Re: Serial Port Issues
On Tue, 07 Apr 2020 08:24:50 -0500
"Martin McCormick" <martin.m@suddenlink.net> wrote:
> deloptes <deloptes@gmail.com> writes:
> > Chris Rhodin wrote:
> >
> > > Tonight I'll look at the serial port ioctls and see if I can spot
> > > a difference there. I also try enabling flow control and
> > > fiddling with
> > the
> > > signals to see if that unstops it.
> >
> > Are you sure that this is enabled in the BIOS, also some serial
> > ports like HP have special connectors and layouts.
> > Best would be to look at the manual first.
> >
> > I have attached a USB to the server. From there I can log in to the
> > firewall. I can not use the same port in the opposite direction.
> >
> > However it is strange that you do get the connection only in one
> > direction.
> > It could be some kind of special connector
> >
> > Otherwise I used those to enable the service
> >
> > https://wiki.debian.org/systemd#Virtual_and_serial_console_changes
> >
> > https://www.thegeekdiary.com/centos-rhel-7-how-to-configure-serial-getty-with-systemd/
> > https://wiki.archlinux.org/index.php/Working_with_the_serial_console
> >
>
> Another possible fly in the ointment could be a hardware
> issue. Some RS-232 ports are old-school and aren't happy with
> any voltage range other than +12 for one state and -12 for the
> other while there are serial ports that can handle state changes
> of + or - 3 volts so they safely handle logic-level signals and
> also can handle the old-school RS-232 levels. Then there are
> some that handle logic-level signals and would figuratively melt
> if you hit them with +-12 volts.
They're not 'old-school', they're badly designed. The spec says,
unequivocally, plus and minus 3V-15V and has done for a great many
years.
The 'logic' level data, quite common these days, is RS-422 or
RS-485. It should not ever be compatible with RS-232. Most serial data
originates at logic level (anything from 2.8V to 5V) and ends up at
logic level somewhere else, so there is little point in converting to
and from RS-232. But things like radio modules often run on 3.3V or
lower, so it may be necessary to convert to and from RS-422 levels.
Many modern 'RS-232' devices, particularly the really cheap USB-serial
modules, in fact use 3.3V/0V or 5V/0V logic levels, as the term 'RS-232'
has just come to mean 'low-speed serial' to many people.
--
Joe
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