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I need self power from serial port lines, anyone?

Discussion in 'Hardware' started by AlbertB, Apr 17, 2008.

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  1. AlbertB

    AlbertB Thread Starter

    Joined:
    Nov 24, 2002
    Messages:
    2,438
    Hi. I have a requirement to enable software which I am writing to control communications via a serial port to a circuit box in a specific way.

    There is a small, very simple hand built piece of hardware connected at the other end. The PC has to indicate a short countdown with a few beeps at the box. It then has to read back a confirmation from the box made with a push button switch.

    I'm trying to set up my reporting system whereby I can toggle one serial port line to indicate the beeps. A piezo sounder in the box can be driven direct from that. The confirmation signal was to be a change of state on one of the other lines. I have a simple circuit built which will pull a line high (or low) but here is the difficulty, it must draw remote power from the serial lines themselves.

    I could send my timer on the RTS line and receive confirmation on the DTR line which can both be SET/CLRd directly in software. But which lines are left which I can use to steal power for my +ve and -ve supplies?

    DSR and CTS function seems to not be as controllable in the sense of setting one permanently high and the other permanently low.

    I had thought that the TD line stayed high at all times when not transmitting data which I will never need to do so I could use it for my -ve line. First measurements show that this seems to not be the case. Other than that I have reached the end of my current knowledge.

    I don't really have enough experience to see a way through this problem, but surely someone knows a solution. Any thoughts anyone?
     
  2. cwwozniak

    cwwozniak Trusted Advisor Spam Fighter

    Joined:
    Nov 28, 2005
    Messages:
    64,256
    First Name:
    Chuck
    A Google search for RS-232 Powered gives a number of good leads to methods of powering equipment from an RS-232 port. Here is one page found by Google that my help ...

    Get power out of PC RS-232 port
    http://www.epanorama.net/circuits/rspower.html
     
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