Nav,

As you know, many talkers can send out different strings, and different versions of nmea... just to add to the complexity of NMEA.

My understanding was that Richard wanted to display two items on his echo sounder:

1. Windspeed/Direction ($--MWV) and
2. Position ($--GLL) or ($--GGA)

To keep things easy, he needs to turn off all other sentances coming out of both the GPS.

Instead of detecting the $ (start of string) and LF (end of string) he could just let the MCU just toggle every couple of seconds, say three.

That way, if a sentance does get chopped off, the echo sounder should just ignore the bad string.

The other thing to consider is the time it takes the echo sounder to bring up an alarm when an string input is missing.

This is how I would do it.

PS
(Thinking more about it, you could do this using a JK Flip Flop, and couple of Gates)