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TTL / RS-232 conversion circuit

Discussion in 'Electronic Basics' started by Will, Jul 1, 2005.

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  1. Will

    Will Guest

    Hi All,

    I am interfacing a ham radio to a computer to program it and I have been
    looking at various TTL / RS-232 conversion circuits. However I happen to
    have a commercial ADMS programming cable I use with my Yaesu VX-7R radio
    (ham radio). This cable also handles this type of conversion so I am
    thinking why reinvent the wheel when I can utilize this existing circuit
    which is already integrated into the D-sub shell of this cable.

    Thus my question:

    These generic circuits appear to RS-232 logic levels (+/-12VDC) on pins 2 &
    3 of a DB9 connector and converts them to 0 & 5VDC. Which TTL voltage
    corresponds to a logic 0 and which to a logic 1? On the RS-232 side, the
    data pins use negative logic, so +3 to +12 VDC is logic 0 and -3 to -12VDC
    is logic 1. Does the TTL data input use positive logic instead ( logic 1 is
    +5VDC and logic 0 is 0VDC)?

    What is the industry convention when terminating the TTL side of this cable
    to a stereo plug. ... is the tip conductor logic 1 or logic 0? Once I
    determine this, then I will use this existing conversion cable and simply
    wire the stereo plug conductors to their respective TXD and RXD pins on my
    Yaesu FT-857.

    Lastly, what happens if one connects the TTL level TX conductor to the TXD
    pin on the radio side accidentally instead of the RXD pin on the radio?
    i.e. wires crossed in error

    Thanks for any and all info!

    73's Will
  2. Conventions differ on this between different signals
    in the RS-232 standard. The modem control and
    status signals are normally active low in the TTL realm.
    If you want to deem that "TTL high = logic 0", that is
    up to you. The data signals are normally considered
    to be at the mark level for a high level in the TTL realm.
    That is level usually considered a '1' when considered
    to be a number at all.
    All TTL<-->RS-232 translators that I have seen are
    inverting, and they are used for both the data and the
    status/control signals with every serial interface device
    I have seen. Without having looked at your equipment,
    I can say that if your translators invert, they are quite
    likely to be doing the right thing.
    I've not seen stereo plugs used for RS-232. But it makes
    no sense to call the tip logic 1 or logic 0. Why would such
    a limited number of conductors be wasted conveying a
    constant value?
    Go for it, I say.
    No permanent harm, just some frustration and maybe
    mystefication when it does not work as you want.
    You're welcome.
  3. I can't say for sure if it's a standard but cables for RS232/TTL to
    glucose testers use the tip as RX and small ring is TX and last is
    common or ground .If you want a simple 1 chip solution look at the
    DS275 from Dallas.

  4. Will

    Will Guest

    Hi Larry / the group,

    Thank you for the reply.

    I've done some more experimenting with my commerical grade cable used with
    the Alinco DJ-X10 wideband radio.

    I've run a number of tests to using a standard 9-pin RS-232 cable (no
    conversion) and measured/monitored the TXD and RXD lines on a dual channel
    scope to serve as a reference.

    As expected, RS-232 data lines idled in a marking state of -11V. Then using
    a Windows Hyperterminal session @ 4800 bps, 8 data bits, 1 stop bit, set my
    time base on my scope to reflect 208 us/DIV sweep so each bit duration
    consume one graticule division. I then monitored the voltages on this line
    as I held down and pressed various keys while observing their binary
    equivalent on an ASCII chart. I observed 10 bits as expected (1 start bit,
    8 data bits, 1 stop bit) and the levels behaved as I would expect.

    I captured the screen shots of my scope for each of the characters I used.
    I then hooked up my RS-232 / TTL conversion cable for the Alinco unit (not
    actually connected to the radio ... hence "unloaded") and observed the

    1. No logical start or stop bits are observed.
    2. On some keys, the binary codes produced predictable results, i.e. the
    letter U which has alternate bits set (01010101). The TTL output was
    comparable, but at ~ 3.5V.
    3. TTL levels ranged from 0 to 3.5V unlike RS-232 levels (+/-11V)
    4. Not all voltage sequences while monitoring the TXD TTL-level conductor
    matched its corresponding bit sequence observed when using the RS-232 (non
    conversion) cable.
    5. As I recall the RS-232 standard (many years ago for me), the whole
    concept of serial communicaitons was asynchronous in nature and hence each
    byte was enveloped by a start and stop bit(s) to sync the tx'er and the
    rx'er. However, on the TTL side, it appears as though there is no concept
    of start and stop bits ( at least logically). Yes, I understand that
    there's either 0 or 5V. That's it. But there is no asynchronous start/stop
    bits utilized on the TTL side.
    6. Since the TTL-side voltage transitions did not always correspond to the
    RS-232 equivalent (ie voltage transition intervals), I am wondering if some
    sort of encoding scheme is used (NRZ, NRZI, etc) ???

    What should data in the RS-232 realm on pin 3 look like after being
    converted into TTL level voltages? In some cases, the RS-232 logic level 1
    (-11V) seemed to correspond to 0V in the TTL realm. In other cases, it
    seemed to correspond to 5V. ?


    PS: As for the stero plug, the tip did correspond to RS-232 RXD and the
    ring conductor did correspond to RS-232 TXD. Since RS-232 is full duplex
    (TXD/GND and RXD/GND pairs), I guess it stands to reason that this
    conversion circuit still maintains two separate and distinct paths for
    sending and receiving data.

    PPS: Not sure I follow about the constant voltage level waiting a conductor.
    Since the TTL signals seem to correspond in some relation to the originating
    RS-232 data byte, the TTL level signal was only constant in voltage for the
    duration of a bit, but it changed in magnitude (0 or 5V) accordingly.
  5. You could try a wrap jumper..With TTL side connected jumper RS232 TXD
    to RXD and scope signals. Then disconnect TTL and connect RS232 and
    jumper TTL TXD to RXD and scope signals. You should see the original
    and converted signal in each direction.
  6. You're welcome.
    I suggest you use a dual trace o'scope. I believe you will find,
    if you trigger on one signal while looking at the other, (and do
    not use the "alternating" display mode that analog o'scopes
    usually have), that they have the same logical state at all times
    except for the effects of propagation delay.

    You asked: "What is the industry convention when terminating the TTL side
    of this cable to a stereo plug. ... is the tip conductor logic 1 or logic 0?"
    The question makes little sense, so neither does the answer. What you
    meant to ask may have made sense, but those words do not.

    [Snip confusing top-posted stuff.]
  7. Mike

    Mike Guest

    Hi Larry / group,
    Actually, I see what you mean now re: the tip. I guess it was one of those
    things where its clear in my mind but not properly captured on "paper". I
    should have wrote: is the tip conductor TXD or RXD. My apologies. From
    my testing so far, I have found that the tip is in fact RXD, thus ring is
    TXD. Sorry for the confusion!

  8. mike

    mike Guest

    YMMV, 'cause there ain't no standard that everyone adheres to.
    My experience has been that I can always make radio interfaces work
    with inverting ttl translation.
    I also use opto-isolators with appropriate reverse protection and
    current limiting. Works for low data rates and helps with my paranoia
    about smoking my laptop.

    Return address is VALID but some sites block emails
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