USB CDC Communications for Dummies!

[mackrackit]

Just curious about post 57. What helped?

[cncmachineguy]

I ment post 54!

Sorry for the confusion

[mackrackit]

Back to your question about speed....
I think it runs at USB speeds....

Universal Serial Bus Features:
• USB V2.0 Compliant
• Low Speed (1.5 Mb/s) and Full Speed (12 Mb/s)

[Demon]

Just tech notes about USB speeds:

The original USB 1.0 specification, which was introduced in January 1996, defined data transfer rates of 1.5 Mbit/s "Low Speed" and 12 Mbit/s "Full Speed". The first widely used version of USB was 1.1, which was released in September 1998. The 12 Mbit/s data rate was intended for higher-speed devices such as disk drives, and the lower 1.5 Mbit/s rate for low data rate devices such as joysticks.

The USB 2.0 specification was released in April 2000 and was standardized by the USB Implementers Forum (USB-IF) at the end of 2001. Hewlett-Packard, Intel, Lucent Technologies (now Alcatel-Lucent), NEC and Philips jointly led the initiative to develop a higher data transfer rate, with the resulting specification achieving 480 Mbit/s, a fortyfold increase over the original USB 1.1 specification.

The USB 3.0 specification was published on 12 November 2008. Its main goals were to increase the data transfer rate (up to 5Gbps), to decrease power consumption, to increase power output, and to be backwards-compatible with USB 2.0. USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus. For this reason, the new version is also called SuperSpeed. The first USB 3.0 equipped devices were presented in January 2010.

http://en.wikipedia.org/wiki/Universal_Serial_Bus

[dhouston]

I don't know whether I'm too dumb or not dumb enough to grasp all the intricacies of CDC but, if I understand it, there are virtual serial port drivers for interfacing on the PC side. When these are used, there's likely to be a slight speed hit as the data needs to be 'translated' from its RS232-ish form before transmitting and the reverse is true when receiving.

This may not be noticed for normal communications but can be readily apparent when doing something like a firmware download which usuall includes short bits of data and CRC which is ACKed or NAKed from the embedded device. Of course, the faster the version of USB, the less this is a big factor.

[mister_e]

For CDC, baudrate is a meaningless parameter. You can set hyperterminal/vbSerialPort a whatsoever baudrate, the usb engine will emulate a serial communication as fast as it can

[dhouston]

For CDC, baudrate is a meaningless parameter. You can set hyperterminal/vbSerialPort a whatsoever baudrate, the usb engine will emulate a serial communication as fast as it can

I'm not talking about transmission speed and I agree that baudrate has little meaning in this context. However, I have had a lot of experience downloading firmware files which might be as big as 64KB and there is a very noticeable difference between sending them via Serial->USB->Serial vs. sending them Serial->Serial with the latter being much faster as a result of the translation that must occur (on both ends). CDC should only need to translate on the PC end but I suspect there will be a significant speed hit never-the-less on certain applications.