http://forum.arduino.cc/index.php?topic=227807.new;topicseen#new
Read Reply#22 & 26 on the above post.
NMEA is RS485 , NOT TTL. You can't connect the A & B NMEA signals to TTL . You need a MAX485 IC to convert RS485 to TTL
Post the datasheet for you NMEA device or a photo of it taken with your cell phone that shows the phoenix connectors on it and
the label, particularly the ones that say XXa & XXb and post the photo and the datasheet or a link to the manual.