Re: -5v and -12v Serial Ports

On Fri, 01 Jun 2007 20:16:58 -0700, stratus46@xxxxxxxxx wrote:

I believe the original spec for RS-232 levels was minimum +/- 3v.

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.

Jon
.

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