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 <phco...@gmail.com>: > > 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 > > _______________________________________________ > Grub-devel mailing list > Grub-devel@gnu.org > https://lists.gnu.org/mailman/listinfo/grub-devel _______________________________________________ Grub-devel mailing list Grub-devel@gnu.org https://lists.gnu.org/mailman/listinfo/grub-devel
