The documentation you’re looking for is linked from the developer page for the dotbox:
http://www.multitech.net/developer/products/multiconnect-mdot-box-and-evb/

If using LoRa Demo, Survey Single, or Survey Sweep mode: http://www.multitech.net/developer/software/dot-box-and-evb-software/data-packet-format/

If using Survey GPS mode:

Survey GPS

Hi Jeffrey,

As you already discovered, you need to use byte format not UTF-8. UTF-8 will not represent values above 0x7f in one byte. The following link details UTF-8.
https://tools.ietf.org/html/rfc3629

As for your payload, the debug output is displaying it as decimal but our documentation is in hex. Your payload is:
14, 255, 255, 15, 8, 6, 2, 111, 5, 0, 77, 11, 1, 208
From the link http://www.multitech.net/developer/software/dot-box-and-evb-software/data-packet-format…
0x0E or 14 decimal is ‘Acceleration 3-Axis Measured’. So it contains 3 bytes of data. In your case, 255, 255, 15. The next set of data starts with 8 which is pressure and has 3 bytes. So 6, 2, 111. Next is 5 which is lux and is given in 2 bytes as 0, 77. Last is 11 or 0x0B for temperature with 1, 208.

Again looking at the data-packet-format page, you see that the temp value is 0.0625 Degrees C (Signed MSB). First take 1, 208 and convert to a full 16 bit value. The first byte is trivially 0000 0001. The second byte is decimal 208 or 1101 0000 or 0xd0.
Combining the two bytes yields a hexadecimal value 0x1d0 or 464 decimal… 464 x 0.0625 = 29°C.

Kind regards,
Leon

This post really helped me. Belated 2017 thanks… sharing back my resultant TTNv3 payload decoder for box/evb lora-demo: hope it is of use to someone…

function decodeUplink(input) {
var data = {};

var acc_x = input.bytes[1];
var acc_y = input.bytes[2];
var acc_z = input.bytes[3];
var press = (input.bytes[5]<<16) | (input.bytes[6]<<8) | input.bytes[7];
var lux = (input.bytes[9]<<8) | input.bytes[10];
var temp = (input.bytes[12]<<8) | input.bytes[13];
data.acc_x = acc_x * .0625;
data.acc_y = acc_y * .0625;
data.acc_z = acc_z * .0625;
data.press = +((press * 0.25 / 1000).toFixed(2));
data.lux = lux;
data.temp = +((temp * 0.0625).toFixed(1));

return {
data: data
};
}