Xon/Xoff Flow control using fputs

Xon/Xoff Flow control using fputs

Post by Timothy Cra » Sat, 08 Mar 1997 04:00:00

To anyone that could help.                                                

A vendor writing software for us is having the following problem.  
Their process is tasked with writing verification messages out of a serial
port (RS-422) to another computer system (SMS).  The other computer
system currently uses Xon/Xoff control protocols to control message flow.  The
vendor is using a fputs(msg_str, sms^which_sms-1|) command to write out the
port to the SMS system.  Unfortunately, the vendor does not know how to check
the port to insure that an Xoff has not been received.  Consequently, the
vendor's process keeps writing to the port, filling the port buffer and
eventually backing into the process and killing it.  Any hints on what the
vendor could do to insure that port can take the data?            

As far as I can tell (I'm looking at the source code), I don't think he can
use a "canput" command, or other queue commands to check the status of
the port.  I could be wrong, though.                                          

The unix system is the latest version of Interactive Unix.

Thanks in advance for all replies!

Timothy Crane           http://members.tripod.com/~tcrane/index.html
"Useless laws weaken the necessary laws."
Charles de Secondat Montesquieu, De L'Esprit des Lois, 1748


1. Cannot keep XON/XOFF flow control

I have a Telebit WorldBlazer modem with FAX capability. I've been
trying to get "efax" to work, with little success. From what I can
tell, XON/XOFF flow control will just not work. I have all tried all
the possible register settings, with no success. With a 9600 bps mod.
rate I am getting thruputs of greater than 12,000 bps; as well as
a warning about extra XON/XOFF characters at the start of transmision.

As the man page states, this seems to indicate that flow control is
not functioning; I can verify this just by watching the lites on the

Then I picked up some cheap commercial software (E-FAX) that I installed
and ran on the DOS/Windows partition, and faxes flow just fine. (with
just a plain old &F8 setting - what you would expect). When I switch
back over to Linux, IFF I do not reset the modem, the Linux e-fax
works just fine, also. But ONCE the modem is reset (for SLIP/UUCP), I
can not get XON/XOFF flow control to kick in again.

All register settings for the modem are identical, before and after
the reset. XON/XOFF just won't work after.

I don't think it is the modem, since on the DOS side it functions
just fine.

And I the writer of "efax" is on vacation till September sometime.

Any one know what is going on, and how to fix this? Help?

Bruce Parkin

2. script question

3. Serial printer, XON/XOFF flow control, anyone?

4. ISDN Bridge/Router Experience???

5. XON/ XOFF flow control

6. Alaris Mantissa video card

7. flow and xon/xoff (terminfo(5) ?)

8. ipchains logging

9. XON flow control with lpr serial printing

10. XON/XOFF Question

11. Digi Board problems with Xon Xoff

12. Serial Port XON/XOFF problems

13. xon xoff protocol for serial devices