Servo - April 2009

array entries into the correct memory slots of a digital packet. However, for now, let’s insert the parameters manually:

//READ ID DIGITAL PACKET
xmit_buff[0] = 0xFF; //sync character
xmit_buff[1] = 0xFF; //sync character
xmit_buff[2] = 0x01; //unique ID
xmit_buff[3] = 0x04; //# of PARMS+INSTRUCTION+CHECKSUM
xmit_buff[4] = iREAD_DATA; //instruction
xmit_buff[5] = ID; //parameter 1
xmit_buff[6] = 0x01; //parameter 2
xmit_buff[7] = 0xF4; //checksum

Both of the digital packets we assembled will trigger a response from the AX- 12+. In the case of the PING digital packet, we should receive a status message containing the AX- 12+’s ID, an ERROR byte, and a checksum byte. If the PING operation is successful, here’s what a returned status digital packet looks like from a programmer’s point of view:

xmit_buff[0] = 0xFF;
xmit_buff[1] = 0xFF;
xmit_buff[2] = 0x01;
xmit_buff[3] = 0x02;
xmit_buff[4] = 0x00;
xmit_buff[5] = 0xFC;

//sync character
//sync character
//payload value = ID of AX-12+
//# of PARMS+INSTRUCTION+CHECKSUM
//error byte – 0x00 = none
//checksum

This status message contents are confirmed in Screenshot 3, which is the PING response data I received from an AX- 12+ with an ID of 0x01. I also took the liberty to capture the response for the READ_DATA digital packet in Screenshot 4. Here’s the programmer view of the status message data captured in Screenshot 4:

xmit_buff[0] = 0xFF;
xmit_buff[1] = 0xFF;
xmit_buff[2] = 0x01;
xmit_buff[3] = 0x03;
xmit_buff[4] = 0x00;
xmit_buff[5] = 0x01;
xmit_buff[6] = 0xFA;

//sync character
//sync character
//unique ID
//# of PARMS+INSTRUCTION+CHECKSUM
//ERROR byte – no errors
//parameter 1
//checksum

More To Come

I think you’ve got the idea. So, next time we’ll add some meat to our firmware potatoes and code up a number of functions that will give us dominion over the AX- 12+ Dynamixel robot actuator. In the meantime, I’ll post the preliminary AX- 12+ PIC18F2620 firmware I used here to communicate with an AX- 12+ as a download package on the SERVO website ( www.servomagazine.com). Be sure to have your AX- 12+ controller hardware ready to roll as next time we’re going to concentrate on the firmware. SV