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    USB to RS232 exactly similar to COM1

    Hi All,

    I have purchased a few USB to RS232 converter but non of them couldn't give me exact COM port of a desktop PC. In my desktop PC running windows xp voltage of pin 3 of COM1 with respect to 5 (Ground) is -11 v when you turn the PC on and nothing connected to COM1 measuring ohmeter and pin 2 with respect to 5 is 0 v but non of the usb to RS232 converters I purchased does not have these voltages (only one model has both 2&3 5v with respect to 5 and the rest zero) and that is why when I connect my handled Niton XRF device to desktop PC it is working OK but when I connect to my laptop via USB to RS232 converters I purchased it is not working. Anyone knows what is the reason of this difference and which converter should I use to get my device to work? Manufacturing company selling C2G brand converter but it is not available locally. I want to find a replacement converter in local market.

    thx

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    Re: USB to RS232 exactly similar to COM1

    Hi,

    Read RS232 specifications.
    There is no need for 11V. If Im not mistaken, then even 3V / -3V levels comply with RS232.

    If your connected hardware does not work with levels below 11V, then this device does not comply with RS232.

    Klaus
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    Re: USB to RS232 exactly similar to COM1

    Device works with COM1 of my desktop computer and also with COM1 of my old heavy laptop. This means it comply with RS232. As far as I know COM1 is RS232. I checked voltages of pin 3 with respect to pin5 in both COM1 ports in idle condition. On my desktop it was -11 v and on my laptop it was -7 v. This means RS232 idle level is 1 (not 0) and between -5 and -15 volts as can be seen here (https://www.lammertbies.nl/comm/info/RS-232_specs.html) and this is the one I measured in both my old laptop and desktop computers COM1 port. Right?
    Last edited by steve_rb; 15th October 2019 at 07:44.



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    Re: USB to RS232 exactly similar to COM1

    Hi,

    This means it comply with RS232
    No.
    It just says it works with your computer, but it does not say anything about compliance.
    *****
    True: RS232 idle state = high = 1 = negative voltage (lower than -5V)
    *****
    ...It seems I was mistaken with my statement about voltage levels.
    They need to be at least +/-5V loaded at transmitter side.

    *****
    * There are several different pinnings at the connectors
    * some DTE device do not wire all RS232 signals
    * some DTE devices have different control signal (RTS, RTR, DTR) status at power up. They might change when opening the serial device at the PC software.
    * some devices draw their operating current from the RS232 signal lines...maybe causing too much current

    Klaus
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    Re: USB to RS232 exactly similar to COM1

    Quote Originally Posted by KlausST View Post
    ...It seems I was mistaken with my statement about voltage levels.
    They need to be at least +/-5V loaded at transmitter side.
    The +/- 3V is correct for the RS232 receiver, so for short cabling it should work if the transmitter gives at least +/- 3V.

    I don't think the voltage levels is the problem here.

    Some older software can only handle COM1-4 correctly, bus USB serial ports can get COM5 or higher.
    In Windows 7 (I don't know Windows 10) you can go to the device manager and force the USB serial port to be COM1-4.



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    Re: USB to RS232 exactly similar to COM1

    Problem is almost solved but not quite. I have a USB-to-TTl converter and it gives me +5 volt at pin 2 and 3 with respect to pin 5. I have added a MAX3232 rs232-TTL converter and now I am having -9 v at pin 2 with respect to pin 5 and 0 at pin 3 but I need revers. I mean I need pin 3 to be at -9 and pin 2 to be at 0 with respect to pin 5. This will be recognized by my old device. I have connected TXD,RXD,GND and VCC on usb-to-TLL to the same signal pins on MAX3232 rs232-TLL converter. How can I make pin 3 to have -9 and pin 2 to have 0? This is exactly what I have on COM1 of my desktop PC. Have I done something wrong? People have claimed MAX3232 can give true RS232 but in my case it is almost true but voltages are reversed at TXD and RXD ?



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    Re: USB to RS232 exactly similar to COM1

    What has not been clear here so far is the purpose of this port in your application, whether only communication or communication + power supply. Some older native COM port-based programmers intricately pull their power from the RS-232 bus itself, and this was no problem with native interfaces which as stated above have a reasonably higher voltage level than those commonly seen in USB converters.
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    Re: USB to RS232 exactly similar to COM1

    Quote Originally Posted by steve_rb View Post
    Problem is almost solved but not quite. I have a USB-to-TTl converter and it gives me +5 volt at pin 2 and 3 with respect to pin 5. I have added a MAX3232 rs232-TTL converter and now I am having -9 v at pin 2 with respect to pin 5 and 0 at pin 3 but I need revers. I mean I need pin 3 to be at -9 and pin 2 to be at 0 with respect to pin 5. This will be recognized by my old device. I have connected TXD,RXD,GND and VCC on usb-to-TLL to the same signal pins on MAX3232 rs232-TLL converter. How can I make pin 3 to have -9 and pin 2 to have 0? This is exactly what I have on COM1 of my desktop PC. Have I done something wrong? People have claimed MAX3232 can give true RS232 but in my case it is almost true but voltages are reversed at TXD and RXD ?
    You must have connected the MAX3232 wrong. The RS232 outputs from MAX3232 are on pins 7 & 14, and one of them should be connected to pin 3 (TXD) of a male DSUB-9 if you want to emulate a PC serial port (DTE).



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    Re: USB to RS232 exactly similar to COM1

    Hi,

    draw a sketch.

    Klaus
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    Re: USB to RS232 exactly similar to COM1

    Yes I got the pin number wrong. Now it is OK and device working OK. Thank you all.



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