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RS232 to TTL

Discussion in 'General Electronics Discussion' started by dashy1981, Jan 13, 2014.

  1. dashy1981

    dashy1981

    45
    1
    Jun 19, 2013
    Hi Friends,

    I have one old COM to TTL converter(IC Part : WP91380L1 Maxim 9920), which I have used for connecting Rx and Tx of micro at 5V logic.

    But at present my micro is at 3.3V logic. Can I use my old converter for it ?

    I googled a lot about the datasheet of the chip mentioned above, but it was nowhere to find.
    Any help, deeply appreciated.

    Cheers
    dashy
     
  2. Arouse1973

    Arouse1973 Adam

    5,164
    1,078
    Dec 18, 2013
    if you feed 5 volts into a micro input which is running from 3.3V you are likely to damage the internal ESD protection diodes. And don't think about putting a series resistor to limit the current through these diodes as this can cause latch up and you don't want that. Another option would be to run the micro from 5Volts if that's possible. Is the part number you gave correct, that number seems to be a current sense device?
    Adam
     
  3. dashy1981

    dashy1981

    45
    1
    Jun 19, 2013
    Thats what i also thought... strangely the part number is correct, I too was directed to some chinese distributors(by searching for WP91380L1) but nowhere could i find the data sheet. Googling for Maxim 9920 gave datasheets for current sensing devices :-( :-(
     
  4. dashy1981

    dashy1981

    45
    1
    Jun 19, 2013
    Continuing on above I am planning to use a voltage divider to convert 5V to 3.3V. according to my calculation I need to connect ckt as below:

    ---- 5V signal
    |
    |
    |
    z
    z R1
    z
    |
    |---------- to 3.3V signal
    |
    z
    z R2=2R1
    z
    |
    |
    ---- GND

    Is it all I need ? what about current limiting resistor and the values I choose for R1 and R2 ?

    Pls reply.

    Cheers
    dashy
     
  5. Arouse1973

    Arouse1973 Adam

    5,164
    1,078
    Dec 18, 2013
    Yep that will work.
    But with any resistor in the signal path you will have an RC time constant between the micro port pin and the signal line. So before you work out the value of the resistors properly you need to know 1) what the capacitance of the micro port is. 2) You then need to find out what the shortest pulse width is of your data. If you try say a 1K for R1 and 2K for R2 this will be ok with a port pin Capacitance of 1nF(This is probably excessive) and rise and fall time of 1us and pulse width of 100us. This is just an example for you to try and work out this your self. Keep the ratio for the two resistors the same and you can go up and down in value to get this right. Don't go too low as your driver circuit might not be able to supply into the signal line. If you need to go low in value of resistors then your better off with a high speed buffer with level shifting.
    Thanks
    Adam
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    Some 3.3V devices are "5V-tolerant" (specifically designed to be able to accept 5V signals) and will not be damaged. You need to look on the data sheet for your microcontroller. (Or you could tell us what it is.)

    Maxim (and others) make lots of RS-232 interface devices that can be powered from 3.3V and use 3.3V levels on the logic-level side.

    I've never heard of WP91380. The 9920 is probably a date code (1999, week 20).

    The ideal ratio between R1 and R2 is 1.94 not 2.0, because the 3.3V rail is not 3.333V. But it's not critical; the pin doesn't need to be driven to exactly 3.3V and it won't be damaged by a slight overvoltage (typically 0.3V is fine).

    Typical pin input capacitance is around 10 pF. Resistances of 12k and 18k will be fine unless you're running at a high speed (more than say 115200 bps). These will add a constant load of 125 µA on the 5V rail.
     
  7. Arouse1973

    Arouse1973 Adam

    5,164
    1,078
    Dec 18, 2013
    Yes well put Kris, better than my explanation, but I didn't have any 1940 Ohm resistors at the time :)
     
  8. dashy1981

    dashy1981

    45
    1
    Jun 19, 2013
    Thx Kris and Adam, for replies.
    @kris: I'm using a Beaglebone black board. details for its serial port are provided here:
    http://elinux.org/Beagleboard:BeagleBone_Black_Accessories#Serial_Debug_Cables
    But since I have none of the cables listed there, I was trying to make my old cable with some unknown chip WP91380, work with it.

    Fortunately my adapter was a two side pcb, so did not take much time to figure out that the chip is a replacement for MAX232CPE kind of chip, as it matched pin to pin.

    Going forward I used 10K and 20K voltage divider to connect RX of TTL side(from adapter) to Rx of my board. Tx of board I connected directly to TTL side of adapter since 3.3V will be regarded as High for TTL as well. The setup worked flawlessly with 115200 baud.

    So for now I can use my old adapter with this board.
    @kris: anything I am missing or any improvements ?
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    Sounds all good to me.
     
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