Xiaofan, a client has asked if a USB memory stick (which I think
is what you're calling a thumb drive) can be added to a 4550 to
log analogue readings and general data

I've had a look through some of the links you provided, especially
those that mentioned mass storage. I'm aware that the 4550 is not
a host, as you point out in the quote below. Do you know if anyone
has a work-around ? Perhaps a peripheral adapter ? A thumb drive
would be most convenient for end users (write with PIC, read with
PC) so I'll pretty much do what it takes if it's possible 

=================

http://forum.microchip.com/tm.aspx?m=304436

Description:   
Will the full speed usb PIC18Fxx5x device, talk to a usb thumb-drive? 

Resolution:   
The PIC18Fxx5x (such as the PIC18F4550) cannot be a usb host

This means that you can build any device that is a slave device.  This
basically means that you can connect the PIC18F4550 to a PC, and
program it to be a lave.  However you can not connect a device to
the PIC18F4550 that would need to connect to a PC. This means
you could put code on the '4550 to read a memory card and provide
the data to a PC, but you could not program the '4550 to read a
thumb drive, as the thumb drive needs a host, such as a PC

The reason the thumb drive doesn't work is that the typical usb
thumb drive is a 'slave only' device just like the PIC18F4550. Thus
the PIC18F4550 cannot talk directly to a thumb drive is a slave
device can not talk to a slave device

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