This is an interesting project Jeff. Congratulations in getting this far.

I was wondering if there wasn't a bit (no pun intended) of redundancy in your serial protocol. You're currently sending 3 bytes of data, an on/off byte, an X co-ordinate, and a Y co-ordinate.

How about using the LSB of each co-ordinate byte as the on/off bit - so in your first post

1
128
139
1
120
130
0
115
120

would become

1000 0001 - 129 - Laser on + X co-ord of 128
1000 0100 - 139 - Laser on (no change) + Y co-ord of 139
0111 1001 - 121 - Laser on (no change) + X co-ord of 121
1000 0010 - 131 - Laser on (no change) + Y co-ord of 131
0111 0010 - 114 - Laser off + X co-ord of 114
0111 1000 - 120 - Laser off (no change) + Y co-ord of 120

There is a slight (one unit) loss of resolution, but you save a loop through the interrupt each time to compensate.

Alternatively how about sending the offset instead of the absolute co-ordinate each time (assuming that you have no wild variances. Using the same data as above, and assuming you've already reached 129, 139 by stepping there...

Use bit 7 as laser on/off
Use bit 6 as increment (1)/decrement (0)

1000 1000 - 88h - laser on, decrement X co-ordinate by 8
1000 1001 - 89h - laser on, decrement Y co-ordinate by 9
0000 0101 - 05h - laser off, decrement X co-ordinate by 5
0000 1010 - 0Ah - laser off, decrement Y co-ordinate by 10


I can't help feeling that you may be able to squeeze say 3 sets of data into 2 bytes of information.