SPI question

Just a little enhancement to reduce the code size: i guess its unnecessary to define the var fault. You stored the information in the object (see line 17.) why not use the object as default?

trtn.fault = (d.charCodeAt(3) & 7);
    switch (trtn.fault) { ... }

and you can combine these lines:

var trtn={};
trtn.temp=(d.charCodeAt(0)<<6 | d.charCodeAt(1)>>2)*0.25;
trtn.fault=0;
-------------------------
var trtn={
    temp: (d.charCodeAt(0)<<6 | d.charCodeAt(1)>>2)*0.25
    fault: 0
};

Sorry, to be a little bit niggling :-) Memory is a limited resource! (it's hard to say this in a time of 2014)