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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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 Once i had programmed similar problems using plain c under real mode, what is rather outdated.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

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.

This looks like a fairly difficult task. How do you plan to calculate resistance using capacitance? I would suggest you create a wheatstone bridge setup like

Here is another link for serial port.
http://electrosofts.com/serial/index.html

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?

It won't work and you will probably fry the port. Why are you trying to do this?

> 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.

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.

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.

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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