On Mon, Sep 5, 2011 at 3:51 PM, Aleš Nesrsta <star...@volny.cz> 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 either. _______________________________________________ Grub-devel mailing list Grub-devel@gnu.org https://lists.gnu.org/mailman/listinfo/grub-devel
