Serial port operation -

Bob Goodwin bobgoodwin at wildblue.net
Wed Nov 30 00:31:15 UTC 2011


On 29/11/11 18:46, Maciek Borzecki wrote:
> 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,

        It works perfectly with one computer.

> 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.

        All I need is for RTS to toggle, no data is being transfered.

> 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)).

        I don't know how to deal with these commands, will have to read
        further.

        Thanks,

        Bob


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