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USB to RS232

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Wig
Wig Posts: 14,139 Forumite
I'm just wondering how a USB to RS232 (serial port) cable works?

USB has four "pins" 5 if you count the external bit which might act as a ground. And serial port has 9 pins & an external bit too. So how does the cable manage to adapt the signal?

Comments

  • isofa
    isofa Posts: 6,091 Forumite
    If flow control isn't required, 2 or 3 connectors via RS-232 will work fine for transfer, only when flow-control is needed does more (5 connections I believe) become important, I doubt many devices ever used the full 9 pin connections. So I guess (I'm not certain!) that is why those cables work... (unless of course a specialist driver is used to swap the signals back and both from the transmit and receive pins etc).
  • espresso
    espresso Posts: 16,448 Forumite
    Part of the Furniture 10,000 Posts Combo Breaker
    Wig wrote: »
    I'm just wondering how a USB to RS232 (serial port) cable works?

    USB has four "pins" 5 if you count the external bit which might act as a ground. And serial port has 9 pins & an external bit too. So how does the cable manage to adapt the signal?

    Basically USB has four connectors +5V D+ D- 0V The outer external part of the plug/socket is not used but a supply voltage is useful to power attached devices e.g. Flash drives etc.

    RS232 has many other signals but only TX RX and Gnd are really needed to transmit and receive data.

    USB is a later standard where all devices work to the same standard.
    :doh: Blue text on this forum usually signifies hyperlinks, so click on them!..:wall:
  • unrich
    unrich Posts: 814 Forumite
    I imagine there is a small interface circuit powered by the 5v.
  • Nilrem
    Nilrem Posts: 2,565 Forumite
    Part of the Furniture 1,000 Posts
    unrich wrote: »
    I imagine there is a small interface circuit powered by the 5v.

    yup, they aren't directly compatible so the USB to Serial connectors have a conversion chip in them (same with the usb to parallel etc connectors) .
  • Yep, there's processing going on in there. I have a USB-IDE cable which lets me plug in an old hard disk drive. That's got 40 pins on the IDE end.
    USB 2.0 is a pretty good standard actually. It's easy for manufacturers to write device drivers, or use existing ones, and the throughput is really fast - you can send two high def TV channels down one USB connection.
    :)
  • espresso
    espresso Posts: 16,448 Forumite
    Part of the Furniture 10,000 Posts Combo Breaker
    Yep, there's processing going on in there.............

    It's not processing as such, more level changing. the processing is done by software not hardware. USB data signals are logic 0 = 0V logic 1 = 3.6V whereas RS232 uses signals that are plus or minus 3 to 15 volts i.e. they swing above and below 0V. The level conversion chip used is commonly a MAX232 details here.

    USB to IDE adapters are a completely different thing.
    :doh: Blue text on this forum usually signifies hyperlinks, so click on them!..:wall:
  • Belnahua
    Belnahua Posts: 1,493 Forumite
    Cashback Cashier
    We use these http://www.misco.co.uk/applications/SearchTools/item-details.asp?EdpNo=235832&CatId=760 at work, and they generate a new COM port; so we have com1 thru 4. Which is very useful when you have old legacy equipment using RS232 and new PCs with only one serial port!
    A friend is someone who understands your past, believes in your future, and accepts you just the way you are.
  • Wig
    Wig Posts: 14,139 Forumite
    Ok, that's understood now. Thanks everyone.
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