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Seial cable and power in one? — Parallax Forums

Seial cable and power in one?

george miyagigeorge miyagi Posts: 48
edited 2005-07-05 14:46 in BASIC Stamp
Is it possible to use some of the unused pins on the serial adaptor to wire power through so instead of having 2 cables sticking out from the stamp end of the project, just having one, and then running the power source into the other end of the cable? will this interfere with communication to the stamp?
tx

Comments

  • allanlane5allanlane5 Posts: 3,815
    edited 2005-07-05 02:00
    1. Yes, you should be able to do this.

    2. An RS-232 cable is a long and resistive piece of copper. What this means is, if you run too much current down it, it will first get warm, then get hot. If you run TOO much current down it, it will melt.

    I should think 500 mA would be acceptable -- but other's should give their input. If you really screw this up, you'll have a fire hazard for sure. But I don't know the guidance for how much current can go on which guage of wire. The current demand really depends on what you have at the far end of the cable. I've seen a design that powers a remote keypad and LCD display, using power in the serial cable.

    3. I'd put like 12 volts on it at the 'source' end. Then have your standard 50 uF electrolytic, Linear Regulator, 10 uF electrolytic circuit on the 'destination' end. This will allow the cable to drop a few volts between source and destination, while still having enough voltage at the 'destination' end to allow your regulator to work. A 9-volt DC should work also.

    4. If you put enough capacitance at the 'destination' end, you should not get too many noise-inducing spikes on the power supply line at the 'source' end.

    5. Make VERY sure you build the cable properly, to insure the PC end of the cable does NOT implement the pins you use for power (it MUST get ground). You don't want to blow up your PC's serial port, after all.

    6. You will have to do something on the BS2 end of the cable to route the power from your chosen pin to a Vdd connection.
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-07-05 02:23
    This could end up being very tragic.... just bind the power cable to the serial cable.

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    Jon Williams
    Applications Engineer, Parallax
  • george miyagigeorge miyagi Posts: 48
    edited 2005-07-05 02:33
    Scary! Even if it's only 9V running through? It's more to do with only wanting to have one connection, because the object the cables are pluging into are so small
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-07-05 03:11
    The build custom cables -- and do so very carefully. If you cross-up either end with your power supply you could end up damaging the serial port of your PC.

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    Jon Williams
    Applications Engineer, Parallax
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-07-05 14:46
    ·· I agree with Jon.· You could very easily damage your PC serial port if you make even a small mistake.· And while you're concerned about wiring for a small project, remember, the inside of the DB9 shroud is a small tight space, easy to cross a wire inside.·

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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