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USB Serial Protocol


New member
I am currently evaluating a CFA635 for integration in my custom system. From capturing traces with a USB bus analyzer when the CFA635 is connected to a PC, I can see the initialization sequence and the transactions that occur to run a command from the Windows demo/test program to the CFA635.

The USB transaction sequence for running a command matches what is described in the documentation. For example, I see the command/response packets when I write to the LCD from the test program. I do however see the device respond with a 2-byte packet every 15 msec rather than just continuously NAK. After several of these 2-byte packets, the actual command response is sent. What are these 2-byte interim packets?

I cannot find documentation for some of the USB transactions that I see during initialization. I see the standard USB setup get descriptor requests and the device response appropriately, but then I see several non-standard setup requests sent to the device. Can someone point me to some documentation for these non-standard setup requests? Are they custom to the CFA6xx or are they part of some "standard" USB-serial protocol? Since my system is running a custom OS, I cannot use the Windows or Linux driver directly.

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CF Tech

Please take a look at the FT232BM. This is the chip we use, and they supply the serial emulation drivers for Windows and Linux. To use the chip outside of that environment, you would need to study the documentation from FTDI.