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Low profile RS232-DB9

Discussion in 'Electronic Design' started by [email protected], Mar 19, 2007.

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  1. Guest

    I am designing a very small board that has a RS232 connector, the
    typical DB9 connectors are HUGE! does anyone have suggestions for a
    very small, low profile (perhaps even SMT- since this is not going to
    be a high use socket)

  2. Tim Wescott

    Tim Wescott Guest

    A surface-mount socket with some other mechanical support can be just as
    strong as through-hole -- frankly I'd be suspicious of a DB-9
    through-hole that didn't have screws or at least clips.

    Do you have to use a DB-9, or just a nine pin thing? There are some
    pretty small connectors out there if you're willing to step away from
    the older standards.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  3. Guest

    Honestly I HATE the DB9 connectors and I'd happily do away with it.
    However Serial cables are the most widely available for PC-device (no
    I can't do USB unfortunately).

    what others do you suggest? I only need 2 wires rx/tx ?
  4. Guest

    And ground. I used a 1/8" stereo headphone jack in one of my projects.
    I picked up a few 1/8" to DB-9 cables at the junk store so it all
    worked out.
    And don't top post. My cat hates that.
  5. Tim Wescott

    Tim Wescott Guest

    Well, three -- rx, tx and ground.

    If it doesn't need to be exposed to the outside you can use anything you
    want -- I often use a four- or five-pin header, and make up a matching

    Modular telephone jacks are a popular choice for compact equipment that
    needs to run on serial. You still have to make up a matching cable, but
    it's done all the time.

    If you absolutely positively have to be standard, then you just have to
    use a DB-9, and let the tail wag the dog, of course.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  6. 3.5mm audio (microphone type) jacks are a common alternative to the DB-9. A
    few minutes with a Digikey catalog should produce some adapter cables which
    will give you the most common pinouts. Another advantage is that they're
    cheap compared to a DB9.

  7. Gerhard

    Gerhard Guest

    What about using a PS2 (female) connector on your board
    Connections Tx, Rx, Gnd & +5v standard as per PC mouse interface.

    Then also do the following.

    Leave the TTL <-> RS232 level shifter (MAX232)
    off the board and obtain some of the MAX232 wired into
    the connector shells.
    This cable is expensive but one normally only needs a few.
    Cable with MAX RS232 can probably be obtained at $10-$15.
    Replace the cable with a PS2 extension cable.
    Cut the female socket off the eaxtension and wire the
    9-Pin shell with MAX 232 onto the cable.
    This TTL <-> RS232 cable can be very short and you
    can still use a long RS-232 extension.

    The onboard serial interface is much cheaper without MAX 232 and caps.

    The PS2 (miniature DIN) is not the most robust connector but
    I haven't seen reports of connector failures on PC mother boards.

    Gerhard van den Berg
  8. John Larkin

    John Larkin Guest

    We use a 2.5 mm stereo headphone jack for RS-232 on several of our
    instruments where we don't have room for a D9, like this one:

    And then we make extra bucks selling the adapter cable!

  9. Jamie

    Jamie Guest

    Many small devices use a stereo 1/8 jack as the connection
    for RX/TX and common.
  10. Jamie

    Jamie Guest

    You work for M$ ?
  11. Guest

    I think the modular phone plug is a better idea. What I don't link
    about the stereo phone plugs is they tend to short upon insertion.
    Granted, the short is momentary.
  12. mpm

    mpm Guest

    The RS-232C spec did not specify a connector or pin assignments for
    any particular connector.
    (And anyone whose ever tried to attach "anything serial" to a Kaypro
    already knows this!!)

    So basically, you can use whatever connector you like.
    But when you make the selection, try using something that won't be
    confused with some other standard. For example, it would be a poor
    choice to use a USB-style cable for RS-232C, although technically,
    that would not violate the RS-232C standard.

    Later editions of 232 attempted to standardize the physical interface,
    but by then, the genie was out of the bottle. Others here have given
    you some good ideas. I would have to concur with the surface mount
    comments. 9-pin 232's get a lot of user abuse. Keep that in mind.

  13. mpm

    mpm Guest

    I think the modular phone plug is a better idea. What I don't link
    Actually, in the original official RS232C spec, you could short any
    pin to any other pin for as long as you wanted and it would cause ZERO
    damage to the hardware. (Look it up!)

    It wouldn't work of course, but you were essentially guaranteed you
    wouldn't blow anything up!

    Now that said, we all know there were lots of products that weren't
    "truly" RS232 compatible....

  14. Rich Grise

    Rich Grise Guest

    When I was working at FMC, designing controllers for naval missile
    launchers and gun mounts, we made RS-232 drivers/receivers using
    discretes, that really WERE short-proof. ;-)

  15. Guest

    How the chip handles short circuits depends on how the power supply
    rails are created. For chips with on-board charge pumps or switchers,
    the power supply rails are stiff enough to do the job (i.e. 232 and
    drive serial mice), but not all that stiff should the output be
    shorted. It is a cheesy protection scheme. For the chips I've
    designed, the short circuit conditions were considered "indefinite".
    Thus you could short it and it would survive, but a short for 24&7
    until the cows come home would be a different story.

    Using discretes doesn't necessarily save your ass. For instance, if
    the power supply rails are stiff, the drivers could get some serious
    current. It's been my experience that most discrete designs are not as
    rugged as a well designed chip. This is because often chips have
    protection circuits not shown in the datasheet. This is to take care
    of screw ups in the design-in phase. That is, if you short some pins
    that in real life won't get shorted but might get shorted by a clumsy
    scope probe AND the chip fails, the product won't get designed in. The
    customer will think the part is flaky. Such short circuit protection
    is brute force, i.e. set high enough that it will never effect
    performance of the part, and often high enough that the short can't be
    left there infinitely.

    There is a reason most datasheets only show simplified schematics or
    just a block diagram. There is much not being shown.

  16. There are NO DB9 connectors. The proper designation is DE9.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  17. jasen

    jasen Guest

    I've never heard of rs232 being damaged by a short-circuit

    I've seen some equipment use a 1mm pitch 3x2 pin header, atleast I think
    it'a a serial port, the actual label is "console" - it's on a linux
    based ADSL modem.
  18. jasen

    jasen Guest

    The only times I've seen a MC1488 or 1489 fail were during an electrical
    storm. (none of my dodgy serial cables eved damaged anything)

    I never connected mains voltages to a serial port though - did the
    naval ones need to withstand something like that?

  19. Jamie

    Jamie Guest

    The worse thing you can do is have messy cables where the serial cable
    happens to wrap around or be in line with the AC cables during a near
    by strike.
    the induced currents will transfer to the serial and over load it.
    I for one, lost 2 keyboards this way..
  20. Rich Grise

    Rich Grise Guest

    Well, you've got me on that one - maybe for that they depended on the
    difference in connectors. In any case I know they were terribly robust,
    because they were designed to be used on a Navy ship, and survive under
    combat conditions.

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