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Re: Timeout broken with serial port

From: address@hidden
Subject: Re: Timeout broken with serial port
Date: Tue, 6 Sep 2011 15:36:01 -0500

On Mon, Sep 5, 2011 at 3:51 PM, 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).

An idle transmitter should be in the range -3V...-12V.  But here there
is no transmitter.  I would not be at all surprised if some hardware
implements "cable detect" using a weak pull to 0V.

Still, data shouldn't be reported by the receiver unless the voltage
is in the valid range, and the input should not be left floating

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