LabVIEW Programming Real-time Control of KEITHLEY 6517A Electrometer

LabVIEW provides a way for any program to call another LabVIEW program, that is, to use the latter as a subroutine (i.e., subVI). Therefore, in the main data acquisition program of the experimental station, the simulated 6517A is embedded as a subVI called by the main program (of course, this subroutine can run independently without relying on the main program). According to the requirements of the main program, the subVI of 6517A has the following characteristics:
concise code and high execution efficiency; it can fully realize the functions of each button on the real panel, and can basically display the various information displayed by 6517A synchronously; independent window, does not visually interfere with the main program window, so as to facilitate user use.
For complex uncompiled LabVIEW programs, the code is required to be as concise as possible, which is beneficial to improve the running speed. In addition, considering that the adjustment of 6517A parameters is only a small part of the data acquisition and control of the experimental station, we did not use the LabVIEW subroutine provided by KEITHLEY with the instrument. For 6517A, there are three ways to realize the functions of the panel buttons: one is to use the function selection commands in its SCPI commands, such as using various commands in the SENSe command group to select the measurement content (voltage, current, resistance and charge) and various parameters related to the measurement of these quantities [1]. This method is comprehensive and can directly realize a certain function with a sufficiently long command, which may require pressing multiple keys to realize, and the user does not need to know the display information of the 6517A front panel (the LabVIEW subroutine provided by KEITHLEY is written in this way). The disadvantage is that the code command is long and complicated to write, and all SCPI commands must be very familiar. Another way is to use the SYSTem:KEY〈NRf〉 command in the SYSTem command group. Each function key on the 6517A function panel corresponds to a number. For example, executing: SYSTem:KEY 15 is equivalent to manually pressing the V (voltage) key once. Using this method basically "copies" the buttons on the front panel on the computer monitor. Although setting a parameter requires multiple button presses and the selection menu needs to be displayed synchronously on the computer, the code is concise and the interface function provided by LabVIEW can be used to completely simulate the front panel of 6517A in appearance. It is a good method. The third method is to use DDC language, which has a concise syntax, but is difficult to remember, difficult to maintain, and not powerful. We did not consider it when writing. ?

The entire 6517A subroutine is a large whileloop. Within the loop are the commands and display subroutines corresponding to all the buttons on the front panel. The condition for the loop to end is that the Boolean button "RETURN" = false. This structure has two functions. Since the execution of the LabVIEW program is parallel, it can accept commands from any virtual button at any time. The other is that the display on the computer screen is basically synchronized with the display of the electrometer. The subroutine is set to "open when called and close after completion", which does not visually affect the use of the main program. That is, after pressing RETURN, the subroutine window is closed.
2.2 Buttons
Figure 1 is the interface of the 6517A subVI. The character "Ω" in the figure has been replaced by "Ohm". It tries to imitate the front panel of the actual instrument, except that the POWER switch of the 6517A is replaced by the RETURN button. The mechanical action of each button is set to "Latch when Released", so that every time the button in the figure is pressed, the program sends a corresponding command to the 6517A through the GPIB interface to complete the manual key press action. Then the button floats up and will not cause malfunction. The program corresponding to the button is a "true?false" case structure. Write the command in True CASE: SYSTem: KEY 〈NRf〉 (Figure 2), and leave it blank in False CASE. When you click the button with the mouse, 6517A will complete the corresponding key action according to the command. As shown in the command in the figure, the GPIB write control sends: SYST: KEY 15 to 6517A, and it is completed by pressing the "V" key once.

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In order to better display the unit of the measured value, the 6517A display line has some special symbols, such as the temperature symbol ℃ on the secondary display line, the resistance unit Ω (ohm) on the main display line, and the magnitude unit μ (i.e., 10-6). However, actual tests show that the signal received by the computer from the 6517A can only correctly display the numbers 0 to 9, letters A to B and a to b in the standard ASCII code. Therefore, using only the above command will display garbled characters and cannot correctly reflect all the display information of the 6517A. After analysis, the author obtained the corresponding encoding of non-ASCII codes after passing through the GPIB interface as follows (Table 1).

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