(long post)

>as well as having examined the NMEA 0183 spec and all I have to do now is
>to figure out how to extract a five byte string out of a sentence that
>begins with 4 unique recognizable ASCII characters and is followed by a
>number of commas. At 4800 baud or faster I guess I would have to UART all
>the data in, then parse for my favorite characters, which will fall
>between say, the 7th & 8th comma.

Not pic, not nmea, but normally you do not keep data you do not need,
instead build a state machine that recognizes the characters one by one.
You then examine each received character and do something with it
depending on where you are in the state diagram of the fsm (you get to
draw this - you know - bubbles with text and arrows).

Then you write have code like:

        state = START;  // have to start somewhere

loop: // forever, or at least while wanting to decode input
        c = read_char();
        switch(state) { // test state against some constants
          case START:
            if(c == FIRST_CHAR)
              state = FIRST_SEEN;
            break; // not seen, just stay in START and look at next char
          case FIRST_SEEN: // seen, expect 2nd
            if(c == SECOND_CHAR)
              state = SECOND_SEEN; // got it
            else
              state = START; // bad char seen, try again from start
            break;
          // more cases like this ...
          case BEFORE_FIRST_USEFUL_GROUP: // ! this is important for ea. group
            if(isuseful(c)) { // start storing here, continue in next state
              isuseful[0] = c; // it's good, remember it
              isusefulcounter = 1; // got the 1st already
              state = FIRST_USEFUL_GROUP; // in the group now, *next* state
            } else
              state = START; // bad char, try from start
            break;
          case FIRST_USEFUL_GROUP: // with each c. in this group it except
                                   // the first!
            if(isuseful(c)) {
              if(isusefulcounter >= MAX_USEFUL) { // got too many ?
                 scream_and_die(); // one of these will work for you
                 state = START;
                 break;
                 // eat_sliently();
              }
              isuseful[isusefulcounter++] = c; // a good un', loop for more
            } else {
              if(c == SEPARATOR) { // say, comma, or space ?
                // done reading a group of interesting chars, use them
                do_someting_useful_with(isuseful[]); // bad syntax, good idea
                state = BEFORE_SECOND_USEFUL_GROUP; // ea. group has a 'before'
              } else
                state = START; // bad char, you know what to do
            }
            break;
          case BEFORE_SECOND_USEFUL_GROUP: // cont. as above
          // more like this as needed ...
          default: // catch all, no such thing as an undefined state
            printf("This state (%d) does not exist. Go to sleep.\n", state);
            state = START; // you may want to stop the program instead
        }
        goto loop; // or equiv. interrupt-driven 'loop'

Now this is verbose. Yacc and Flex produce similar code but in a much more
compact way, representing the states and transitions as bytecode. They are
not recommended for small pics (the code is huge). To write your own you
can do it by hand (one off) or write a tool that produces such a state
machine from a definition (like yacc does). A one off starts with a
drawing of the state machine (bubbles and arrows) and then you implement
it, each bubble is a state (and has a case statement) and each arrow out
of a bubble is equivalent to a tested condition in that case code (falling
out the bottom of the case also represents an arrow). Relatively easy
troubleshooting involves counting the states (must be == to the number
of bubbles in the drawing) and the exits from each case (must == the
number of arrows *starting* from the bubble whose case is being examined).

good luck,

Peter

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