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> Is This Wrong With Max232cpe?
anhnha
Posted: April 28, 2012 06:10 pm
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I used MAX232CPE without any capacitor and power supply is tied.Then I tied the TX of comm port to one pins the the RX of the comm port to another pins of MAX232.Next transmit data to comm port.But i am confusing that I can receive data exactly when TX and RX is tied any pin of MAX232
I have tested MAX232 it is good.Would you tell me is this is true?Or there something wrong here?
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MikeGyver
Posted: April 29, 2012 01:29 am
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No charge pump capacitors?
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anhnha
Posted: April 29, 2012 01:35 am
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Thanks for your help.
I had used caps and tested it but there are serval things make me confused.I can receive even without power supply.Then I remove all caps and power of max232 and tied TX and RX to any pin of max232, say VCC and GND and it can receive data exactly.I wonder if this is true for max232 or not?
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draget
Posted: April 29, 2012 04:14 am
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Its a level shifter/driver.

Measure voltages/watch the waveform and you will see why the capacitors are supposed to be there.
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anhnha
Posted: April 29, 2012 05:55 am
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At first I had connected all components as the datasheet and i found that I only need tied TX to one pins and RX to another pins of max232 to transmit data without power supply.
Then in order to debug this circuit i checked by removing every component step by step.Finally, when all caps and power is removed, I sitll received the data exactly.
It seem that all pins of max232 is connectted to each other without power and resulting in TX and RX is connected to each other.
It make me confuse if it is true for this chip or not.Previously I thought the chip is blown but when i checked it under the following guide, all results is relatively right.
"Power up the MAX232 and check voltage across supply pins to insure correct voltage input.
The following two tests check each "TTL in - RS232 out" converter.
Put 0 volts into pins 10 and 11, then check pins 7 and 14. Should have about 10 volts output on each.
Put 5 volts into pins 10 and 11, then check pins 7 and 14 again. Should have about -10 volts output on each.
If that succeeded, then check each "RS232 in - TTL out" converter. Connect pin 7 to pin8. Connect pin 13 to pin 14. This will be a "loopback test".

Put 0 volts into pins 10 and 11, then check pins 9 and 12. Should have about .6 volts output.
Put 5 volts into pins 10 and 11, then check pins 9 and 12 again. Should have about 4.5 volts output."

Therefore I it make me confuse that chip is work or not.

This post has been edited by anhnha on April 29, 2012 05:59 am
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draget
Posted: April 29, 2012 06:05 am
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So you did test it, and you put 5V in and got -10V out when it was connected properly with a power supply and capacitors?

But the device still worked when you removed the power? You definitely didn't get +/- 10V out with no power...

What is the receiver you are sending the RS232 to?
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anhnha
Posted: April 29, 2012 09:05 am
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QUOTE (draget @ April 29, 2012 07:05 am)
So you did test it, and you put 5V in and got -10V out when it was connected properly with a power supply and capacitors?

But the device still worked when you removed the power? You definitely didn't get +/- 10V out with no power...

What is the receiver you are sending the RS232 to?

QUOTE
So you did test it, and you put 5V in and got -10V out when it was connected properly with a power supply and capacitors?

Yes.When I put 5V in TTL inputs and got -9.21V out when all components is connected as datasheet.
QUOTE
But the device still worked when you removed the power?

I still send and receive data when power off.
QUOTE
You definitely didn't get +/- 10V out with no power...

Yes. although the power off and I didn't get +/- 10V out, I still send and receive data exactly.
QUOTE
What is the receiver you are sending the RS232 to?

I used DEV-C++ transmit and receive data to/from com port.
Thanks.
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Ice-Tea
Posted: April 29, 2012 09:49 am
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Assumption: input has ESD diode to Vcc. Connection this input to a high signal will feed the chip through the ESD diode.
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Gorgon
Posted: April 29, 2012 11:34 pm
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The commport is supplying the -voltage via the Rx input from the PC. This will drive the tx to a negative voltage. This may work as long as you don't receive anything from the PC when transmitting.

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anhnha
Posted: May 04, 2012 04:32 am
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Thanks for all help.
You are right.
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