See: http://wiki.hackspace.org.uk/wiki/Projects/LHS_Graphs_and_Visualizations#Space_occupancy

Serial control format

Commands - no newline after char, only last char in buffer used:

'X' - beam off
'L' - beam on
'S' - poll for status
'R' - reset counter
'I' - increment
'D' - decrement
'M' - silence the current alarm

Status response - sent on poll:

[K|A][I|O|N][X|L][count]\n

'K' - Okay - beams are not obstructed
'A' - Alarm - beams are obstructed
'I' - Last event was a person coming in.
'O' - Last event was a person going out.
'N' - No previous events (just started).
'X' - Beams are off.
'L' - Beams are on.
count - number of occupants.