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Can anyone tell me how I can read the resistance between pins 4 and 6 of the serial port using VB or c++?

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Last Post by hammerhead
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hi guy40az,

i am afraid this is not an easy task. Once i had programmed similar problems using plain c under real mode, what is rather outdated. Today i am using professional library for port and memory mapped IO on XP. Maybe you will succeed by using this interesting library: http://www.codeproject.com/KB/system/serial.aspx
good luck!

krs, cliff

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krs, cliff

Thanks for the link I think I can us the info to make it work.

I was also thinking about using a rc circuit and measure the time it takes to charge the capacitor and I could figure out the resistance that way.

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My plan was to calculate the resistance of the RC circuit. The formula T=R*C.

T=time
R=resistance
C=capacitance


So if I know the capacitance and the time it takes to charge the capacitor I can then calculate the resistance.

Do you think this would work?

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> Can anyone tell me how I can read the resistance between pins 4 and 6 of the serial port
Serial ports are digital.
Try using the analog game port used for reading analog joystick positions.

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My electrical is rusty but i think t=RC will work. But I suggest you to do lots of research before proceeding with it. Pins from the serial port have physical voltages of their own. This is a fairly difficult problem and you may end up frying your port. You are better off with a multimeter.

Serial port behaves erraticaly and produces +3 to +25 V for logic 1 and -3 to -25 for logic 0. I suggest you use a Max232 converter IC to scale them down to static +5V and 0V.

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It won't work. The serial port has a maximum output current and will try to charge the cap with an unlimited amount of current.
We still don't know why he wants to do this.

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Once the capacitor get charged it will start acting as an open switch. Even I am curious as to why the op wants to do such a thing.

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