Servo - May 2009

return returncode;

SCREENSHOT 2. This is an MPLAB IDE Watch window view of the recv_buff array contents and the status message bytes I chose to parse and retain. As you can see by the value of rc, everything worked as designed.

Note that the read_control_table function incorporates our core send_ packet and get_packet functions to retrieve the Dynamixel AX- 12+’s Control Table contents. Here’s the code I used to fill the recv_buff bytes you see in Screenshot 3:

 parm_len = 3;
break;

//***********************************************
//*MAIN MAIN MAIN MAIN MAIN
//***********************************************
void main()
{

init();
cleaner();
rc = read_control_table(0x01);
++scratch8; // debugger break point

I didn’t show you all of the status message bytes in Screenshot 3. However, you can look at the LENGTH byte at offset 3 to get an idea as to how many bytes follow it. The 0x34 LENGTH byte tells us that 52 bytes including the checksum and ERROR byte follow the LENGTH byte. Add the LENGTH byte, the ID byte, and the pair of sync characters to the 52 bytes in the digital packet and you get a complete digital packet length of 56 bytes.

I’m sure you can see the advantages of being able to read and interpret the AX- 12+’s Control Table. It’s also important to be able to write to an actuator’s Control Table. So, let’s assemble a function that is capable of modifying the write-enabled Control Table memory slots:

void write_control_table_item (char act_id, char
offset, unsigned int table_data)
{

char parm_len;

parms[0] = offset;
parm_len = 2;

switch(offset)
{

case ID:
 parms[1] = make8(table_data,0);
break;
case BAUD_RATE:
 parms[1] = make8(table_data,0);
break;
case RETURN_DELAY_TIME:
 parms[1] = make8(table_data,0);
break;
case CW_ANGLE_LIMIT:
 parms[1] = make8(table_data,0);
 parms[2] = make8(table_data,1);

}
send_packet(act_id,iWRITE_DATA,parm_len);

There’s a case entry for every writable Control Table entry. Thus, the write_control_table_item function is too lengthy to print here. So, I’ve only included just enough code to show you how it works. Note that the parms buffer is loaded before the WRITE_DATA instruction is transmitted. Most of the Control Table entries only require a pair of instruction parameters. I’ve modified the parm_len value accordingly for Control Table entries that require more than two instruction parameters. To demonstrate how the write_control_table_item function works, let’s modify the RETURN DELAY TIME value. In Screenshot 3, it is currently set to 0xFA (offset 10 in the recv_buff). This code will modify the value to 0xF0:

SCREENSHOT 3. Note the Return Delay Time value of 0xFA at offset 10. This value represents a time of 500 µs. We’re going to change that in Screenshot 4. There are 52 (0x34) bytes in the status message following the byte at offset 3. The byte immediately following the LENGTH byte (0x34) is the ERROR byte (0x00).