port shutdown

[John Boteler]

>From article <894 at bbx.UUCP>, by russ at bbx.UUCP (Russ Kepler):
> In article <5542 at ozdaltx.UUCP>, root at ozdaltx.UUCP (root) writes:
>> I'm running into a strange problem since I've started using
>> a new modem.  The message,
>> 
>> "garbage or loose cable on serial dev 8, port shut down" [...]
> 
> OK, this one has been running around for a while and I haven't
> seen anyone else answer...
> 
> If the message is appearing on serial post that are actually
> internal modems a fix exists from SCO:

If the modem is external, and it has a Hayes command set,
it is possible that this message is being caused by 
echoing from the modem or result codes being generated
by the modem.

Solution: Turn the echo and result code reporting off for dial-in use.

As 'getty' initializes the port, if the modem receives any
characters, it will echo them back to the host, which then echoes
back to the modem...infinitely. More commonly, result codes are
generated by the modem in response to 'getty' initializing
the modem; the result is the same. 

'getty' can't believe that any modem would have the audacity to
actually send data to it without 'carrier detect' asserted. 
It therefore assumes that with 'carrier detect' absent,
there must be either electromagnetic interference on the 
serial line or a loose connection causing invalid data to be
sent, and reports same.

Note: Telebit Trailblazers are made for UNIX systems, and
have incorporated a dual mode result code command for
just this reason. Practical Peripherals modems allow two
setup configurations to be stored internally for quick
switching between dial-out and dial-in modes. There are 
others, also.

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