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product:usb2ax:advanced_instructions
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product:usb2ax:advanced_instructions [2013/04/02 05:41] xevel |
product:usb2ax:advanced_instructions [2018/04/02 22:16] xevel [SYNC_READ] |
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| ===== USB2AX: Advanced instructions ===== | ===== USB2AX: Advanced instructions ===== | ||
| - | The USB2AX can react to some specific commands as if it were a Dynamixel device. | + | The USB2AX can react to some specific commands as if it were a Dynamixel device, and much more. |
| ---- | ---- | ||
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| SYNC_READ is used for reading data from multiple Dynamixel actuators with one command. | 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. | 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 [[https://github.com/Xevel/usb2ax/tree/master/pc_software/usb2ax_DynamixelSDK|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. | **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. | ||
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| **How does it actually work?**\\ | **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).\\ | + | 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.\\ | 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. | This significantly decreases the effect of USB latency. | ||
| - | Limitations | + | **Limitations**\\ |
| - | + | A SYNC_READ command reads the same registers in each servo.\\ | |
| - | You can use this instruction only when you want to read the same bytes 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. | The USB2AX limits the maximum number of servos to read from (N) to 32, and the maximum data length (L) to 6 bytes. | ||
| - | **Instruction Packet** | + | **Instruction Packet**\\ |
| <0xFF><0xFF><ID><Length><Instruction><Param 1><Param 2><Param 3> ... <Param N+2><Checksum> | <0xFF><0xFF><ID><Length><Instruction><Param 1><Param 2><Param 3> ... <Param N+2><Checksum> | ||
| - | ID : 0XFD or 0xFE | + | ID : 0xFD or 0xFE |
| Length : N + 4 (N: number of Dynamixel actuators to read from, value between 1 and 32 inclusive) | Length : N + 4 (N: number of Dynamixel actuators to read from, value between 1 and 32 inclusive) | ||
| - | Instruction: 0X84 | + | Instruction: 0x84 |
| Param 1 : Starting address of the location where the data is to be read from | 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 2 : L (L: length of the data to be read, value between 1 and 6 inclusive) | ||
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| - | **Status Packet (Return Packet)** | + | **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> | <0xFF><0xFF><ID><Length><Error><Param 1> ... <Param L><Param L+1> ... <Param L*N><Checksum> | ||
| Line 100: | Line 103: | ||
| - | == Example == | + | ** Example **\\ |
| Reading the Present Position and Present Speed for 4 Dynamixel actuators with IDs of **0**, //1//, __2__, ''7''. | Reading the Present Position and Present Speed for 4 Dynamixel actuators with IDs of **0**, //1//, __2__, ''7''. | ||
| - | **Instruction Packet**\\ | + | Instruction Packet\\ |
| 0XFF 0XFF 0XFD 0X08 0X84 0X24 0X04 **0X00** //0X01// __0X02__ ''0X07'' 0X44\\ | 0XFF 0XFF 0XFD 0X08 0X84 0X24 0X04 **0X00** //0X01// __0X02__ ''0X07'' 0X44\\ | ||
| - | **Response Packet**\\ | + | Status Packet\\ |
| 0XFF 0XFF 0XFD 0X12 0X00 **0X50 0X01 0XFF 0X01** //0X20 0X00 0X00 0X02// __0X10 0X00 0X10 0X02__ ''0X00 0X00 0XFE 0X01'' 0X5C | 0XFF 0XFF 0XFD 0X12 0X00 **0X50 0X01 0XFF 0X01** //0X20 0X00 0X00 0X02// __0X10 0X00 0X10 0X02__ ''0X00 0X00 0XFE 0X01'' 0X5C | ||
| + | ** Performances ** | ||
| + | |||
| + | 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// | //Based on documentation by Richard "iBot" Ibbotson// | ||
product/usb2ax/advanced_instructions.txt · Last modified: 2018/04/02 22:16 by xevel
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