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USB to UART serial converter.

Discussion in 'General Electronics Discussion' started by HellasTechn, Aug 9, 2017.

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  1. HellasTechn

    HellasTechn

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    Apr 14, 2013
    No... i also had output scopings.
     
  2. HellasTechn

    HellasTechn

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    Apr 14, 2013
  3. dorke

    dorke

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    Jun 20, 2015

    Not enough info.
    But lets see, they are using the CP2102.
    The UART side is for connecting to a Modem(DCE equipment).
    Therefor the way you use it depends on what kind of equipment you connect it to :

    1. If to DCE(like modem) connect directly,and connect DTR to DTR as well.
    2. If to DTE(like PC) connect crossed,and the DTR(of the adapter) to both DCD and DSR.
     
  4. Electrobrains

    Electrobrains

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    Jan 2, 2012
    Once I had a bit similar problem in a complex circuit with micro controllers and amplifiers. To save energy, I wanted to put the amplifiers in standby mode, but I needed the processors running. It did not work.

    The reason might be the same as for you and the solution could be easy.

    When you remove the supply voltage, the internal (positive) rail of the circuit will drop below the positive input signal level. Inside the circuit you will have diodes that are normally reverse biased towards that rail and possibly other parts of the circuit.
    Through the input, you will then "supply" a current into the circuit. That current goes wherever the path is open and can cause both strange behavior or even damage.
    Just looking at the rail, could easily be understood that a strange behavior would appear when the IC would be powered with the input pulses!

    My solution was to add a simple "resistive isolation barrier" between the two circuit groups.
    In my case I put 470Ω in series with the input pins of the powered down circuits. That was low enough to keep the signals running without too much voltage drop and by slowing down the edges too much. It was also high enough to isolate the input signal from the powered down circuitry, without causing any negative influence.

    So my suggestion:
    Try to put some different resistors in series with SW3 and SW4
    (for instance in the range between 10Ω and 4.7kΩ)
    It's easy to test and might bring the solution
     
    CDRIVE likes this.
  5. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Yes i want it to talk to PC serial port.

    I will connect it on a pc usb port and through TERMITE serial comm program i will transmit characters while i will hook the Oscope to the txd and rxd pins to see which is transmitting.

    P.S.
    I do not understand electrobrain's post :( ...
     
  6. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Both RXD and TXD give me +3.5 Volts. While i transmit data the TXD pin pulses to 0V which is normal.
    Why the RXD line gives me +3.5 Volts ? also shouldn't the TXD line be +5 Volts when at idle state ?
     
  7. Electrobrains

    Electrobrains

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    Jan 2, 2012
    Try with some different R (for instance R=100Ω, R=1kΩ etc.)
     

    Attached Files:

  8. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I get it now...
    The question on my post #69 had nothing to do with the problem mentioned in post #1 !!!
    :)

    As for the problem mentioned on post 1 a quick fix is the simple transistor inverter circuit like the one on post #61.
    I dont know ... theese max3232 chips are just acting weird.
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Probably because EB is unaware that you've shifted this thread from the MAX232 to a CP2102.
    Personally I love EB's Resistor fix :cool:for the MAX3232. It makes sense.

    That said your better off staying with the Direct USB/TTL (CP2102) module because your project doesn't require the bidirectional communication that the MAX3232 provides.

    Chris
     
  10. HellasTechn

    HellasTechn

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    Apr 14, 2013
    True... In my case the cp2102 and the max3232 concern two different projects. It is just that i didnt want to make an other thread about rs232 :)
     
  11. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Do you really think that an in series resistor will fix the low impedance problem of the powered off max chip ?
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    It makes sense to me but let's let EB take the reins because he's actually implemented it.

    It's all that crisp clean air in Switzerland. It makes the brain work better! :)

    Chris
     
  13. dorke

    dorke

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    Very important !!!
    The CP2102 is not an rs232 compatible device,you have to add a rs-232 transceiver!!!
    connecting it directly to a RS232 port will destroy the IC(in case the output of that port is larger than 5.8V,or smaller than -0.3v, which it will be!).

    The device output of 3.5V at idle is fine since it is a CMOS device operated internally by a 3.3-3.45V voltage regulator.
     
    Last edited: Aug 22, 2017
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    On another note and since this thread focuses on both the RS3232 and the CP2102, here's a level shifter if you need to level shift from 3V to 5V logic level. It's none inverting so a logic1(3V) in = a logic1 (5V) out. I believe your USB/TLL(CP2102) module provides a +5V Vcc output.
    Chris
    upload_2017-8-22_8-48-26.png
     
  15. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I figured that it could not receive rs2
    Yes it does. Thank you for the circuit.
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I don't believe that Ivan has the stand alone chip. I think he said he purchased (Ebay) the complete USB/TTL converter. You'll have to search back through this thread to see where this thread switched tracks.

    Chris
     
  17. dorke

    dorke

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    Jun 20, 2015
    Oh,
    He absolutely has the sand alone chip,that is what's on the module( #69).
    It connects directly to USB and produces CMOS(TTL compatible) interface to the "rs-232 side".
    From #73 you can confirm that the levels are 0-3.5v.
    That isn't RS-232 compatible thus a rs-232 transceiver is absolutely needed !
     
  18. HellasTechn

    HellasTechn

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    Apr 14, 2013
    i was looking at the diagram and cant figure out what the switches are there for also is vin the 3.5V signal output from the converters RXD pin ?

    R2 and r3 are going to GND right ?
     
  19. HellasTechn

    HellasTechn

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    Apr 14, 2013
    In other words it is a USB to TTL serial converter. It can not accept rs232 level signals. Come to think about it, when i bought it i thought it was usb to rs232 converter and the fact that was not lead me to put it aside.
     
  20. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I know i am mixing the posts right now but here is the question. According to your diagram Chris, the idle state of the rs232 is 15 volts (or could be up to 18 for hardware rs232 i think) while at the following link Christ clearly states that the idle state of a hardware rs232 is negative. look at his post #9

    https://www.electronicspoint.com/threads/help-with-data-switch.259040/


    So it is the exact opposite than what is shown on the diagram on your post #61
    The -15V will be +5V for logic 1 and the +15V will be 0V for logic 0
     
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