Re: Serial Port Configuration
- From: floyd@xxxxxxxxxx (Floyd L. Davidson)
- Date: Wed, 15 Aug 2007 23:53:51 -0800
Rikishi 42 <fsck_spam@xxxxxxxxxx> wrote:
On 2007-08-15, Floyd L. Davidson <floyd@xxxxxxxxxx> wrote:
Ooops, my bad.
You are right. RS-232 typically operates at signal voltages of 3V or 5V, but
RS-232 does not typically operate at 3-5V. They
typically operates at 10 to 15 volts, and on virtually
all PC's they use +/- 12 VDC.
You guys just caused a major flashback, here. Major. :-)
I studied the CCITT norm on RS-232 signaling, some 20-odd years ago. At that
time, I also looked into the hardware side of it. In those days, I used it
to connect PC-XT's to a Intergraph Vax and later to some microcomputers,
like the Synclair QL. Oh, and the pen plotters and first laser printers used
RS-232, too.
From memory (allways dangerous, but that's all I have here) an originalRS-232 was expected to signal using +/- 12V. It had to be able to 'endure'
24V at least, maybe 32V. And it had to be able to get a signal out of a
tension as low as +/- 3V. That was for the old DB25 connector.
Your memory is fairly good!
Note that the DB25 connector has virtually nothing to do
with it though. No connector is specified by the RS-232
Standard, and a huge variety of connectors have been
used. The DB25 was very common in the computer industry
initially, to the point where most people think that it
is part of the standard. Then when things started to
get smaller and PC's began using a 9 pin connector (it
is in fact a DE9 connector), everyone called it a DB9!
The "DB" references the physical size though, and a DB9
would look just like a DB25 (with 16 pins removed).
The signal voltages are specified for the transmitter as
5 to 15 volts, while for the receiver it is 3 to 25
volts. That is with a load resistance of 3000 to 7000 Ohms.
The transmitter voltage with no load must be below 25 volts.
And the output impedance when power is off must be greater
than 300 Ohms.
The receiver input range is specified equal to the
driver specifications. Nominal input is 15 volts.
Impedance is 3k to 8k. It must survive 25 volts input,
and should "work" with as low as 3 volts. (See below for
the reason quotes are around the word work.)
The specification also states that the device must be able
to sustain a short between any two pins without damage, and
that the maximum current will be 0.5A.
When the DB9 came into use, the typical voltage dropped to +/-5V, probably
because PC's motherboards dropped in tension, too. But the high and low
limits still had to be respected. This drop in tension might not have been
part of any standard, but was common practise, especially on luggeables,
portables and laptops.
The ability to work at +/- 5 volts was always part of
the standard, but actually using that voltage was not
and is not common or recommended. It is true that many
compact devices, battery operated devices, etc. etc.
will use lower voltages, but few will attempt to work a
5 volts with a production unit, though 9-10 volts is
often encountered.
The reason for that is because lower voltages have
several effects. The most obvious is that it reduces
the maximum bit rate which will work over any given
length of cable. Indeed, with some cables and some
devices that might mean that it won't work at all with
anything practical! A ten foot run of cable might be
restricted to bit rates lower than 300 b/s, for
example... and if your application requires a 56.7Kb/s
rate, it dies (even though the RS-232 implementation
technically does meet specs).
Groan... I'm getting old for this.
Back to lurking mode, then.
I've always found RS-232 interesting, mostly because so
many design engineers have no practical experience...
they read the specs and figure it means what it
says. :-)
They design to the specs; while at the same time very
few technicians have any theoretical experiences, and
have never read the specs. Techs are often totally at a
loss to understand what the design engineer intended
because it is some oddball offbeat never used by anyone
else implementation!
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.
- References:
- Serial Port Configuration
- From: ashamael
- Re: Serial Port Configuration
- From: Ton Nijkes
- Re: Serial Port Configuration
- From: Floyd L. Davidson
- Re: Serial Port Configuration
- From: Ton Nijkes
- Re: Serial Port Configuration
- From: Floyd L. Davidson
- Re: Serial Port Configuration
- From: Rikishi 42
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