Multi-Tech Systems DT101, DT102 Appendix D - Flow Control Background, DataTalker Owner’s Manual

DataTalker Owner’s Manual

Appendix D - Flow Control Background

Flow control refers to techniques used by computer devices and the DataTalker to stop and restart the flow of data between them. Flow control prevents a channel device or DataTalker from receiving more data than it can handle. Flow control initiated by the DataTalker is called DataTalker-initiated flow control. It might be needed, for instance, if a DataTalker is connected to a minicomputer that can output more data than the DataTalker can handle. Flow control by the channel device is called channel device-initiated pacing. Pacing might be required, for instance, by a printer channel device that cannot print data as fast as the DataTalker can send it, or which may need to go off-line when it runs out of paper. To state it simply, flow control is something the DataTalker does to the channel device, while pacing is something the channel device does to the DataTalker.

DATATALKER-INITIATED

FLOW CONTROL

	DATA
Channel		DataTalker
Device

CHANNEL DEVICE-

INITIATED PACING

DATA

ChannelDataTalker Device

Flow control stops the input of data to the DataTalker

Pacing stops the output of data from the DataTalker

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 RS-232C interface (pin 5) is brought low to stop data and high to restart it. When you select a flow control method with a DataTalker command you are also selecting the corresponding pacing method.

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