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--- product:usb2ax:advanced_instructions [2013/04/02 04:52]
xevel [SYNC_READ]
+++ product:usb2ax:advanced_instructions [2018/04/02 22:16]
xevel [SYNC_READ]
@@ Line 1: / Line 1: @@
 ===== USB2AX: Advanced instructions =====
-//Based on documentation by Richard "iBot" Ibbotson//
+The USB2AX can react to some specific commands as if it were a Dynamixel device, and much more.
-The USB2AX can react to some specific commands as if it were a Dynamixel device.
 ----
@@ Line 13: / Line 11: @@
 | 0x06  | RESET       | Reboot USB2AX                              |  0                 |
 | 0x08  | BOOTLOADER  | Reboot USB2AX in bootloader mode           |  0                 |
-| 0x84  | SYNC_READ   | Read from several Dynamixel simultaneously |  4 or more           |
+| 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.
@@ Line 54: / Line 52: @@
 ==== SYNC_READ ====
-SYNC_READ is a command 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.
-FIXME
+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.
-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!
+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!**
-<TODO>
+**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.
-SYNC_READ is a command only the controller can understand.
+**Limitations**\\
-Using this command without such a controller to interpret it will either not have any effect or, if you have a servo with an ID of 0xFD connected on the Dynamixel bus, it might be responded to with a 0x20 error.
+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.
-In the controller, SYNC_READ is converted into multiple separate READ commands.
+**Instruction Packet**\\
-This allows for a significant performance boost when reading the same values in multiple actuators.
-The communication time is decreased by the SYNC_READ instruction since many Dynamixel bus read instructions can be performed in a single controller command and response. This makes a significant performance increase where the controller connection may experience high latency compared to the dynamixel bus speed.
-You can use this instruction only when the lengths and addresses of the control table to be read from are the same.
-Note : the USB2AX limits the maximum number of actuators to read from (N) to 32, and the maximum data length (L) to 6 bytes.
-**Instruction Packet**
   <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)
-  Instruction: 0X84
+  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)
@@ Line 95: / Line 84: @@
-**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>
@@ Line 111: / Line 103: @@
-== Example ==
+** Example **\\
 Reading the Present Position and Present Speed for 4 Dynamixel actuators with IDs of **0**, //1//, __2__, ''7''.
-**Instruction Packet**\\
+Instruction Packet\\
 XFF 0XFF 0XFD 0X08 0X84 0X24 0X04 **0X00** //0X01// __0X02__ ''0X07'' 0X44\\
-**Response Packet**\\
+Status Packet\\
 XFF 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//

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