Xon-Xoff Handshake - HP 7470A Interfacing And Programming Manual

Xon-Xoff Handshake
With the Xon-Xoff handshake method, the plotter controls the data
exchange sequence by telling the computer when it has room in its
buffer for data and when to shut off the flow. The plotter uses buffer
threshold indicators (an Xon trigger character and an Xoff trigger
character) to prevent buffer overflow.
OVERSHOOT (DUE TO TIME REQUIRED TO
REACT TO XOFF TRIGGER CHARACTER)
BUFFER FULL l 0
XOFF THRESHOLD
(XOFF TRIGGER
/\ r\
CHARACTER SENT) 3 4 5 '
TOTAL
_ BUFFER
XON THRESHOLD SPACE
\/
2 5
(XON TRIGGER AVNLABLE
CHARACTER SENT}
BUFFER EMPTY TIME 255
Xon-XoffThreshold Levels
As data is sent to the plotter by the computer, it is stored in the buffer
and simultaneously acted on by the plotter. The preceding figure is
representative of the way the Xon-Xoffhandshake works; the numbers
represent the following:
1. Data enters the buffer faster than it can be acted on by the plotter,
and the buffer starts to fill.
2. The plotter begins processing the input data faster than the computer
sends it, and the buffer starts to empty.
3. The data enters the buffer at a faster rate than the plotter can
process it. The amount of data stored in the buffer reaches the Xoff
threshold level, at which point the plotter sends the Xoff trigger
character stopping the flow of data from the computer.
4. Due to a finite delay between the time the plotter sends the Xoff
trigger character and the time it takes the computer to react, a slight
overshoot may occur. For this reason, the Xoffthreshold level should
always be specified at least as large as the data block size or the
RS-232-C/CCITT v.24 INTERFACING 10-19