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-5v and -12v Serial Ports

Discussion in 'Electronic Basics' started by Bart, Jun 2, 2007.

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  1. Bart

    Bart Guest

    Hi All,
    A buddy of mine got a new computer, replacing his old 486. He uses his
    computer's serial port to "talk" to older equipment and now he's having
    problems. His new computer has a PCI card with a serial port and I told him
    it may be that the PCI (who's bus doesn't support -12volts) has less of a
    voltage peak to clearly send the data like his old 486 machine. His cables
    are kinda long and I figure he was pushing it even with his old computer and
    now that his serial is only +/-5v he can't quite get the data across.
    Is there a clear line in history of computers when this change from -12v
    to -5v was crossed in serial ports on PC's? I told him that any computer
    with ISA slots, even if the serial port was on the motherboard would support
    the -12v serial port he needs.
    Am I way off base?
    Thanks for any input,
  2. Guest

    I believe the original spec for RS-232 levels was minimum +/- 3v.
    Calling Jim Thompson for enlightenment. I'm surprised about the +/- 5
    thing. I thought most everybody used a MAX232 (or knock off) to make
    +/- 10 from the 5 volt supply.

    ISA slot? Where would you find one of those? I needed an ISA slot for
    an EPROM programmer 4 or 5 years ago and the best I could do was a
    late 90's Biostar board on eBay for $35. I also keep a Pentium 166
    machine and a K6-2 550 'just in case'

  3. Puckdropper

    Puckdropper Guest

    wrote in

    You can get PCI to ISA adapters... but I don't know how well they'll
    work... DAGS if you're interested.

  4. I keep ISA-based machines around, mostly because it is very easy for
    me to design and wire up a custom ISA board that works and ... very
    difficult for a hobbyist like me with reasonable tools at hand to do
    that for PCI. I just don't have the stuff I need for reflection wave
    technology. So ISA is important to me.

    The ISA, once PCI came about, was supported by the south bridge. The
    simpler ISA transactions can be popped over to the PCI bus okay, but
    ISA DMA in particular has no equivalent on the PCI bus and there is no
    way to map it correctly without "sideband" channels to let the main
    chipset know what in the hell is going on. A PCI-to-ISA adapter, if
    it doesn't also have access to sideband wires, won't be able to do all
    DMA transactions, for certain. And if the motherboard wasn't designed
    to handle ISA at the start, it's likely those lines aren't available
    in a special connector and it's also likely the BIOS can't set things
    up. If a BIOS is included in the adapter, it will itself need to
    contend with a wide variety of possible motherboards, which also seems
    "hard" or "unlikely," too. I'd just assume "no ISA DMA" in that case.

  5. My vague memory included that figure, but also a few others, too. Like
    3k-7k ohm for the receiver resistance. But I also remembered a
    minimum 5V thing and I was confusing it in my mind with the 3V you
    mention above. So I looked up the RS-232-C spec. Turns out it is
    more interesting than that:

    (1) The open-circuit output voltage of the driver cannot exceed 25V.
    (2) The driver must be able to sustain a short circuit to any other
    wire in the cable without damage and the short circuit current to any
    other wire cannot exceed 0.5 amps.
    (3) Signals are MARK, "1", when the cable pin voltage is < -3V and
    SPACE, "0", when the cable pin voltage is > +3V and everything else in
    between is "not defined."
    (4) The receiver's load resistance must be < 7k ohm when any voltage
    from 3V to 25V is applied BUT also > 3k ohm for any voltage < 25V
    (which includes the 0V to 3V range, as well.) So my recollection of
    the 3k-7k seems about right.
    (5) While it is true that the MARK and SPACE have the voltages
    defined in (3) above, a driver hooked up to a receiver meeting the
    loading in (4) above must present at least +5V and no more than +15V
    (for a SPACE) or at least -5V and no more than -15V (for a MARK.)
    (This is to provide the minimum of 2V noise margin.)
    (6) The slew rate cannot exceed 30V/us.. except when transitioning
    through the "not defined" region where it must take no more than 1 ms
    of time or no more than 4% of the bit time, whichever is smaller.
    (7) The shunt capacitance of the receiver shall not exceed 2500pF --
    and that includes the cabling from the driver, too.
    (8) The impedance of the driver, powered off, shall exceed 300 ohms.

    I think the -D spec is about the same, except that the timing through
    the "not defined" region has a few extra cases -- none of which
    materially affect much.

  6. Don Bowey

    Don Bowey Guest

    I tossed most of my reference material a few months ago, but as I recall,
    Mark is negative and Space is positive. Would you please verify this in your

    RS-232 was +- 20V. It was used as a Standard, but was always a Recommended
    Standard, thus the RS.


  7. Jamie

    Jamie Guest

    12+/- was very common voltage to use and many devices used the power
    from the port to power them self.
    Many devices were made to force the output to 12+/- line level drive
    voltage from a serial port to ensure proper operations
    normally they spec it as 2.5 Volts +/- as the threshold for on/off with
    0 volts as undefined.
    If you look up the spects on RS-232 line levels for older equipment,
    you may find voltages higher than 12 volts. 15 volts rings a bell to me.
    at work, we have a interface box that must be placed between the
    serial port and PLC to boost the voltage when used on a laptop.
    These old units required 12+/- volts like found in most PC's that had
    real serial ports. When using some PC's this box isn't needed.
    There are some Laptops that have higher voltage serial ports for
    industrial use. they work fine with out the aid of this helper unit.

    the output is normally inverted and this is where the level of
    output was generated.
    You may want to test the voltage on it.
  8. Don Bowey

    Don Bowey Guest

    On 6/2/07 12:47 AM, in article ,


    I found a few interesting things:

    1. You had already posted, correctly) the polarities. (my senior moment).

    2. A copy of ANSI EIA 232-D, which I thought I had tossed. (another s.m.)

    3. An assortment of other Standards and USW Technical Publications.
    (I think my ego got in the way of dumping some of these)

    Do you want any? I won't toss them until I hear from you.

  9. Jasen

    Jasen Guest

    most PCs use one of Jims MC1488s (or similar) to do the +/-12V, as PCI
    (and ISA) slots make those voltages avaialable.

    I'm surprised that this recent PCI serial card doesn't.
    they can still be had, but not at a regular retail pc parts place,
    try someone who supplies to the embedded market.

  10. Outside of my parts box, I haven't seen a 1488 or 1489 in a PC for
    quite some time. They are though, if nothing else, nice little space
    heaters. I forget which of the two, but one of them runs almost too
    hot to touch long.

  11. Thanks! I was starting to wonder about me.
    I don't have the actual spec, itself. So I'm interested.
    I think I might. You can write to me at the email address that goes
    out with my posts, if you want to let me know what else there may be
    or otherwise work out details.

    Thanks for considering the idea,
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