Scanse Sweep V1 - Appendix

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APPENDIX USER’S MANUAL SWEEP V1.0

Authenticating Receipts (Does not apply to Data Block)

Authentication of a valid receipt is accomplished by a checksum, which uses the Status (2 bytes) and Sum of

Status (1 byte) from the receipt. To perform the checksum, the received Sum of Status bytes is checked against a

calculated sum of the 2 Status bytes. The protocol design allows for performing the checksum visually in a

terminal, which requires all bytes in a receipt to be legible ASCII values (ex: '00P'). Therefore, performing the

checksum in code is not intuitive. It works like this:

• The status bytes are summed

• The lower 6 bits (Least Significant) of that sum are then added to 30Hex

• The resultant value is compared with the checksum byte

//statusByte#1 + statusByte#2

let sumOfStatusBytes = status1_byteValue + status2_byteValue;

//grab the lower (least significant) 6 bits by performing a bit-wise AND with 0x3F (ie: 00111111)

let lowerSixBits = sumOfStatusBytes & 0x3F;

//add 30Hex to it

let sum = lowerSixBits + 0x30;

return ( sum === checkSumByteValue );

Example: Consider the common case of '00P' (decimal -> [48, 48, 80], hex -> [0x30, 0x30, 0x50])

0x30 + 0x30 = 0x60 // sum of the status bytes

0x60 & 0x3F = 0x20 // retrieve only the lower 6 bits

0x20 + 0x30 = 0x50 // calculate the ASCII legible sum

0x50 = 'P' // translate to ASCII

Parsing Data Block 16-bit integers and floats

The Data Block receipt includes int-16 and float values (distance & azimuth). In the case of distance, the value is a

16-bit integer. In the case of the azimuth, the value is a float which is sent as a 16bit integer and must be

converted back to a float.

In either case, the 16-bit int is sent as two individual bytes. The lower order byte is received first, and the higher

order byte is received second. For example, parsing the distance:

//assume dataBlock holds the DATA_BLOCK byte array

//such that indices 3 & 4 correspond to the two distance bytes

let distance = (dataBlock[4] << 8) + (dataBlock[3]);

For floats, start by using the same technique to acquire the 16-bit int. Once you have it, you can perform the

conversion to float like so:

//assume dataBlock holds the DATA_BLOCK byte array,

//such that indices 1 & 2 correspond to the two azimuth bytes

let angle_int = (dataBlock[2] << 8) + (dataBlock[1]);

let degree = 1.0 * ( (angle_int >> 4) + ( (angle_int & 15) / 16.0 ) );