rts/cts - a tutorial on flow control

[Martin Minow]

... "What is really wanted is a standard method of *positive* flow
control.  I.e., 'I'm ready for another 347 bytes now'."

The "negative" flow-control methods work (XOFF/XON) work satisfactorily
if sufficient space is allocated for overrun.  For example, (assuming
9600 Baud transmission) the receiver can allocate a 400 byte buffer, sending
XOFF when there are 100 bytes unprocessed bytes in the terminal input
buffer.  This gives the host about 600 msec. to stop output.
The terminal might send XON when, say, only 50 bytes are in the input
buffer.  (Actual numbers are made up and shouldn't be taken as
a recommendation -- a university-level course on simulation might
give you tools for selecting real values.)

As has been already noted, it is important to implement flow-control
algorithms at a low-level in the operating system, preferably within
the terminal device interrupt-service routine.

Martin Minow
decvax!minow