Problem with max232

Discussion in 'General Electronics Chat' started by rogercores, Jul 14, 2012.

  1. rogercores

    Thread Starter New Member

    Jul 14, 2012
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    I'm new here, hence forgive any mistake i might commit unknowingly.

    I've read a lot of threads about max232 and related problems, but i didn't find any substantial solution hence i have to post this.

    I have 2 Max232CPE and one Max232N. I connected the 10uF caps as given in datasheet and rechecked their polarity. My DB9 is good.when i connect the "CPE" i get garbage value on the flashmagic terminal. For "N" there is no o/p at all. I dont have multimeter and other test equiments, so i connected led to pin 12 of max232.For both "CPE" and "N" led blinks when data is sent. for both when i short 11 and 12 there is no o/p.

    For the "CPE" when 11 isn't short with 12, it gives garbage.Is this because of wrong caps or some other issue?

    what are best ways to fully test max232 chips.

    i heard about ds275, and that it doesnt require caps, is it good for 89c51,89s52 and P89v51rd2 ???

    plz reply,
    thnx
     
  2. upand_at_them

    Active Member

    May 15, 2010
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    C4 is backwards in your circuit. The + lead goes to GND and the - lead goes to pin 6.
     
  3. BMorse

    Senior Member

    Sep 26, 2009
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    the caps used for the max232cpe DO NOT need to be polarized, using 0.1uf non polarized caps (used for decoupling) would work just fine.....
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    Isn't really an effort to use 1uF caps.
    Because they are very cheap. I have MANY of these around here.
     
  5. upand_at_them

    Active Member

    May 15, 2010
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    Yeah, 10uF is higher than needed. I have two of these circuits setup on my workbench right now and they both use 1uF caps. Also, at the PC COM connector I have pins 1, 4, and 6 tied together, and pins 7 and 8 tied together.
     
  6. rogercores

    Thread Starter New Member

    Jul 14, 2012
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    I corrected C4, didn't show any progress. I'll get the .1 and 1 uF caps then.....
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    Couple of suggestions, what baud rate are you using? Can you post a photo of your board?
    Also show a schematic of your MCU circuit. What are you using for a crystal?
     
  8. rogercores

    Thread Starter New Member

    Jul 14, 2012
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    The baud rate is 9600, I've attached the photos.
    i'm not using any mcu i'm testing max232, although i plan to use the ic for connection with P89v51RD2 once it works properly.

    also, is it good to use ds275. heard of it a lot, and that it doesnt require caps......
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Disconnect pins 14 and pin 13 from the MAX232 and connect pins 2 and 3 from the DB9 connector for a loop-back test and let is know how that works.

    I believe you have the connections to the DB9 reversed. TXD from the DB9 pin-3 should go to pin-13 on the MAX232. RXD from the DP9 pin-2 connects to pin-14.

    The MAX232 spec sheets show 1μF caps. Use those if you have any.
    In any case, the cap on pin 2 should be positive at pin 2 and negative to GND.

    Put a cap across VCC and GND, +ve at VCC, -ve to GND.
     
  10. rogercores

    Thread Starter New Member

    Jul 14, 2012
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    just replaced caps value from 10uF to .1uF, got rid of 7805 and used a 5v adapter.
    everythings working fine, still i m not clear why i faced that problem...
    loopback test is positive,
    when i didnt connect the pin 11 to anything it gave garbage, but when i connected it to ground, garbage didnt continue...
    could this be a problem??:confused:
     
  11. BMorse

    Senior Member

    Sep 26, 2009
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    why are you still using polarized caps???? anyway, pin 11 is a TTL level input... so I do not know why it is solving your "issue".

    Double check all your connections and replace the caps with non polarized ones... polarized do not seem to always work well with these IC's.... I have spoken to MAXIM engineers themselves before and they did say non polarized will work better because of the built in charge pumps, etc.....

    RS232_TTL.png

    once you double check you connections, and you place a jumper across pins 11 and 12 of the MAX232 IC and you should see exactly what you send to it...
     
  12. rogercores

    Thread Starter New Member

    Jul 14, 2012
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    cant get non polarized ones here,
    loopback (jumper across 11 and 12) test works fine,
    but sends garbage if pin 11 is not connected to anything...

    i'll try to get the non polarized one caps
     
  13. BMorse

    Senior Member

    Sep 26, 2009
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    hmmmm, thats odd that you can get polarized ones and not non polarized... anyway, I can't really tell from the pictures but, what side of the capacitor is connected to pin 6? It should be the - side of the cap with the positive side connected to ground, since this is the -10 volt charge pump output.... and the capacitor you have hooked up to pin 2, definitely needs to be non polarized, you have it connected in reverse, so what is happening is the capacitor's electrolytic plating is breaking down and the leakage current is increasing. It is becoming less of a capacitor and more of a resistor. I know this is how it is shown in the datasheet, but, as I have said, years ago when I started playing around with these IC's I had issues using polarized caps, I contacted them and I was told by their engineers that non polarized will work better for the charge pumps to work properly..... but, if you can not get non polarized capacitors from a source, is there any electronics you can salvage them from? They are usually plentiful on a lot of boards since these are usually used for decoupling power supply lines and such......
     
    Last edited: Jul 18, 2012
  14. rogercores

    Thread Starter New Member

    Jul 14, 2012
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    i'm still trying to get the non polarized caps,
    meanwhile i was successful to program p89v51rd2
    i'll definitely use non polarized when i mount on a board....

    thanx for all your help....
     
  15. MrChips

    Moderator

    Oct 2, 2009
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    The MAX232 data sheet suggests that 1μF electrolytics are fine, just watch the polarity. Some converters will accept 0.1μF caps.
    MAX233 require no caps (except for supply decoupling). Shop around.

    [​IMG]
     
  16. MrChips

    Moderator

    Oct 2, 2009
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  17. rogercores

    Thread Starter New Member

    Jul 14, 2012
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    just went testing my ICs and found out that max232n work well, even programming p89v51rd2, but every max232cpe give garbage for the same circuit.

    is there any difference between cpe and N???
     
  18. BMorse

    Senior Member

    Sep 26, 2009
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    yeah, the size of the caps......
     
  19. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    But the op asked about the MAX232CPE and MAX232N..... the CPE requires .1uf....... as I have said the datasheets show polarized, but expect issues.... use non polarized, no issues...... I am currently using a MAX232CPE to transmit data to a PC from a PIC16F887, no issues at all using .1uf NON-polarized caps....
     
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