Packet Formats

Numerical Values

All numerical values received and transmitted to the controller are transmitted Most Significant Byte (MSB) first.

Number Value Scaling

Numerical values may be represented in one of two ways:

  • Integer with scaling
    • The floating-point number is multiplied by the Scaling factor, then converted to an integer.
    • As an example, consider the Set Current command, which accepts a target current of datatype int32 with scaler of 1000
    • To command a current of 10.5 amps, you multiply by the scaler (1000) to get 10500.0, then send that as an integer (10500).
    • For reading telemetry from the device, you do the opposite – convert to a floating point number, then divide by the scaler.
  • Direct floating point

UART Interface

The external system sends command messages to the MC and receives telemetry from the MC. Command and telemetry messages use the proprietary protocol listed below.

Byte LengthValue
Start byte10x02 for short packets that are 255 bytes or less
0x03 for long packets
Packet length1 if short packet
2 if long packet		Number of bytes of the Data section of the packet.
Data1..nFirst byte is always PID. Any number of bytes may follow.
CRC216-bit CRC of the Data portion of the packet
Stop byte10x03

All variables transmitted over this interface are transmitted in Most Significant Byte order, also referred to as big-endian.

All packets start with a Start Byte that is determined by the length of the packet. A typical packet is less than 255 bytes, so the start byte is usually 0x02. If the packet is longer than 255 bytes, then the start byte is 0x03.

The packet length is the number of bytes contained within the Data portion of the packet. The packet length is 1 byte long if the Data length is 255 bytes or less, and 2 bytes long if the Data length is more than 255 bytes.

The Data section of the packet is always started with the Packet Identifier. An arbitrary number of packet data bytes may follow.

The CRC calculation is listed below.

CRC Calculation

The CRC is always calculated over the Data bytes of the packet.

The CRC algorithm is the CRC16-CCITT using the polynomial 0x1021: X16 + X15 + X2 + 1. The CRC seed value is 0x0000.

The algorithm in C-language is listed below.

// CRC Table
const unsigned short crc16_tab[] = { 0x0000, 0x1021, 0x2042, 0x3063, 0x4084,
  0x50a5, 0x60c6, 0x70e7, 0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad,
  0xe1ce, 0xf1ef, 0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7,
  0x62d6, 0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de,
  0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485, 0xa56a,
  0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d, 0x3653, 0x2672,
  0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4, 0xb75b, 0xa77a, 0x9719,
  0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc, 0x48c4, 0x58e5, 0x6886, 0x78a7,
  0x0840, 0x1861, 0x2802, 0x3823, 0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948,
  0x9969, 0xa90a, 0xb92b, 0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50,
  0x3a33, 0x2a12, 0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b,
  0xab1a, 0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41,
  0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49, 0x7e97,
  0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70, 0xff9f, 0xefbe,
  0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78, 0x9188, 0x81a9, 0xb1ca,
  0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f, 0x1080, 0x00a1, 0x30c2, 0x20e3,
  0x5004, 0x4025, 0x7046, 0x6067, 0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d,
  0xd31c, 0xe37f, 0xf35e, 0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214,
  0x6277, 0x7256, 0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c,
  0xc50d, 0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
  0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c, 0x26d3,
  0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634, 0xd94c, 0xc96d,
  0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab, 0x5844, 0x4865, 0x7806,
  0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3, 0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e,
  0x8bf9, 0x9bd8, 0xabbb, 0xbb9a, 0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1,
  0x1ad0, 0x2ab3, 0x3a92, 0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b,
  0x9de8, 0x8dc9, 0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0,
  0x0cc1, 0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8,
  0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0 };

unsigned short crc16(unsigned char *buf, unsigned int len) {
  unsigned int i;
  unsigned short cksum = 0;
  for (i = 0; i < len; i++) {
    cksum = crc16_tab[(((cksum >> 8) ^ *buf++) & 0xFF)] ^ (cksum << 8);
  }
  return cksum;
}

The stop byte is always 0x03.

CAN Bus Interface