MAX232E: MAX232E

Pins 11 and 12 must not be connected to the RS-232 bus. See figure 10 in the datasheet:

HI, 

I have C3 connect to GND. I have serial port DB9 pin3 connect to MAX232E pin13. I could not see any signal from MAX232E pin12. Please let me know if I should have a pull up or pull down resistor on MAX232E pin12?

Pins 8 and 13 have internal pulldowns.

What are the voltages at pin 13 and 12?

Hi Kaipun,

What is the voltage at pin 13 (from pin 3 of the DB9 connector) - is there a data-signal confirmed being sent to the device?

Is there any signal being seen at 12? 

Pin 12 doesn't require a pull-up or a pull-down (it shouldn't hurt adding a pull-up) however, since the output is meant for TTL or CMOS logic - which are push-pull architectures meaning there doesn't need to be pull-up to drive the signal - pull-ups/pull-downs would be used if you want a known idle voltage when nothing is being driven. 

Please let me know!

Best,

Parker Dodson

To Clemens and Parker,

[kp] Pin 13 gets +5V and -5V from DB9 pin3, pin12 was always high. With DB9 disconnect DB9, pin13 pull high to +5V (via 4k7), i got a 0V at pin12; pull low to pin13 will get 5V at pin12. It shows pin13 was working well from 0V to 5V but it was not if it goes from -5V to 5V (my view). The baud rate was 9600 during the test.

Can you show an oscilloscope trace of pins 12 and 13?

Connecting bus voltages to a logic pin might have damaged the chip. Can you try another?

Will be getting the oscilloscope for pin12 and pin 13.

Please let me know if the following waveform is correct:

1. C1+ : Square wave from 5V to 10V

2. C1-: Square wave from, 0V to 5V

3. C2+: Square wave from 0V to 10V

4. C2-: Square wave from 0V to -10V

I have only purchase 3pcs. 2pcs was showing the same results.

Hi Kaipun,

Those results for charge pump seem okay. 

The VCC is 5V

A 5V square is output at C1-  should result in ~2x the input voltage on C1+. This charge is then stored in the storage capacitor at V+. This is acting as a positive charge pump.

For C2 a 10V square wave should be output from C2+ and the C2- should be ~ -1 * C2+ voltage - which is then stored in the storage capacitor at V-.

Based on the results you gave above that seems okay.

When you get the scope shots please share them as it may help id a problem - as the initial setup where logic pins could have experience bus voltages could have damaged the part. As it should be able to receive at +/-5V signal at the RIN pins. 

For the one part that isn't giving those charge pump values - what values are you measuring? 

Best,

Parker Dodson

Hi Parker,

Vcc to Gnd was using a 10uF only.

I have redo the circuit using my final purchased parts. It was working fine now, I am able to see output from pin12 with input to pin 13 (RS232). I may have really damaged the chips while testing the TTL/CMOS pins with higher voltage.