DTR Flow Control

David Elliott dfe at tgwbd.org
Mon Feb 25 20:16:47 CST 2002

On 2002.02.25 16:38 lawson_whitney at juno.com wrote:
> On Mon, 25 Feb 2002, Michael Cardenas wrote:
> >
> > Attached is the relevant part of the output of +file,+comm.
> >
> > It looks like the app is using ClearCommError to check for the number
> of
> > bytes recieved. I've read that the TIOCOUTQ ioctl is buggy and not very
> > reliable, but I'm not sure if that's been fixed or not in newer
> kernels.
> >
> My comms apps have been banging on ClearCommError for that since August
> 1997, but without the overlapped nonsense.  It works for me or I
> wouldn't have gotten your letter.  This app appears to ask for
> rts/cts flow control - at least that is what we give it.  Dumb
> question:  is there a modem cable or null-modem cable involved?  If so,
> is it a good one, with wires for each signal (properly crossed for a
> null-modem cable), or is it a 3 wire Windows cable?  Wine actually does
> use (cause the serial driver to use) rts/cts flow control if the app
> asks for it, and it won't work on a 3 wire cable.  I don't believe
> Windows actually does hardware flow control at all, no matter what the
> app asks for.
Hmm.. I am pretty sure that Windows 9x and NT do use RTS/CTS flow 
control.  The standard win3.11 drivers don't IIRC, but 95 and above and NT 
always do if you have the hardware flow control radio box checked in the 
properties for the modem (and the app doesn't override it itself).


One thing I would like to mention is that I think almost all Gateway 2000 
486 computers have an incorrectly wired 25-pin serial port.  DTR is not on 
the correct pin on the 25-pin D connector but is on the correct pin on the 
10pin connector that is on the motherboard (which to my knowledge is the 
same for almost every motherboard that uses this type of serial 
connection).  If you upgraded the motherboard in one of these old cases 
and did not use the new serial ports that came with the new mobo this may 
cause you some trouble.  Fortunately my USR v.Everything has a DIP switch 
setting to turn off DTR control.  The downside is that with DTR turned off 
you cannot just drop DTR to hang up but must instead use +++ATH.  
Fortunately I got rid of that 486 board many computers ago.  
(Un)fortunately I no longer use the v.Everything modem for much since I 
have a cable modem.  It's almost a shame too, it's a really nice modem, 
connects with everything under the sun.


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