Initializing the USB2AX¶

On being plugged in, the USB2AX registers itself in the /dev directory. In all probability it will register itself as /dev/ttyACM0, however it may come up as /dev/ttyACM1. Running dmesg will help debug any problems:

$ dmesg | grep -B 7 ACM
[12917.687364] usb 3-3.4: new full-speed USB device number 66 using xhci_hcd
[12917.700656] usb 3-3.4: New USB device found, idVendor=16d0, idProduct=06a7
[12917.700662] usb 3-3.4: New USB device strings: Mfr=1, Product=2, SerialNumber=220
[12917.700664] usb 3-3.4: Product: USB2AX
[12917.700666] usb 3-3.4: Manufacturer: Xevelabs
[12917.700668] usb 3-3.4: SerialNumber: 6403635373035121E161
[12917.701516] usb 3-3.4: ep 0x82 - rounding interval to 1024 microframes, ep desc says 2040 microframes
[12917.701784] cdc_acm 3-3.4:1.0: ttyACM0: USB ACM device

In the above output, the device has registered as ttyACM0 – the device ID is thus 0 and the interface can be initialized with the default constructor

Controller()

Or

Controller(device_id=0)

Otherwise, initialize the Controller with the indicated device_id, e.g. for ttyACM1

Controller(device_id=1)

Bus scanning¶

When you instantiate a usb2ax.Controller it will scan the bus for connected servos and print out information relating to them. A typical output looks like this:

usb2ax: Initiating scan...
usb2ax: USB2AX          : /dev/ttyACM0
usb2ax: AX-12           : 1
usb2ax: AX-12           : 2
usb2ax: AX-12           : 3
usb2ax: AX-12           : 4
usb2ax: AX-12           : 5
usb2ax: AX-12           : 6
usb2ax: AX-12           : 16
usb2ax: AX-12           : 17
usb2ax: AX-12           : 18
usb2ax: Model no.       : 16897
usb2ax: Firmware version: 3
usb2ax: Success!

There should be a line for every connected servo. The model no. and firmware version at the end of the list relate to the USB2AX device itself.

If not all of the expected servos are listed that is likely to be a problem. Ensure that all servos are set to communicate at 1Mbps and that there are no ID conflicts – the servos are all configured with ID 1 to start with. To change this, connect each servo one at a time, and simply use

with Controller() as dxl:
    dxl.write(1,"id",<NEW ID>)

Sync read delay timing¶

When using the sync_read() function it is best to instantiate the Controller object with the keyword argument fix_sync_read_delay set to True:

with usb2ax.Controller(fix_sync_read_delay=True) as dxl:
  ...

The USB2AX SYNC_READ functionality supported by this library is a useful way to get quicker reads from multiple servos. It allows the USB2AX device to send multiple read requests to different devices, then collate all the responses and send it back to the client application as one packet. This reduces the latency introduced by the USB to serial port conversion.

When an individual READ instruction is sent to a servo, the servo waits a short time to send a reply. This timing can be set in the servo’s control table. By default, Dynamixel AX-12 servos have this set to a value of 250. This appears to be an unreliable setting for use with the SYNC_READ function – ideally it should be much lower. To avoid any problems, PyUSB2AX can fix the return delay time setting automatically if fix_sync_read_delay is set to True when instantiating Controller. This will (if necessary) change the return delay settings for poorly configured Dynamixels to a sensible value. If fix_sync_read_delay is False and a poorly configured Dynamixel is connected, you may see an output like this:

usb2ax: Initiating scan...
usb2ax: USB2AX          : /dev/ttyACM0
usb2ax: AX-12           : 1
usb2ax: Delay time is 250 -- you cannot use sync_read
usb2ax: To fix this automatically call initialize with fix_sync_read_delay=True

Any calls to sync_read with throw a SyncReadError. Running with fix_sync_read_delay=True produces

usb2ax: Initiating scan...
usb2ax: USB2AX          : /dev/ttyACM0
usb2ax: AX-12           : 1
usb2ax: INFO: Servo 1 return delay set to 1 to make compatible with sync_read

After this, sync_read should work with no problems.

Control tables¶

The Dynamixel servos have internal control tables which are used for control. The tables below show the parameters that can be read and written via these tables. The parameter name can be used as an argument to read, write, sync_read and sync_write. For example, if you pass “goal_position” as the argument to write, the control table address 0x1E will be updated, causing the servo to move to the specified position. Details of the meaning of each control table parameter are given in the Dynamixel manual.

PyUSB2AX should support AX-12/18 and MX-28T servos, which have slightly different control tables. Most of the addresses are the same, so for example if writing to “goal_position” it is not important what model of servo is attached.

In theory (this is untested) this library can talk to both AX and MX servos to the same bus. It can even sync_read and sync_write to both types of servos at the same time, provided the parameter addressed is the same in both types of servo. Calling write to “goal_acceleration” on an AX-12 servo for example will raise a usb2ax.UnknownParameterError. This error will also appear if sync_write is called with the parameter “goal_acceleration” on a collection of servos where any of the servos is an AX-12.