Subject: Re: is TIOCMGET supposed to reflect reality?
To: Tom Yu <tlyu@MIT.EDU>
From: Charles M. Hannum <>
List: tech-kern
Date: 09/27/2004 18:03:45
On Monday 27 September 2004 13:50, Tom Yu wrote:
> That's an interesting point.  I notice that com.c always enables modem
> status interrupts on the 8250/16405/16550 UARTs.  Do line drivers
> attached to PC UARTs tend to be biased in a way that makes the modem
> status lines less susceptible to noise?  Or are PCs fast enough these
> days that nobody cares about stray modem status interrupts? :-)

Neither, and in fact this is occasionally a problem (such as with a certain 
series of modems that under certain conditions will oscillate all the status 
lines at 150KHz).  I was going to fix that at some point, but I forgot about 
it.  I guess I'll put it back on my todo list.  :-)

> Also, at some point in the past (around rev 1.39), z8530tty.c did have
> DCD_IE and CTS_IE unconditionally enabled, but there was a change in
> rev 1.53 to only enable them if the tty layer expects to track the
> status lines.  It looks like this change was in response to some
> problems on mac68k machines.

IIRC, the "printer" port on older Macs had these signals disconnected and 
floating, so you could get the previously cited condition.