Serial port operation -

Maciek Borzecki maciek.borzecki at
Tue Nov 29 23:46:28 UTC 2011

At Tue, 29 Nov 2011 18:18:28 -0500,
Bob Goodwin wrote:
> On 29/11/11 17:58, Maciek Borzecki wrote:
> > At Tue, 29 Nov 2011 17:48:45 -0500,
> > Bob Goodwin wrote:
> >> On 29/11/11 16:14, Tim wrote:
> >>> On Tue, 2011-11-29 at 14:14 -0500, Bob Goodwin wrote:
> >>>> I have two F-15 computers with a program that should be toggling
> >>>>           RTS and DTR on the serial port. One works as expected this one I
> >>>>           am on now does not.
> >>>>
> >>>>           I haven't had much to do with the serial port and don't know how
> >>>>           to troubleshoot it.
> >>>>
> >>>>           Can someone offer a suggestion where to begin?
> >>> Obvious first line of thought:  Are you using two different serial
> >>> cables?  And, if so, is the non-working one fully wired?
> >>>
> >>
> >>
> >>          I eliminated that possibility and swapped the cables, one worked
> >>          "out of the box" this one doesn't. I was hoping it would be some
> >>          configuration problem but don't know where to look. One is
> >>          F16-32bit, this one 64 bit, however they are different
> >>          computers, this one an HP, the other Dell, both of roughly the
> >>          same vintage.
> > Disable flow control and connect through a null modem cable to another
> > PC. Fire up minicom, set both to 8N1, same speed for both and you
> > should be able to see characters pop up on the other end while typing.
> >
>         I would have to make a null modem cable, probably have parts for
>         that but it seems improbable that that will work if I am not
>         seeing the voltage switch at RTS and DTS. I hate to make a
>         bigger project out of it than necessary. I see that minicom is
>         apparently installed by default so if need be it is there but
>         it's been years since I messed with that ...

If you've never seen it work with those PCs, then disable hardware
flow control if possible. I've seen it to fail occasionally with some
newer hardware (as if anything aside from data and ground would not be
wired). By disabling hardware flow control RTS/CTS signalling is not
used at all. 

If it's not possible to disable hardware flow control for that
particular application, then start with the lowest layer in the stack
- check the cabling. After that you may try to see see if the port is
getting configured correctly (run it under strac and look for ioctls -
verify with manpages tty_ioctl(4) and termios(3)).

Maciek Borzecki

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