Re: Timeout broken with serial port

[Aleš Nesrsta]

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...).

Best regards
Ales
 

Michal Suchanek wrote:
> 2011/8/24 Vladimir 'φ-coder/phcoder' Serbinenko <address@hidden>:
> > On 24.08.2011 11:16, Michal Suchanek wrote:
> >> Hello,
> >>
> >> I was experiencing a mysterious error with grub. It would not boot
> >> with timeout set.
> >>
> >> It turns out that setting serial IO disables timeout.
> > Check your contacts. Bad contacts are known to send noise on I/O which
> > GRUB interprets as keypresses.
> 
> I did not have the cable connected at all at the other end. Connecting
> it to my notebook resolves the problem.
> 
> Still it is definitely a bug in Grub that it interprets noise as keypresses.
> 
> Is there not DTR line?
> 
> If every switch with serial console failed to boot when there is
> nothing connected to it there would be no internet. This is not how a
> serial enabled device should behave.
> 
> Thanks
> 
> Michal
> 
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