#: 17511 S12/OS9/68000 (OSK)
    11-Feb-93  20:19:23
Sb: #17498-Xon/off
Fm: Bill Dickhaus 70325,523
To: Bob van der Poel 76510,2203 (X)

Bob,

The xon/xoff values in the descriptor are normally used by the driver to
provide software flow control _from_ the computer to the external device. In
other words, whatever is connected to that serial port can use xon/xoff to
control output from that port. With a directly attached terminal, ^S and ^Q
could be used to pause output (if those were the actual xon/xoff values in the
descriptor, which they usually are). Someone dialed into your system through a
modem could also use xon/xoff, as could another computer. Sometimes even line
noise can cause an apparent port lockup, when in fact the driver simply honored
what it saw as an xoff.

You are correct in not wanting to use software flow control with the modem, for
exactly the reason you mentioned, there might be xon/xoff in the data.

Software flow control (xon/xoff) is normally used end to end, computer to
computer, terminal to computer, etc. Hardware flow control or hardware
handshaking, is usually local between two devices, such as a computer and a
modem.

With the newer modem protocols, the modems do flow control between each other
as part of the protocol, local flow control (between a modem and the local
device, such as a computer or terminal) is handled according to the local modem
setup, and xon/xoff flow control is a result of settings (like in the
descriptors) at either or both ends of the connection. All of these flow
control scenarios are independent of each other (though they may influence each
other).

Bill