Re: Timeout broken with serial port

[starous]

<Michal Suchanek"> wrote:
> On 5 September 2011 22:51, Aleš Nesrsta
> <address@hidden> wrote:
> > Hi,
> >
> > I think it is not GRUB related problem, more
> > probably there is some HW
> > > problem on Your serial port. Try to check idle
> > voltage on RxD pin of
> > > serial port (best with oscilloscope... :-) ). Or
> > You can have some
> > > unwanted "leakage" between pins of connector or
> > wires in cable (check
> > > resistance between wires of cable) etc.
> >
> > Normal serial port should NEVER do anything if
> > there is nothing
> > > connected to it. According to RS232 (V24)
> > specification there should be
> > > voltage in range from -12V to -3V on RxD pin in
> > idle state (in simplest
> > > case there is some internal pull-up resistor
> > connected from RxD to -Vcc
> > > directly inside UART or something else...).
> > In this normal case nothing is received by
> > serial port and nothing will
> > > be interpreted as keypress in GRUB, i.e. GRUB
> > will boot normally.
> > >
> > In Your case, probably something causes some
> > noise on RxD pin or there
> > > is bad voltage level on RxD pin (e.g. >= +3V or
> > near to 0V etc.) when
> > > nothing is connected - so serial port interprets
> > it as receiving some
> > > character(s).
> > When You connect Your serial port to another
> > device (notebook in Your
> > > case), RxD is set to correct idle voltage from
> > connected idle TxD, so
> > > the problem is corrected.
> >
> > From my long praxis with serial ports on
> > different PCs and another
> > > (non-PC) devices, manufacturers very very often
> > don't respect RS232
> > > (V24) specification (mainly for RxD/TxD signals
> > voltage levels). It
> > > often caused bad situations, some of them were
> > in fact the same as Your
> > > case.
> >
> > Using of DTR line is fine but if You have some
> > HW problem on RxD line,
> > > there could be the same problem also on DTR
> > (RTS, CTS ...) line and
> > > final effect will be the same...
> > Additionally, lot of devices are working without
> > using any additional
> > > link/modem state signals (i.e., only signals
> > RxD, TxD and GND are used),
> > > it is simplest and almost working solution (and
> > working well - of course
> > > it depends of kind of communication and it also
> > expects that RS232
> > > specification is not violated and all HW is in
> > order...).
> > >
> 
> FWIW I am using
> 
> description: Motherboard
> product: DQ45CB
> vendor: Intel Corporation
> physical id: 0
> version: AAE30148-302
> 
> Since the BIOS does not support serial boot it is
> not tested for this
> I guess. I don't have an oscilloscope so I can't
> tell what is going on
> on the wires.

If You can, try at least simply measure voltage between pins 2 (RxD)
and 5 (GND) - if You have commonly used 9 pins connector on PC.
Common ("minus" - often black color) measuring wire should be
connected to pin 5 (GND). If You will measure voltage  outside the
range approx. <-12..-3V> then most probably there is something wrong
with Your serial port - from HW point of view.

There is also question (but maybe I don't understand Your original
post) - why do You use serial module in GRUB when You don't need
serial port ?
I think, if serial module will not be loaded in GRUB, You should
don't care about serial port even if it is broken or buggy...

> 
> Is there some alternative to using the onboard
> port? Does Grub support
> some serial addon cards?

AFAIK, GRUB supports at least USB serial port converter.
Or probably any addon card which looks from HW side as standard COM
port...

> 
> Also since this seems to be a common problem with
> PC hardware perhaps
> a warning in the manual and the configuration file
> should be added.

Hm, why not, it is maybe not bad idea - but I am not maintainer of
related GRUB parts (serial port nor configuration etc.), maybe
somebody else will discuse it with You...
But, generally, there could be many partially or fully broken HW
parts on PC and it is impossible to warn everybody against all of
them... :-)

BR,
Ales

> 
> Thanks
> 
> Michal


> 
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> 


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