Serial Communications

For those who are interested in the low-level technical nitty gritty of transmission of NMEA sentences, the following links may be helpful.

NMEA 0183 transmission is based on the asynchronous communications technique.
An explanation can be found here:

 http://penguin.dcs.bbk.ac.uk/academic/technology/physical-layer/asynchronous/

Voltage-levels represent data-bits.  Although the principles of asynchronous communication always apply, there are different standards that define the exact electrical characteristics.

For NMEA0183  the RS422 standard applies, but many manufacturers do not strictly adhere to this standard. The RS232 standard is the most common one,  primarily for computer communications, but it is also widely used for marine electronics applications. As there is considerable overlap in the electrical characteristics defined for the different standards,  in practice communication between devices using RS422 and RS232 is often possible, as long as the baudrate, the character length in bits, the number of start and stop bits and the parity are the same.

For standard NMEA transmission, these are: 4800 bps (bits per second), 8 bits, 1 start and 1 stop bit, no parity. For transmission of AIS data, the baudrate is 38400.

Explanations of the RS422 and RS232 standards can be found on many sites on the net, but here is a link anyway:

http://www.lammertbies.nl/comm/info/RS-422.html