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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.
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A serial port USB is intended to interface with a modem or with a similar communication device. This USB gateway usually identifies hardware that is more or less compliant to RS-232 standard.
USB has become the preferred serial communication protocol between digital products like computers and peripheral devices as it can transmit data across longer cables and offers higher transmission rates. UART, however, is still is used today for certain applications and is often seen in older devices.
USB Host functions to support Communication Device Class (CDC) Abstract Control Model (ACM) USB Devices. The CDC ACM class in the USB Host Component is used for attaching communication devices to your system.
In strict theory, any port using serial communications (almost any modern bus - including USB, which stands for "Universal Serial Bus", if my memory serves me) is a "serial port". However, in most cases, when people refer to "the serial port", they actually refer to a port that complies to RS-232. Save this answer.
USB allows data to travel on the average of ten times the speed of the normal parallel port. It is also faster than a serial port. The average serial port transfer rate is 150 kbps; the USB port is up to 12 Mbps. USB 2 is forty times faster, with a maximum transfer rate of 480 Mbps.
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.
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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.
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