[ On Monday, May 6, 2002 at 09:33:48 (-0700), Don Yuniskis wrote: ]
> Subject: Re: SparcStation ipx versus Sparc Classic
>
> > "Greg A. Woods" <woods@weird.com> wrote:
> > [ On Saturday, May 4, 2002 at 20:40:38 (-0700), Don Yuniskis wrote: ]
> > > Subject: Re: SparcStation ipx versus Sparc Classic
> > >
> > > I suspect folks doing this will note no change in results.  For those
> > > using PC's/laptops, I imagine the "solution" would be a pulldown
> > > on the input to the driver -- since it is undoubtedly being driven
> > > from a Combo and floats until the Combo is configured by the
> > > BIOS.
> >
> > That's what that resistor is designed to do....  Are you sure you
> > experimented with various values?
> 
> No.  The resistor on teh RS232 "connector" biases the input of the
> *receiver* to a MARK state.

Well, DUH.  :-)

I meant only that the resistor fixes BREAK-detection problems caused by
a _floating_ line.

Obviously the resistor alone is not going to change the levels on the
line when the it is "purposefully" driven to a SPACE state (regardless
of whether this happens by accident on power-up or power-down, or on
purpose when the terminal device intends to send a SPACE bit, or indeed
a series of them to purposefully cause a BREAK condition.

> By contrast, teh resistor added to the RS232 connector tries to
> bias teh *input* to the SPARC's *receiver* to the MARK level.
> But, if the input to the RS232 *driver* inside the pc/laptop is
> "telling" the driver to put a SPACE level on that wire, then the
> resistor will have no effect.  It is as if the laptop was intentionally
> driving the line to SPACE... intentionally trying to BREAK...

Well, DUH, again!  :-)

If the resistor completely prevented a SPACE level (eg. if you
hard-wired the line to a -12vdc source with little or no resistance)
then of course no driver would be likely able to raise it to the SPACE
level and not only would an intentional BREAK condition be impossible,
but so would all data transfer in that direction!  (You might even do
some damage to the remote line driver, but that's not the point of
course....)

You can't get rid of an intentional BREAK condition (regardless of
whether it's intended by the operator or simply caused by some poorly
designed terminal device's UART or line driver) with a resistor alone,
and _I_ certainly wouldn't want to either!  (A sophisticated RS-232
repeater might be able to eliminate all BREAK conditions from being
detected by any host while still allowing data to be transmitted to the
host, but why you'd ever want to do that is beyond me -- just fix the
cause of the unintentional BREAKs!)

-- 
								Greg A. Woods

+1 416 218-0098;  <gwoods@acm.org>;  <g.a.woods@ieee.org>;  <woods@robohack.ca>
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