The USB2AX can react to some specific commands as if it were a Dynamixel device, and much more.
The USB2AX responds to the following instructions:
|Value||Instruction||Description||Number of parameters|
|0x02||READ_DATA||Read data from USB2AX||2|
|0x08||BOOTLOADER||Reboot USB2AX in bootloader mode||0|
|0x84||SYNC_READ||Read from several Dynamixel simultaneously||4 or more|
Please note that it will silently ignore the other Dynamixel commands (PING, WRITE_DATA, REG_WRITE, ACTION), and in fact will transmit them on the Dynamixel bus.
WARNING the USB2AX uses the ID <0xFD>, therefore you can not use it for any another Dynamixel device on the bus.
The packet structure is the same as the one of the regular Dynamixel packets ( see http://support.robotis.com/en/product/dynamixel/communication/dxl_packet.htm ).
Read data from the USB2AX Control Table.
|0(0x00)||Model Number (L)||Lowest byte of Model Number||R||0x01|
|1(0x01)||Model Number (H)||Higher byte of Model number||R||0x42|
|2(0x02)||Firmware Version||Version of the firmware in use||R||-|
|3(0x03)||ID||ID of USB2AX||R||0xFD|
Identical to the command you would use to read data from a Dynamixel device with an ID of 0xFD.
Reboot the controller.
Identical to the command you would use to reset a Dynamixel with an ID of 0xFD to factory settings.
Reboot the controller in bootloader mode, for firmware update.
Similar to the command you would use to reset a Dynamixel with an ID of 0xFD to factory settings, except with an instruction of 0x08.
SYNC_READ is used for reading data from multiple Dynamixel actuators with one command. This allows for a significant performance boost when reading the same values in a lot of devices.
An (obsolete) implementation in the DynamxelSDK v1 library is availalbe in the GitHub repository. The newer revisions of the Dynamixel SDK are not compatible with this definition of the SYNC_READ, which pre-dates the official one.
Warning: Various implementations of custom Dynamixel multiple-read commands named SYNC_READ can be found over the Internet, this one is based on the one found in the arbotix_python ROS package by Michael Ferguson. It is not part of the original Dynamixel protocol, and is only supported by some advanced third party Dynamixel controllers, like the USB2AX. Please only refer to your controller manufacturer for support, do not contact the servo manufacturer ROBOTIS!
How does it actually work?
SYNC_READ is a command only the controller can understand: using this command without such a controller will either not have any effect or, if a servo with ID 0xFD is present on the bus, might return an error 0x20.
The controller converts SYNC_READ into multiple separate READ commands to get data from each servo, then sends back to the computer a single big packet containing all the data.
This significantly decreases the effect of USB latency.
A SYNC_READ command reads the same registers in each servo.
The USB2AX limits the maximum number of servos to read from (N) to 32, and the maximum data length (L) to 6 bytes.
<0xFF><0xFF><ID><Length><Instruction><Param 1><Param 2><Param 3> ... <Param N+2><Checksum>
ID : 0xFD or 0xFE Length : N + 4 (N: number of Dynamixel actuators to read from, value between 1 and 32 inclusive) Instruction: 0x84 Param 1 : Starting address of the location where the data is to be read from Param 2 : L (L: length of the data to be read, value between 1 and 6 inclusive) Param 3 : The ID of the 1st Dynamixel actuator ... Param N+2 : The ID of the Nth Dynamixel actuator Checksum : The usual checksum of Dynamixel packets
WARNING Some choices of implementation of the Dynamixel SDK make it impossible to simply send this packet and expect a valid response.
Status Packet (Return Packet)
<0xFF><0xFF><ID><Length><Error><Param 1> ... <Param L><Param L+1> ... <Param L*N><Checksum>
ID : 0xFD Length : ( L * N ) + 2 (L: length of the data to be read, N: number of Dynamixel actuators to read from) Error : 0x00 ( Only Range Error supported, set if value of L or N was invalid) Param 1 : 1st Value read from starting address of 1st Dynamixel actuator ... Param L : Lth Value read from starting address of 1st Dynamixel actuator Param L+1: 1st Value read from starting address of 2nd Dynamixel actuator ... Param L*N: Lth Value read from starting address of Nth Dynamixel actuator Checksum : The usual checksum of Dynamixel packets
Reading the Present Position and Present Speed for 4 Dynamixel actuators with IDs of 0, 1, 2,
0XFF 0XFF 0XFD 0X08 0X84 0X24 0X04 0X00 0X01 0X02
0XFF 0XFF 0XFD 0X12 0X00 0X50 0X01 0XFF 0X01 0X20 0X00 0X00 0X02 0X10 0X00 0X10 0X02
0X00 0X00 0XFE 0X01 0X5C
On tests performed with the reference implementation (see above), reading the present positions (2 bytes) of 8 servos took around 2.7ms, on a Win7 64 bits desktop plateform. Each additional servos will add betwee 0.21 and 0.34ms.
Based on documentation by Richard “iBot” Ibbotson