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Re: Timeout broken with serial port

From: Michal Suchanek
Subject: Re: Timeout broken with serial port
Date: Tue, 6 Sep 2011 08:55:15 +0200

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.

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

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.



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