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RS232 convertor - Why?

Discussion in 'Electronic Design' started by [email protected], Dec 2, 2006.

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  1. Guest

    Hi All,

    I used to enjoy electronics as a hobby many years ago but sort of left
    it behind in the last 10 years.. :(

    I have now rekindled the hobby by building myself a GPS attachment for
    my camera (Nikon D200)
    Soldering skills are not what they used to be but and a few attempts
    later and some trips back to the shop for more veroboard and
    components, I got it working.

    To the question:
    The GPS device pumps out serial data. Fine. Happy with that.
    The Nikon accepts serial data. Fine.
    Why is it that all the circuit diagrams I find on the net stay I need a
    MAX232 IC in between? Are the levels different? If so, which is higher?
    Would sticking the TX from the GPS straight into the camera cause a
    loss of that ever important smoke (Electronic components work on smoke;
    once it escapes, they work no more.)

    Compliments of a few sites I have found on the net, I built the RS232
    circuit and the camera detected the GPS data and all is well.

    My question stems from the fact that I have this lovely little GPS
    receive all with SMT and a smart little smoked box. Now I have to have
    an ugly veroboard black box attached as well. (or stick everything
    thing into a larger box)

    TIA for any help...

    Crispin Proctor
  2. And if you asked where this sort of question belongs, sci.electronics.basics
    then I likely would provide an answer.

    This is not a design issue. This is a basic issue. Grasp what RS-232 is
    about, and you've got your answer.

  3. Well,

    There are lots of tastes in serial data transfer. The RS232 as used by the
    COM-ports in PCs are most commonly known by the general public. The signal
    level vary from +/-5V to +/-12V or something in between. Which is well
    according to the specifications. The serial data from cameras is at TTL
    level so 0-5V (or 3.3V these days.) AFAIK the Nikon D200 uses 0-5V. So the
    point is: What's on the GPS side? If it's true RS232, you'd frie the serial
    interface of your camera when connecting directly. If it's also TTL the
    should have the same levels or you may damage your equipment as well.
    Apparently your GPS speaks RS232 as it is unlikely (to say the least) you to
    have a working connection by now.

    petrus bitbyter
  4. Guest

    The 232 trip level can be any where between -3 to 3 volts according to
    the specifications, but nearly all 232 receivers are designed to be TTL
    compatible. I make it a point not to use those 232 chips with charge
    pumps for short distance interface.

    If you building something just for your own use, check the levels. If
    everything is above ground, skip the 232 chip. If one of the
    transmitters goes beneath ground, you need to insure that you don't
    forward bias the protection diode of whatever circuit you attach to
    that transmitter. Generally a series resistor will do the trick, but
    that does slow things down.

    If you are trying to market some product, all bets are off.
  5. NMEA0183

    NMEA0183 Guest

    NMEA I/O from a GPS receiver is TTL swing but opposite polarity to true RS232.
    So apart from the level shifting - which in MANY cases is not required - the
    inversion in a MAX 232 (et al) is the key ingredient in most of these circuits.

    Regardless of actual signal levels, I'd be betting that the camera has true
    RS232 polarity at its serial port and can direct connect to a PC's com port.
    The odd-man-out in these schemes is the GPS receiver. The NMEA signalling
    system was designed to directly interface (and multi-drop) to other marine
    electronic devices such as autopilots, NOT personal computers. Some later GPS
    receivers have USB output to satisfy the market for direct-PC-connection.
  6. NMEA-0183 is TTL levels, but with RS-232 polarity. Almost all PC
    serial ports will accept the 5 volt NMEA-0183 output from a GPS
    receiver with no problems.

    To convert NMEA-0183 signals to true RS-232 levels, you need an
    inverter and a MAX 232 or other RS-232 line driver, as the line
    drivers invert the signals, as well as changing levels.
    Peter Bennett, VE7CEI
    peterbb4 (at)
    new newsgroup users info :
    GPS and NMEA info:
    Vancouver Power Squadron:
  7. Overlooking my information so far I think you're right. If the serial camera
    interface was supposed to be connected to a PC directly (using the cable
    provided with the camera?), its polarity and levels should be true RS232. A
    lot of modern PCs may accept TTL levels (*not* polarity) but you can't count
    on it. All RS232 drivers I know are inverting ones, so you have to invert
    again when going back to TTL. (AFAIK TTL with RS232 polatity exists but is
    not very common). Nikon provides a cable, MC-35, with build in RS232 to TTL
    conversion ($150,--) apparently to convert the cameras RS232 to TTL and not
    the other way around as I was originally thinking. A circit like that can be
    build into a DB 9 connector cap using modern (SMD) components.

    petrus bitbyter
  8. <snip>

    I'm 'glad' you can make the TTL work into RS-232 but as one of the guys
    who has to clean up after things like that, I'm not inpressed. Example:
    Rosetta Stone RS-232 to RS-422 converter (different I know but wait)
    uses a few transistors inside to do the inversions and level shifts
    required. It 'works' with a short cable but run it into a patch bay
    with longer cables and it falls apart - edges skewed, crosstalk TX to
    RX so that it's useless. So when I hear of somebody intentinally
    cheating to make HIS life easier at the expense of mine, I'm not

  9. NMEA0183

    NMEA0183 Guest

    Gee, Peter, I'll have to go back and rebuild all those GPS<->PC interfaces that
    I've built so far, and which are currently in service { ;^0>

    I'm quite used to using TTL swings directly into RS232 devices, but the polarity
    is not as you indicate on the Rockwell/Conexant Jupiter receivers, for one.
    These receivers do output true NMEA signalling.
    When outputting "NMEA" protocol data (as distinct from Garmin's proprietary
    protocol), receivers such as Garmin II+/III+ output NMEA sentence structure but
    NOT true NMEA signals, which are opposite polarity to RS232.

  10. NMEA0183

    NMEA0183 Guest

    Whoa there Glenn. I *have* used TTL swings into RS232 devices (and vice versa)
    a lot, but only for my in-house stuff and NEVER for release outside my cage.

    I am fully aware of how *iffy* it can be. So any *cheating* (as you put it)
    that I do certainly isn't at your expense or anyone else's. But thanks for
  11. Guest

    Thanks guys. A lot of it is over my head for now, will have to go read
    up but the question was answered.

    As pointed out, Nikon has a MC-35 which is stupidly expensive and
    I managed to make the same thing with a £10 cable from ebay and £5
    worth of parts from Maplin.
    After building it, connecting it and waiting for the smoke, the camera
    started flashing GPS which means it found it. Yippeee.

    Thanks again for the answers and to the two guys (gals?) having a fight
    - chill :)

  12. Paul

    Paul Guest

    Don't buy anything from Maplins. My broadband started to go slow and my
    ISP said I needed a new filter. Maplins said I needed new cable. I
    bought new cable but the broadband speeded up again - must have been
    BT. So I took the cable back. But because it had been machine packed
    there is no way I would return it "in the same condition". The cable
    WAS "in the same condition". But the idiots said the packing had to be
    the same -- impossible!

    If they want to get things back "correctly" they should use packing it
    is possible to pack.

    Every other company I've taken anything back to, in living memory has
    accepted it back with full refund.

    Except Maplins.

    Don't even think of shopping with them.

  13. I'll bet you're so cheap that you would try to return used condoms.

    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
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