具有多个串行端口的USB通信设备，可在所有平台上运行

Question

We have an embedded device that connects to the PC via USB, and it has multiple virtual serial ports (CDC-ACM).

We have this working on Windows. On the embedded device, we have multiple CDC-ACM interfaces. The USB descriptors declare it as a composite device (class=0xEF, sub-class=2, protocol=1), and it has an "Interface Association Descriptor" for each virtual serial port. On Windows, we use an INF file that installs usbser.sys for each CDC-ACM control interface (MI_00, MI_02, etc).

However, as we've found, this method doesn't seem to work for Mac. I've found that I can get it to work for the Mac and Linux, by changing it to a "Communications" class (class=2, sub-class=0, protocol=0), and removing the IADs. (For Linux, testing with Ubuntu, I found that this worked with the Ubuntu Linux kernel 2.6.35-28 or newer. With earlier kernels, only the first serial port worked.) But then, this method doesn't work for Windows.

What method can be used to make a USB device with multiple virtual serial ports, that works on Windows, Mac, and Linux? I think I'd prefer a solution that uses the CDC-ACM standard as much as possible, and avoids the write-your-own-drivers option as much as possible.

Answer 1

The one way I can think of off top of my head would be the device presents itself as an USB hub with multiple separate single-serial-port devices attached to it. This isn't pretty but very bulletproof.

Answer 2

One solution that I've found, which I think could work (subject to further testing on Windows):

Make the device enumerate in the way that works for the Mac:

• Make it "Communications" class (class=2, sub-class=0, protocol=0), not composite device.
• Remove the IADs.

The device should "just work" on Mac and recent Linux, in this configuration. (For Linux, testing with Ubuntu, I found that this worked with the Ubuntu Linux kernel 2.6.35-28 or newer. With earlier kernels, only the first serial port worked.)

Then, for Windows, modify the INF file for the device, to explicitly load the composite device driver usbccgp.sys . I'm a novice with Windows INF files, but here are the relevant snippets from what I could figure out so far:

[CCGPDriverInstall.NT]
Include=usb.inf
Needs=Composite.Dev.NT
AddReg=CCGPDriverInstall.AddReg

[CCGPDriverInstall.NT.Services]
Include=usb.inf
Needs=Composite.Dev.NT.Services

[CCGPDriverInstall.AddReg]
HKR,,EnumeratorClass, 0x00000001,02,00,00

...

[DeviceList]
%DESCRIPTION_DEVICE%=CCGPDriverInstall, USB\VID_FFFF&PID_0001
%DESCRIPTION_AT%=DriverInstall, USB\VID_FFFF&PID_0001&MI_00 
%DESCRIPTION_MENU%=DriverInstall, USB\VID_FFFF&PID_0001&MI_02 

[DeviceList.NTamd64] 
%DESCRIPTION_DEVICE%=CCGPDriverInstall, USB\VID_FFFF&PID_0001
%DESCRIPTION_AT%=DriverInstall, USB\VID_FFFF&PID_0001&MI_00 
%DESCRIPTION_MENU%=DriverInstall, USB\VID_FFFF&PID_0001&MI_02 

[DeviceList.NTx86...1]
%DESCRIPTION_DEVICE%=CCGPDriverInstall, USB\VID_FFFF&PID_0001
%DESCRIPTION_AT%=DriverInstall, USB\VID_FFFF&PID_0001&MI_00 
%DESCRIPTION_MENU%=DriverInstall, USB\VID_FFFF&PID_0001&MI_02 

With the INF file explicitly loading the usbccgp.sys driver, both USB serial ports worked for me on Windows XP SP3 32-bit.

I have done only limited testing so far, so I'd be interested to hear how well this works, or not, for others.

Answer 3

As Apple's drivers don't support composite CDC devices, I'd suggest either making your device reconfigure somehow and making your alternate descriptors plain CDC, or sticking with the composite and using a third party driver (my company makes CDC ACM drivers for OS X which will probably support your device).

It may also be possible to force the issue with a codeless kext.