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USB to Serial adaptor

Discussion in 'Electronic Design' started by Oppie, Oct 9, 2013.

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  1. Oppie

    Oppie Guest

    Does anybody have a favorite USB to RS-232 adaptor that they can recommend?
    Evidently some are not created equal and have had some issues.

    A colleague in France reported that he had found some discrepancies in how
    the grounding was accomplished:
    "It appears that the RS232 plug body is NOT always bound to USB plug body,
    nor to the pin 5 (ground) of the rs232 plug,
    so that finally there is not the same ground reference between the PC, the
    RS232/USB plugs and then the RS-232 peripheral.
    This is the origin of MANY communication errors."

    In his case, the adaptor was run on a laptop that had no connection to
    earth. The RS-232 peripheral (which we produce) was earthed.

    tia - Oppie
  2. Tim Williams

    Tim Williams Guest

    I have a Kingwin adapter cable:

    - USB shield connects to braided cable shield.
    - Not sure if cable shield goes anywhere at the other end. No apparent
    electrical connection.
    - Pin 5: < 1.3 ohms to USB ground pin (I measure ~0 ohms to computer
    chassis once plugged in).

  3. Tim Williams

    Tim Williams Guest

    - DE-9 shield not connected.
    - Voltage levels meet RS-232 requirements: symmetrical (+/-) 6-7V, with
    some ripple from the charge pump evident. Not good for a really long
    cable (where the original +/-15V would be better suited), but certainly
    legitimate, not some phony +3/-0V "compatible" interface.

    Chip is Prolific (probably PL2303), which has TTL/LVCMOS interface. Don't
    know what RS-232 driver was used; probably a MAXx232 clone.

    - Works well in my limited experiments; seems to have sufficient baud rate
    precision and selection range. Doesn't seem to transmit frames
    continuously, but that may be a limitation of the program's sending
    capability, or the drivers or USB. Does correctly recieve continuous
    frames (i.e., complete stop bit immediately followed by a start bit).

  4. We've been using these under WIN7, works on XP.

    The DB9 shell is floating. Pin 5 is connected to the USB shell.
    I have yet to discover a problem with them.
    The USB cable is 3 feet long to boot.

  5. whit3rd

    whit3rd Guest

    I've always liked the Keyspan adapters; the single USA-19 works well
    and the dual USA-28 (Macintosh style connections) has full-differential
    transceivers (RS-422) and works at impressively high data rates (I've
    tested up to 240 k baud, it claims 1 M bps).
  6. Syd Rumpo

    Syd Rumpo Guest

    I would avoid FTDI parts like the plague. Often, they work well.

  7. I have had good experience with this one:



    PS Some USB to serial converters do not have good control of the handshake lines, so the f*'' up the timing. AFAIR the MCT behaved well
  8. This is the only model I found that works correctly with any equipment
    that's not broken:

    expensive yes, but worth it.
  9. USB is pretty sensitive to ground bounce. We've wiped out usb ports
    because of that. The shorter usb converters may work because they are
    less sensitive to noise due to the short cable length (one has the
    chip in the USB connector end, the other is 12" long). As for the 4
    port powered hub, might be the power supply is robust and external.

    I wonder if a usb isolator will make the longer cables work.

  10. miso

    miso Guest

    I go out of my way to use FTDI chipsets. I never had an issue with them.
  11. miso

    miso Guest

    Ditto this. I use FTDI with old serial radios. They work fine.

    Prolific is probably second best. But I have had issue with them as OSs
    changed. FTDI keeps the drivers up to date for old products. That is a
    rarity these days.
  12. miso

    miso Guest

  13. Syd Rumpo

    Syd Rumpo Guest

    Nor did I for years. Now I do: their support is poor and (at least some
    of) their parts are borderline.

  14. rickman

    rickman Guest

    Care to elaborate? I have used both Prolific chips and FTDI and never
    had a problem with either. What problems did you find? Which parts are
  15. rickman

    rickman Guest

    I have to be at least a little suspicious of these sellers.

    "This is an Industrial Grade Product using a Philips FTDI Chip Set."

    Since when was Philips involved with FTDI??? Oh yeah, since when was
    $30 a "low" price for a USB serial interface? FTDI chips are used in a
    many, many products at a much lower price, often under $10.
  16. Tim Williams

    Tim Williams Guest

    So, what were you expecting? ;-)

    ....Did any of the adapters have brand names on them, or were they all
    assorted POS-1 specials as the price and source suggest?

    I note my Kingwin doesn't actually have a label on it, but it was from a
    reputable distributor.

  17. Syd Rumpo

    Syd Rumpo Guest

    Specifically the M232R module. It works. New PCB with minor changes to
    unrelated circuitry, it dosen't work - 'not recognised' says the PC.
    Often but not always. Different PC's much the same, different modules
    have differing failure rates. Oh look, here's an addendum to the data
    sheet, well hidden. Still doesn't work all the time.

    I'd used their parts for years, but as soon as I had this problem I
    discovered that I wasn't alone. If I disable the 40MHz clock on the
    host PCB, USB module works, always. The clock doesn't go near the
    module, the power looks clean.

    I dare say we'll get it working, but it seems to be very sensitive to
    something. Phases of the moon, probably.

  18. Jasen Betts

    Jasen Betts Guest

    Turbo Pascal "Division by zero" ?
  19. Charlie E.

    Charlie E. Guest

    I have been trying to find a good USB to RS485 serial adapter. I have
    a small contract to modify some code to add MODBUS capability to a
    product. I found one item on Amazon that had USB to serial
    TTL/RS232/RS485 that really fit the bill, so I got one.

    It came in, and seemed to work, then just kinda stopped. I think it
    is that TTL capability - makes the whole thing really static
    sensitive! I ordered another one, and today right in the middle of
    testing, it has gone bad as well. All my commands suddenly started
    failing CRC. After spending all day (on a Saturday!) trying to 'fix'
    my code, I finally pulled out my old USB to RS232 dongle, and my code
    is fine! Not a fun way to spend a weekend, especially since my
    customer is supposed to demo the MODBUS capability to the customer on

    Well, at least on RS232 it works fine...
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