kern/153289: Modem Ring Signal not reliably detected

Tom Glinos sonilg at
Sun Dec 19 02:10:10 UTC 2010

>Number:         153289
>Category:       kern
>Synopsis:       Modem Ring Signal not reliably detected
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sun Dec 19 02:10:09 UTC 2010
>Originator:     Tom Glinos
>Release:        8.1
U. of Toronto
FreeBSD 8.1-RELEASE FreeBSD 8.1-RELEASE #0: Mon Jul 19 02:55:53 UTC 2010     root at  i386


Modem Ring Signal not reliably detected.

On the hardware side, create a switch that toggles DTR to RI. Also DTR to DSR.

On the software side, run a program to repeatedly scan the modem status register with a TIOCMGET ioctl

A toggle of DTR to DSR is always detected. A toggle of DTR to RI is less than half the time detected. Tested against two different serial hardware, similar behaviour.

The same code with same/similar hardware runs perfectly under LINUX.
Unknown at present. There are plenty of work arounds like changing the harware or OS but anyone still using a modem should be seeing this problem.


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