Canon Usb Protocol - Okii Systems USB Focus Controller User Manual

Canon USB Protocol

Canon EOS digital SLR cameras use a combination of two protocols called Picture Transfer
Protocol (PTP) and Media Transfer Protocol (MTP) to send and receive commands, data, and
images to and from a computer using USB. Various projects, such as Canon's own EDSDK, have
documented many of the USB commands that Canon uses. Okii Systems has also reverse
engineered many of the commands specific to the FC1 USB Focus Controller.
Canon's implementation of USB has several characteristics that directly affect the performance of
the FC1 USB Focus Controller:
1. When a focus command is sent to the camera, a success packet is always returned, even if the
command is ignored. The controller can repeatedly send focus commands, but if the camera is
performing another action, the focus will not change.
2. There is no way to detect an ignored focus command other than seeing that the focus did not
move on the lens or in the live view image. No USB feedback is given when focus commands are
ignored.
3. If the rate of commands is too fast, it can cause the camera to temporarily pause and ignore
further commands. The maximum reliable rate is about 15 commands per second if the lens in
use is mechanically able to move between focus positions at this speed.
4. The time it takes for each lens model to change focus from one position to another varies,
depending on the type of lens motor, the focal length, and the focus step size.
These characteristics make it necessary to adjust the focus command send rate to match the
focal length of your lens and the focus step size. The process is described in the Changing the
Focus Command Send Rate section of this manual.
If a USB bus error occurs, Canon's USB implementation locks out external input, making it
impossible to send additional commands. This can happen for many reasons such as corrupted
USB packets or overflowing the command/event buffers on the camera. If a bus error is
detected, the controller will display a continuous flashing red LED. Power cycling the camera and
controller will resolve the issue.
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