| Introduction to Data Communications |
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14. RS-232D Flow Control (cont'd) |
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14c. Software
Handshaking (Xon/Xoff)
Software Handshaking does not use the RS-232D control signals, it uses the
software commands Xon/Xoff to control the data flow. Do not use software
handshaking with a modem, because you will lose several important function of
the modem such as: RI, and CD.
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Xon
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Transmit On
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- ASCII Character DC1
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Xoff
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Transmit Off
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- ASCII Character DC2
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Software handshaking is a simple flow control method that is used mainly with
DTE to DTE and DTE to Serial Printer connections. The receiving device
controls the flow of data by issuing Xon (okay to transmit data) commands and
Xoff (stop - let me catch up) commands. A good example is the DTE to Serial
Printer connection.
For example, a dot-matrix printer cannot physically print faster than a
transfer rate of 300 bps. Printers are usually equipped with a memory buffer
to store the data before it is printed. The printer buffer allows large chunks
of data to be downloaded to the printer from the DTE, thereby freeing up the
DTE to do other tasks rather than wait for a page to be printed.
When the data is first being downloaded to the printer, the printer issues a
Xon command to the DTE. As the print buffer becomes full (90%), the printer
issues an Xoff command to stop transmitting data until the printer catches up.
When the print buffer becomes almost empty (20%) than the printer issues a Xon
command. This goes on until the complete document is printed.
Since we are using software to control the data flow, we can eliminate a few of
the control lines used in the Hardware Null Modem cable. In its simplest form,
the Null Modem cable consists of SGND, and the TXDs & RXDs crossed.
Usually we find that we have to add a few control lines to fool the DTE's
hardware. There is no standard Software Null Modem configuration for Xon/Xoff.
The exact connection will vary from device manufacturer to device
manufacturer.
14e. Terminals &
PCs
Terminals are considered dumb devices. They can only display data on the screen
and input data from a keyboard. They communicate with a mainframe or
minicomputer which does the number crunching and work. Terminals do not have
hard-drives for storing files or RAM for running programs. Terminals cannot
work by themselves, they are an extension of the mainframe or minicomputer's
display and keyboard.
PCs have microprocessors which are the smarts or brains that can do number
crunching and work. They have hard-drives for storage and RAM for running
programs. They are stand-alone devices.
The purpose of communication programs like Procomm Plus, Kermit, PCLink or
Quicklink II is to turn your PC into a terminal. It is the computer world's
equivalent of a lobotomy.
Copyright Eugene Blanchard Jan 1998, January 2007