Appendix F; Flow Control Background - Multitech MMH900 User Manual

Appendix F

Flow Control Background

Flow control refers to the techniques used by computer devices and
multiplexers to stop and restart the flow of data from each other.
Flow control is necessary so that a channel device does not receive
more data than it can handle or vice versa (the MultiMux receives
more data than its buffers can accommodate). Flow control by the
mux to control data flow from a channel device is called Mux Initiated
Flow Control. Such flow control might be needed if a mux was
connected to a minicomputer that could output more data than the
mux could handle. Flow control by the channel device to control data
flow from the mux is called Channel Device Initiated Pacing. Such
pacing might be required by a printer channel device which could not
print data as fast as the mux might send it or might go off-line for
some reason like running out of paper. To state it simply, "Flow
Control" is something the mux does to the channel device, while
"pacing" is something the channel device does to the mux.
MUX INITIATED
FLOW CONTROL
DATA
Channel
Device
Flow control stops the
input of data to the mux
Flow control can be software or hardware based. In software flow
control, special characters (Xon and Xoff) are used to stop and start
the flow of data. In hardware flow control the Clear To Send (CTS)
signal on the RS232C interface (pin 5) is brought low to stop data
and high to restart it. When you select a flow control method with a
mux command you are also selecting the corresponding pacing
method.
In the example below we have an eight port multi-user minicomputer
connected to a MultiMux on one end of a link and seven terminals
plus a printer connected to another MultiMux on the other end of the
link. The MultiMux at the printer end needs Flow Control and Pace
on the printer channel to stop and restart data from the
minicomputer. The MultiMux at the minicomputer end needs Flow
Control on to all channels to stop and restart data from the
Appendix F - Flow Control Background
CHANNEL DEVICE
INITIATED PACING
Mux
Mux
Pacing stops the output
of data from the mux
DATA
Channel
Device
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