it sounds like you have it figured out.  Please share!

David

Thanks JFerg,

Buying a seatalk<->nmea bridge was what I had hoped to avoid though thanks very much for the pointer to a low cost supplier as I may go that way if I have to.  A Seatalk hack would be an awesome contribution to the sailing-arduino fan base!  I understand that the signal would need to be turned into a 0-5 V TTl or rs232 or whatever and I can do that but modifying the softwareserial library to read the 9 bit seatalk protocall is currently beyond me. If someone were to make a library available, they would attain hero status amongst those of us on a budget and who are sailing older boats with 20 year old instruments!

I am building an arduino based autopilot for my boat and I was hoping to use (as additional input) wind direction (it's a sail boat) and boat speed  (to try to make a surfing mode functionality and to adjust rudder response for one of the PID loops) from my existing ST50 instruments.  I also have a spare raymarine ST50 display that I would like to use to display heading info from my fluxgate compass.  The arduino works well for the compass NMEA and for controlling the linear actuator and with relatively little effort I have something that looks like it will drive the boat quite well (its too cold to test on the water now).   Sailing to the wind and a sparkfun gyro are the next steps.

David