my RS232's low level changes to -2.5V after opening COM!!

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My RS232 communication can not work, so i test its level. After MAX232, it outputs level nearly +10V or -10V, this is right. But after i open the computer serial COM(RS232) through the software, the low level changes to about -2.5V.
As we know, low level should be below -3V that the computer recognizes. So the AVR of my circuit is not able to communicate with the computer.
But nobody has mentioned this strange problem before, they always do this right once at a time. Who can give me some advice?

Our Martians are beginning to learn AVR

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Do you have the Tout from the MAX232 connected to the Tx from the port? You *shouldn't* have - connect Tout to Rx and Tin to Tx.

Neil

Edit: corrected double 'Rx'

Last Edited: Sun. Dec 30, 2007 - 07:24 PM
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i am sure my connection is right, i have checked the connection from the textbook.

Our Martians are beginning to learn AVR

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Can you break your connection and test the voltage separately on the PC and MAX232 ends?

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Something on the tx line is using all the -10 up. Some folks use the RS232 negative volts to adjust contrast on an LCD for example. What else is attached to the -V pin on the max232?

Imagecraft compiler user

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Perhaps you are transmitting? Try using a scope on it...

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Also, is it specific to one computer or is it repeatable on others?

Just sounds like you have your wires swapped. I know you checked this, but double check. Or stick a null cable in there and see if it persists.

Regards,
Paul

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I just use pin2,3 and GND5. Is it necessary to connect the other pins?

Our Martians are beginning to learn AVR

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My MAX232 did not connect with the other part, it just for the RS232 communication. using RS232 cable connecting PC and MAX232 did't make trouble, even transmitting data, but when you use software to open the COM, the overall voltage rises several V. close the COM, it is +6.5V and -10V, open it, it is +11V and -2V

Our Martians are beginning to learn AVR

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maybe i know my problem, i think DB9 male and female are the same, but in contrast. in PCB , the footprint is female, but in real circuit, i use male, so i guess this is the problem

Our Martians are beginning to learn AVR

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Which is what I said in my first post :mrgreen:

It's almost always safest to use a 'straight through' male-female cable, so you have a *female* connector on your PCB, and the Tout of the MAX chip goes to pin 2 on the female connector, and Rin goes to pin 3. Ground goes to pin 5.

If you put a *male* connector on your PCB you need to be aware whether you will be using a straight-through or null-modem (crossover) cable, and wire accordingly. You also need to write it down in big letters on the outside of the box because you *will* forget next time you need to find a cable...

Neil