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neok

Member

From: Cyprus

Registered: 2003-12-14

Posts: 190

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USB latency problem

Hi,

I am writing a libusb application to control a servo via a USB PhidgetServo i/f card, but I am troubled by a latency of the order of 100 mSec in the response of the servo. The servo itself is very fast so the delay must be due to the USB subsystem or perhaps the PhidgetServo card. From tests I have made the delay seems fixed around ~100 mS (the rate at which I send positional requests to the servo can be between 7-60 / sec) but no requests are dropped.

It seems that there is a variable length buffer somewhere that stores positional requests (via the usb_control_msg() function) such that a delay of ~100 mS is assured. I cannot figure out where this latency is occurring and unfortunately its making the control loop unstable and inaccurate. I would very much welcome any help on this subject.

My thanks in advance!

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Regards

Neoklis ... Ham Radio Call: 5B4AZ