Keithley 2701, User Manual

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Model 2701 User’s Manual Remote Operations 10-29
Signal handshaking (flow control)
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 RS232 FLOW menu. When the input queue of the
Model 2701 becomes more than I full, the instrument issues an X_OFF command. The
control program should respond to this and stop sending characters until the Model 2701
issues the X_ON, which it will do once its input buffer has dropped below half-full. The
Model 2701 recognizes X_ON and X_OFF sent from the controller. An X_OFF will cause
the Model 2701 to stop outputting characters until it sees an X_ON. Incoming commands
are processed after the <CR> character is received from the controller.
NOTE For RS-232 operation, *OPC or *OPC? should be used with slow responding
commands. A list of the slowest responding commands and details on *OPC and
*OPC? are provided in Section 12.
XonXoFF is the FACT and *RST default flow control setting.
If NONE is the selected flow control, then there will be no signal handshaking between
the controller and the Model 2701. Data will be lost if transmitted before the receiving
device is ready.
NOTE Even with XonXoFF selected, the computer may lose data from the Model 2701
if the return string is very large (approximately 30,000 or more 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. Your program could provide
some type of error checking for these situations.
NOTE Another solution to the problem is to use the TRACe:DATA:SELected? <start>,
<count> command to return small portions (100 points) of a very large buffer.
With this command, you specify a buffer location (<start>) and the number of
readings to return (<count>). See Section 6 for details.
Hardware flow control
Hardware handshaking can instead be used for flow control. The RS-232 interface
provides two control lines (RTS and CTS) for this purpose (see Figure 10-11 and
Table 10-4.) When the 2701 is ready to send (RTS) data, it will transmit when it receives
the clear to send (CTS) signal from the computer.
2701-900-01.book Page 29 Wednesday, August 3, 2011 9:43 AM