Re: A simple USB question

Op Fri, 08 Jun 2007 10:55:10 +0200 schreef Boudewijn Dijkstra <boudewijn@xxxxxxxxx>:
Op Fri, 08 Jun 2007 10:31:06 +0200 schreef Mouarf <toto@xxxxxxx>:
Le Wed, 06 Jun 2007 16:43:29 +0200, Boudewijn Dijkstra <boudewijn@xxxxxxxxx> a écrit:

Op Wed, 06 Jun 2007 16:12:23 +0200 schreef Mouarf <toto@xxxxxxx>:
Le Wed, 06 Jun 2007 16:00:37 +0200, Boudewijn Dijkstra <boudewijn@xxxxxxxxx> a écrit:

Op Wed, 06 Jun 2007 13:56:00 +0200 schreef vasu <vasu.sharma@xxxxxxxxx>:
Hi! I have an AT91SAM7S-EK kit with an AT91SAM7S256 microcontroller.
This microcontroller supports 4 Endpoints - EP0, EP1, EP2 and EP3. I
am using the USB to communicate with a legacy application running on
the PC. This application can only communicate on a COM port. So, I use
the USB to provide a virtual COM port on the PC with which this
application can communicate. My question is - Can I create more than 1
virtual COM port on the PC with 1 USB connection?

If your legacy driver can only recognize a single COM port per USB device, you could create a 'compound device', which acts to the outside as a USB-hub.

what about a composite device with 2 COM ports? Is it possible?

compound == composite

I've read in "USB Complete" that compound=1 device + hub and composite=1 device with multiple functions I may have badly understood

My copy of the USB 2.0 standard says:
"However, a physical package may implement multiple functions and an embedded hub with a single USB cable. This is known as a compound device."
and
"From the host's perspective, a compound device is the same as a separate hub with multiple functions attached."

Sorry for the confusion. The complete paragraph in section 5.2.3 reads:
"Multiple functions[1] may be packaged together in what appears to be a single physical device. For example, a
keyboard and a trackball might be combined in a single package. Inside the package, the individual
functions are permanently attached to a hub and it is the internal hub that is connected to the USB. When
multiple functions are combined with a hub in a single package, they are referred to as a compound device.
The hub and each function attached to the hub within the compound device is assigned its own device
address. A device that has multiple interfaces[2] controlled independently of each other is referred to as a
composite device. A composite device has only a single device address. From the host’s perspective, a
compound device is the same as a separate hub with multiple functions attached. Figure 5-5 also illustrates
a compound device."

Note that this is the only paragraph in the standard where composite devices are mentioned.

And from the Terms and Abbreviations:
[1] "Function = A USB device that provides a capability to the host, such as an ISDN
connection, a digital microphone, or speakers."
[2] "Interface" is not defined in the Terms and Abbreviations.

So compound != composite.


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