Master-slave communication protocol
***********************************

byte buffer[]: received data

Buffer structure
================

buffer[0]: command
buffer[1]: length of data
buffer[2..length+2] data


Protocol Commands
=================

Requests
--------

00: status request
   length = 0
   no data

01: relay close request
   length = 0
   no data

02: relay open request
   length = 0
   no data

Replies
-------

80: status reply
   length = 4
   data[0]#7: relay status (1: closed, 0: open)
   data[0]#6: reserved
   data[0]#5: reserved
   data[0]#4: reserved
   data[0]#3: reserved
   data[0]#2: reserved
   data[0]#1: reserved
   data[0]#0: sign of temperature
   data[1]: temperature (in 1/2 degrees)
   data[2]: current, MSB 
   data[3]: current, LSB (in mA)

FF: error reply
  length = 1
  data[0]: faulty command number