USB as a COM port vs. as a regular USB


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  1. #1
    jom
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    Default USB as a COM port vs. as a regular USB

    Hello

    My description of my question isn't very clear I suppose but here goes. I've been working with Joe Salbia recently trying to get my 18F4550 PIC to work with his LED VB program. Here is his Wiki entry:

    http://www.protonbasic.co.uk/content...ith-USB-and-VB

    My question is what differences do I need in the PIC programming to get the PIC USB device to show up under "Universal Serial Bus" in the windows Device Manager rather than under Ports as a COM port? Or is the difference in the VB program in Windows? Or both??? The reason I ask is just for my edification as I really know very little about this USB business.

    BTW, with Joe's help I did get my 18F4550 to work after fixing some flubs I had in the hardware. Thanks Joe!

    jom

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    Prolific Poster hadv215's Avatar
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    Default Re: USB as a COM port vs. as a regular USB

    Hi Jom,

    the way Windows treats USB devices depends on the USB device, more precisely, the information in the descriptor file that was used for the device. But why would you want to change that?
    If you want to learn about USB I would suggest you find yourself a copy of one of Jan Axelson's books "USB-Complete USB Complete - Everything You Need to Develop Custom USB Peripherals" or ""USB Complete - Develop Custom Peripherals". But mind you, it's pretty hard stuff.

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  • #3
    jom
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    Default Re: USB as a COM port vs. as a regular USB

    Quote Originally Posted by hadv215 View Post
    Hi Jom,

    the way Windows treats USB devices depends on the USB device, more precisely, the information in the descriptor file that was used for the device. But why would you want to change that?
    If you want to learn about USB I would suggest you find yourself a copy of one of Jan Axelson's books "USB-Complete USB Complete - Everything You Need to Develop Custom USB Peripherals" or ""USB Complete - Develop Custom Peripherals". But mind you, it's pretty hard stuff.
    Change? No...I was just curious. I'm an RF engineer and I've been working with some test equipment and Eval boards recently that act differently when connected (via USB) to my PC. I have a PC driven Synthesizer that connects as a COM port and I have two Eval boards from some ADCs we are working with that connect as a USB and the other as a COM port. I was just curious as to why/how they are different.

    Is there a compelling reason to go with a USB connected (as opposed to a COM port) set up? The ADC eval boards are running data through the USB to PC software that shows FFTs etc etc. Both are running at pretty fast rates so I'd guess the COM port connection runs just as fast as a USB connection but I'm not sure.

    And, yes, I've looked at Axelson's stuff and it is VERY difficult...very similar to Zigbee, and WiFi and other "protocols" that seem (almost purposely so) very vague and abstract. Why that is I have no clue.

    Thanks for any insight you can give.

    jom

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    Prolific Poster hadv215's Avatar
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    Default Re: USB as a COM port vs. as a regular USB

    Quote Originally Posted by jom View Post
    Change? No...I was just curious. I'm an RF engineer and I've been working with some test equipment and Eval boards recently that act differently when connected (via USB) to my PC. I have a PC driven Synthesizer that connects as a COM port and I have two Eval boards from some ADCs we are working with that connect as a USB and the other as a COM port. I was just curious as to why/how they are different.

    Is there a compelling reason to go with a USB connected (as opposed to a COM port) set up? The ADC eval boards are running data through the USB to PC software that shows FFTs etc etc. Both are running at pretty fast rates so I'd guess the COM port connection runs just as fast as a USB connection but I'm not sure.

    And, yes, I've looked at Axelson's stuff and it is VERY difficult...very similar to Zigbee, and WiFi and other "protocols" that seem (almost purposely so) very vague and abstract. Why that is I have no clue.

    Thanks for any insight you can give.

    jom
    The fact that your synthesizer connects as a COM port may be caused by the fact that it has an internal USB<>RS232/TTL converter (like FTDI).
    I think there are a number of reasons to use USB: speed, the lack of COM-ports on modern computers and, if there is that will be only 1.
    Compare that to 255 USB devices

    I agree that COM can go pretty fast, but with USB 3.0 (and versions to be coming in the future) speed can only increase.
    Take a look at these numbers:
    USB 2.0 480 Mbit/s HighSpeed 2000
    USB 3.0 4,8 Gbit/s SuperSpeed 2008
    USB 3.1 10 Gbit/s SuperSpeed+ 2013

    The biggest drawback of USB is that only relatively short cables can be used.

    Regards,
    Harm

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    jom
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    Default Re: USB as a COM port vs. as a regular USB

    Synthesizer: Actually it doesn't. The USB input directly ties into an ATMEGA32U4 which I presume has some sort of USB module internally. This is similar to my set up using Joe's software I alluded to in my first post. I'm using a PIC 18F4550 (internal USB module) which also connects via a COM port. That is what led me to ask the question...since I also thought it should have connected like a USB instead of a COM port. So I presume that my set up connects via the COM port due to the CDC file I'm using and not because of anything to do with the Visual Basic interface?

    Thanks!

    jom

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    Default Re: USB as a COM port vs. as a regular USB

    The PC software will never determine how a USB device operates, it's the other way around, you'll need PC software to communicate with that specific client.

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