usb driver problem

xfratboy xfratboy at gmail.com
Sun Jan 23 10:18:56 EST 2011


I'm coming into this conversation a bit late, but I've been wrestling with
USB/Serial communication in OSX for a several weeks now with no success.  I
doubt if this makes you feel any better.  I'm working with an FTDI chipset
(FT232R).  Communicating through this usb-to-serial-converter works
perfectly in Windows .. If I take that same stack to OSX, it fails
miserably.  It's like my code gets stuck in a sticky goo of zeros and ones. 
Something that has been helpful for my understanding was this info provided
by FTDI support:

----------
This may not be an issue with the serial settings. Understand that there’s a
huge fundamental difference between USB and the old RS232 style interface.
In the good old days, things were set up so that everything was character
based – you send a single character, you receive a single character, and
life was good. With USB, you send a single character, and…. Nothing happens
until one of several conditions are met:
1.)	You fill up a buffer – when the buffer is full, USB automatically
transmits out the data block (not just a single character)
2.)	Your product requests the buffer be sent (even if it’s not “full”)
3.)	The latency timer goes off, and says to send whatever is in the buffer,
even if it’s not full (this could be a source of your lag/latency)

In order to address this, see  section 3.4 and 3.5 page 6 of TN105:
http://www.ftdichip.com/Support/Documents/TechnicalNotes/TN_105%20Adding%20Support%20for%20New%20FTDI%20Devices%20to%20Mac%20Driver.pdf 

Also, keep this in mind when you are writing your application – in visual
basic (for example), you could send out one character at a time, or a string
of characters and with RS232 it wouldn’t make any difference. With USB,
you’d want to send out a string long enough to fill the buffer length so
that USB would send it off instead of waiting for the latency timer to trip.
------

So.... this still doesn't solve my problem with hang-ups and lags, but it
did give me a place to start.  Basically, I have started poking around
inside the FTDIUSBSerialDriver.kext file found in
/System/Library/Extensions/ folder.  If your device is not showing up with
the "DriverNames" function then it's probably not in the FTDI driver's
'info.plist' file.  In which case, you would have to create a new entry in
the info.plist file for your specific device.  That's a whole-nutha-level of
trial and error which I'm staring to figure out a little bit. Feel free to
contact me off topic if you are interested in how to do this or maybe we can
start another thread.



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