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Topic: Checksum calculation help.  (Read 250 times) previous topic - next topic


I will do my best to explain what I am doing, but if I miss something please let me know.

I am using:
Arduino Mega
RS485 Shield.
ROBOTIS Dynamixel 'X' Series ( XM430-W210-R) Smart servos.

The library I am using does not support the 'Acceleration' feature I want to use.
I was able to generate this line of hex that worked,

Code: [Select]
byte M7[] =  {0xff, 0xff, 0xfd, 0x00, 0x03, 0x09, 0x00, 0x03, 0x70, 0x00, 0xaa, 0x00, 0x00, 0x00, 0xff, 0x21};
Serial.write(M7, sizeof(M7));

FF,FF,FD,00 : this is the header with a reserved bit of '0'
03 : This is the address of the Dynamixel In this case ID#:3
09, 00 : This is how many remaing bytes to expect. not including these 2.
03 : This is the command to write data to the servo.
70 : This is the hex value of '112' Or the position in the control table of the servo, in this case it is Velocity.


00 : blank
AA : This is the hex value of '170' the Acceleration I want.
00, 00, 00 : Blank space.
FF, 21 : This is the CRC Checksum.

I am using protocol 2.0

Now with all of that said,

I want to generate the same above command, but instead of ID#3, I want to use the broadcast ID of 254 (0xFE).

The problem I am running into is the checksum (CRC) calculation,

They supply this example code,

Lets just say I am a bit confused on the implementation.

I am asking for pointers on how best to go about this calculation?

I only ever need to generate this once. so not sure if it is worth create the code to do so.



This looks like it may do it.

Code: [Select]
const unsigned short crc_table[256] = {0x0000,
                                0x8005, 0x800F, 0x000A, 0x801B, 0x001E, 0x0014, 0x8011,
                                0x8033, 0x0036, 0x003C, 0x8039, 0x0028, 0x802D, 0x8027,
                                0x0022, 0x8063, 0x0066, 0x006C, 0x8069, 0x0078, 0x807D,
                                0x8077, 0x0072, 0x0050, 0x8055, 0x805F, 0x005A, 0x804B,
                                0x004E, 0x0044, 0x8041, 0x80C3, 0x00C6, 0x00CC, 0x80C9,
                                0x00D8, 0x80DD, 0x80D7, 0x00D2, 0x00F0, 0x80F5, 0x80FF,
                                0x00FA, 0x80EB, 0x00EE, 0x00E4, 0x80E1, 0x00A0, 0x80A5,
                                0x80AF, 0x00AA, 0x80BB, 0x00BE, 0x00B4, 0x80B1, 0x8093,
                                0x0096, 0x009C, 0x8099, 0x0088, 0x808D, 0x8087, 0x0082,
                                0x8183, 0x0186, 0x018C, 0x8189, 0x0198, 0x819D, 0x8197,
                                0x0192, 0x01B0, 0x81B5, 0x81BF, 0x01BA, 0x81AB, 0x01AE,
                                0x01A4, 0x81A1, 0x01E0, 0x81E5, 0x81EF, 0x01EA, 0x81FB,
                                0x01FE, 0x01F4, 0x81F1, 0x81D3, 0x01D6, 0x01DC, 0x81D9,
                                0x01C8, 0x81CD, 0x81C7, 0x01C2, 0x0140, 0x8145, 0x814F,
                                0x014A, 0x815B, 0x015E, 0x0154, 0x8151, 0x8173, 0x0176,
                                0x017C, 0x8179, 0x0168, 0x816D, 0x8167, 0x0162, 0x8123,
                                0x0126, 0x012C, 0x8129, 0x0138, 0x813D, 0x8137, 0x0132,
                                0x0110, 0x8115, 0x811F, 0x011A, 0x810B, 0x010E, 0x0104,
                                0x8101, 0x8303, 0x0306, 0x030C, 0x8309, 0x0318, 0x831D,
                                0x8317, 0x0312, 0x0330, 0x8335, 0x833F, 0x033A, 0x832B,
                                0x032E, 0x0324, 0x8321, 0x0360, 0x8365, 0x836F, 0x036A,
                                0x837B, 0x037E, 0x0374, 0x8371, 0x8353, 0x0356, 0x035C,
                                0x8359, 0x0348, 0x834D, 0x8347, 0x0342, 0x03C0, 0x83C5,
                                0x83CF, 0x03CA, 0x83DB, 0x03DE, 0x03D4, 0x83D1, 0x83F3,
                                0x03F6, 0x03FC, 0x83F9, 0x03E8, 0x83ED, 0x83E7, 0x03E2,
                                0x83A3, 0x03A6, 0x03AC, 0x83A9, 0x03B8, 0x83BD, 0x83B7,
                                0x03B2, 0x0390, 0x8395, 0x839F, 0x039A, 0x838B, 0x038E,
                                0x0384, 0x8381, 0x0280, 0x8285, 0x828F, 0x028A, 0x829B,
                                0x029E, 0x0294, 0x8291, 0x82B3, 0x02B6, 0x02BC, 0x82B9,
                                0x02A8, 0x82AD, 0x82A7, 0x02A2, 0x82E3, 0x02E6, 0x02EC,
                                0x82E9, 0x02F8, 0x82FD, 0x82F7, 0x02F2, 0x02D0, 0x82D5,
                                0x82DF, 0x02DA, 0x82CB, 0x02CE, 0x02C4, 0x82C1, 0x8243,
                                0x0246, 0x024C, 0x8249, 0x0258, 0x825D, 0x8257, 0x0252,
                                0x0270, 0x8275, 0x827F, 0x027A, 0x826B, 0x026E, 0x0264,
                                0x8261, 0x0220, 0x8225, 0x822F, 0x022A, 0x823B, 0x023E,
                                0x0234, 0x8231, 0x8213, 0x0216, 0x021C, 0x8219, 0x0208,
                                0x820D, 0x8207, 0x0202 };

void dxlUpdateCrc(uint16_t *p_crc_cur, uint8_t data_in)
  uint16_t crc;
  uint16_t i;

  crc = *p_crc_cur;

  i = ((unsigned short)(crc >> 8) ^ data_in) & 0xFF;
  *p_crc_cur = (crc << 8) ^ crc_table[i];


And I think I got it,

What I need =  {0xff, 0xff, 0xfd, 0x00, 0xFE, 0x09, 0x00, 0x03, 0x70, 0x00, 0xaa, 0x00, 0x00, 0x00, 0x1b, 0xcd};


For anyone with the same problem in the future,

I used this,


The section CRC-16/BUYPASS was what I used, I had to swap the numbers.
Example if it shows AABB you will switch it to 0xBB 0xAA

So my final header code is this,
Code: [Select]
Serial.begin(1000000); // serial for the USB port monitor / debug //57600 is stock speed.

byte M2[] =  {0xff, 0xff, 0xfd, 0x00, 0xFE, 0x03, 0x00, 0x01, 0x31, 0x42};
byte M3[] =  {0xff, 0xff, 0xfd, 0x00, 0xFE, 0x06, 0x00, 0x03, 0x40, 0x00, 0x00, 0x2e, 0x16};
byte M4[] =  {0xff, 0xff, 0xfd, 0x00, 0xFE, 0x06, 0x00, 0x03, 0x0b, 0x00, 0x03, 0xb8, 0x13};
byte M5[] =  {0xff, 0xff, 0xfd, 0x00, 0xFE, 0x06, 0x00, 0x03, 0x40, 0x00, 0x01, 0x2b, 0x96};
byte M6[] =  {0xff, 0xff, 0xfd, 0x00, 0xFE, 0x09, 0x00, 0x03, 0x6c, 0x00, 0x00, 0x00, 0x00, 0x00, 0x35, 0x65};
byte M7[] =  {0xff, 0xff, 0xfd, 0x00, 0xFE, 0x09, 0x00, 0x03, 0x70, 0x00, 0xaa, 0x00, 0x00, 0x00, 0x1b, 0xcd};

Serial.write(M3, sizeof(M3));
Serial.write(M4, sizeof(M4));
Serial.write(M5, sizeof(M5));
Serial.write(M6, sizeof(M6));
Serial.write(M7, sizeof(M7));

Put that before the dxl.begin.

This will send the velocity of 170 to all servos on the chain, and ping them to start.

I know there is better ways to do this, but like I said, I only needed this once per power cycle.

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