Keithley 6220 Reference Manual page 290

10-26 Remote Operations
Flow control (signal handshaking)
Signal handshaking between the controller and the instrument allows the two
devices to communicate to each other regarding being ready or not ready to
receive data.
Software flow control
Software flow control is in the form of X_ON and X_OFF characters and is
enabled when XonXoFF is selected from the RS-232 FLOW-CTRL menu. When
the input queue of the Model 622x becomes more than 3/4 full, the instrument
issues an X_OFF command. The control program should respond to this and stop
sending characters until the Model 622x issues the X_ON, which it will do once its
input buffer has dropped below 1/2 full. The Model 622x recognizes X_ON and
X_OFF sent from the controller. An X_OFF will cause the Model 622x to stop out-
putting characters until it sees an X_ON. Incoming commands are processed after
the <CR> character is received from the controller.
If NONE is the selected flow control, then there will be no signal handshaking
between the controller and the Model 622x. Data will be lost if transmitted before
the receiving device is ready. The default setting is NONE.
Hardware flow control
Hardware handshaking can be used instead for flow control. The RS-232 interface
provides two control lines (RTS and CTS) for this purpose (see
Table 10-5). When the Model 622x is ready to send (RTS) data, it will transmit
when it receives the clear to send (CTS) signal from the computer.
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NOTE For RS-232 operation, *OPC or *OPC? should be
used with slow responding commands. See Sec-
tion 12.
Even with X_ON/X_OFF selected, the computer
may still lose data from the Model 622x if the re-
turn string is large (>30,000 characters) and one
of the higher baud rates is selected. With no flow
control (NONE selected), the error occurs with a
much smaller return string. Use some type of er-
ror checking in your program to avoid these situ-
ations.
Return to Section 10 topics
Model 6220/6221 Reference Manual
Figure 10-4 and