Re: Serial Port Configuration

Ton Nijkes <ton@xxxxxxxxx> wrote:
Floyd L. Davidson wrote:
Ton Nijkes <ton@xxxxxxxxx> wrote:
ashamael@xxxxxxxxx wrote:


RS-232 serial ports are known to stop 'working' temporarily or even
(i.e. blow up!) when the attached device is (un)plugged or power-cycled.

That is not true.

Is is not true because you never experienced it yourself or because
it is...well...not true?

Let me repeat that one more time: it is *not* true.

The whole idea that power cycling a device will cause an
RS-232 port to "blow up" is hilarious.

Serial devices in general are not 'hot-pluggable' as opposed to USB

Only in the sense that the OS does not support any kind
of auto initialization for RS-232 devices. That is
simply because there is no way to determine
automatically what configuration it should be configured


I hope you understood the use of "only" in that
statement! :-)

As for removing, power cycling, or reconnecting RS-232
devices and then manually starting whatever process is
required to initialize the device, that *is* *very*
commonly done.

I know, I do it myself _all_ the time :-) It works well _most_ of the
time :-(

Exactly. The difference is that you don't know why it
fails now and then... and those who actually work with
this stuff day and and day out for decades do (or

In three words: Electro static discharge. It has
nothing to do with an RS-232 port as such. It's a
simply case of a person walking over and grabbing the
connectors/cords/devices without taking appropriate
steps to discharge any static build up. The same damn
fool thing happens with circuit boards of any kind,
RS-232 or not. (In fact, I've seen circuit cards blown
by people just *walking* past them... and I don't
mean on poorly designed sysytems either.)

Some motherboards/chipsets are very forgiving to RS-232 'hot-plugging'
others are very sensitive.

I've never seen even one that was "sensitive".

I have. I have seen specific brand PCs (Wincor) lock up hard consistently
hot-plugging RS-232 devices.

So a faulty mother board design is used to condemn
RS-232 ports?

(To be honest though, I'm not willing to credit your
ability to correctly observe and report, given the
incredibility of the other statements in these two
articles. But I have never seen a "Wincor" motherboard
and cannot vouch for it either way, so I cannot claim
you are necessarily wrong.)

I have seen other brand PCs (NCR) have their serial ports toasted after
hot-plugging RS-232 devices. Not just one, but many.
Note that this all applies to POS (Point-Of-Sale) PCs and peripherals.
Bad hardware design on some component is entirely imaginable.

Sounds more like poor maintenance practices that need

Never seen sparks coming from an RS-232 connector? I thought that's why
they call it 'hot' plugging ;-)

Incredible. Sparks from an RS-232 connector???? Either
you need to learn more about static discharge, or you
need to learn more about what is and what is *not*
RS-232. You cannot get an RS-232 lead to spark. The
voltages and impedances make it virtually impossible.

That is, assuming proper ESD precautions...

I've been using RS-232 devices for literally decades, on all
sorts of equipment.

Same here.

Did you try to find out whether anyone unplugged or power-cycled the EFT
terminal prior to calling for your help?
Check the output of the 'dmesg' command for any reference to the
word 'UART'.
If the Linux device driver detects a shutdown of the serial port, it will
display a message (I forgot the exact message phrase).

I don't think it does that...


Well it doesn't really 'detect' a shutdown of the port. In fact, the kernel
complains that no UART chip is found at a given I/O-address, although it was
working perfectly fine just moments before... So probably the UART decided
to call it quits. The particular kernel message to look for is:

ttyS%d: LSR safety check engaged!

Unplugging or power cycling of a connected RS-232 device
does not cause the port to shutdown, the UART to
disappear, or a kernel error.

Be very careful about the proper cabling when connecting devices to a
serial port carrying 12V DC on one of its pins; you could blow up your
precious serial device...

Not with 12 VDC you won't. Every pin on an RS-232
interface can deal with significantly more voltage than

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.

is supposed to be able to handle voltages between 3V and 25V (+/-).
So 12V should be fine...i.e. not break anything.

But you advice that the cabling is critical is indeed
correct. There are approximately 2 more ways to cable
RS-232 than there are design engineers to decide which
way it is, so not many are ever the same. :-)


It isn't going to blow anything up, but it isn't going
to work either.

Rule #1 wrt RS-232: Never expect it to 'just work' :-(

Rule #2, when it doesn't, it didn't blow up.

Floyd L. Davidson <>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx