Multi-Tech Systems MMV1600 owner manual Appendix F, Flow Control Background

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

Channel	DATA
		Mux
Device

Flow control stops the input of data to the mux

CHANNEL DEVICE

INITIATED PACING

DATA

Channel

MuxDevice

Pacing stops the output of data from 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 minicomputer so that the mux’s buffer capacity is not exceeded. We chose Xon/Xoff flow control for this example and are setting pace ON for all channel devices.

NODE 1

Channel 1 2

3

Mini- 4

Computer 5

6

7

8

Mux

Link

NODE 2

	Channel
	1		Printer
	2	PC
Mux	3	PC
	4	PC

5PC

6PC

7PC

8PC

MiniComputer Flow Control