Flow Control

A flow-control protocol is selected to ensure that the host does not send data too fast for the target to receive. The need for this can arise because, although the target system may be able to read a record at 9600 baud, the target system may need time to process that record before it can read the next record.

The environment variables dlproto and dlecho are used specify the flow-control protocol.

The following table summarizes the four flow-control protocols supported by this Monitor.

Terminal Emulator Sends Target Returns Set Application
Line terminated by carriage return Echoes same line dlecho = on
dlproto = none
Terminal Emulator waits for full line to be echoed before sending next line.
Line terminated by carriage return Returns line-feed character dlecho = lfeed
dlproto = none
Terminal Emulator waits for line-feed to be echoed before sending next line.
Line terminated by carriage return Returns Xoff and Xon characters dlecho = off
dlproto = XonXoff
Terminal Emulator sends until Xoff. Resumes sending after Xon.
Line terminated by ETX character Returns Ack character dlecho = off
dlproto = EtxAck
Terminal Emulator sends line terminated by Etx. Sends next line after Ack (see note).

Note: The Xon-Xoff protocol is often not as reliable a method of flow-control as it might first appear. This is because many host systems do not shut off the flow of characters immediately upon receiving an Xoff. They may continue to transmit 20 or 30 bytes before finally stopping. Unfortunately these characters will not be seen by the monitor because it is not interrupt driven, and will be busy processing the previous line.

See Also:

stty, hyper term help, terminals and Unix tip help


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