Electron Scattering Cross Sections Experimental Apparatus: : Computer control interface

2.6 Computer control interface

The precise determination of a pre-selected energy range of electrons depends on the precise matching of the electrostatic energy analyzer bias voltage with the voltage of the electron gun power supply. This was accomplished by a computer control interface which controlled the electron gun power supply and the bias power supplied to the ESA. The electron gun and ESA power supplies have two modes of operation: manual mode

Fig. 2.7 Connection diagram to the computer interface card.

and remote mode. This experiment was performed in remote mode, and controlling the programmable voltage signal through the back panel remote control terminals. The programmable voltage signal was obtained from the computer using a national instrument model NI-6703 PCI digital I/O card. Table 2.1 shows the equivalency of the programming signal to the output electron gun and ESA power supplies.

Table 2.1 Programming signal to output equivalency.

Figure 2.7 shows the block diagram of connections from the interface to the equipment. The NI-6307 card has 16 programmable voltage outputs with 16-bit resolution, a 1 mV accuracy²⁷, and 8 digital I/O lines. Connections between the NI-6307 card and the power supplies were made using a 68-pin ribbon cable and a homemade circuit box. Since not all of the I/O lines in the NI-6703 card are needed to control the power supply and the electron gun, a circuit box was designed to convert 68 pins to 25 pins. A detailed description about the pins used from the NI-6703 card and the 68-pin to 25-pin conversion is given in the Appendix.

A parallel cable from the computer to the connections panel on a homemade box, connected electronics equipment to a user controlled switch panel. Figure 2.8 shows the switch panel, which holds 15 switches used to control the electron gun and ESA power supplies. Small electronic circuits were constructed to generate a digital signal pattern for

Fig. 2.8 User control switch panel.

each switch. The circuit diagram and pattern generated from this circuit are shown in the Appendix. This signal was then sent though 4 wires in a 25-pin parallel cable to the computer. Digital signals were read by the NI-6703 card and sent to the computer program which identified the switch pressed. Action was taken according to the user input. The switch panel allowed the user to increase or decrease the energy of the emitting electrons from the electron gun and bias voltage output from two serially connected BERTAN type power supplies to the ESA. NI-6703 card voltage output lines have 1 mV step size which allows changing the energy of the electron gun by 1 eV and voltage supply to the ESA by 0.06 V. Using the computer-controlled interface, the energy of the electron gun and the bias-voltage of the ESA could be matched to the nearest 0.06V.