4 - Principles of Operation
A3 Front Panel Circuits
As shown in Figure 4-1, the supply's front panel assembly contains a circuit board, a keypad, a liquid crystal display (LCD), and a rotary control (RPG) for the output voltage and current. With the exception of the RPG (A3G1), the A3 Front Panel board is an assembly-level replaceable part. A separate front panel binding post board is also included on the unit. It is also available as an assembly-level replaceable part.
The A3 front panel board contains microprocessor circuits, which decode and execute all keypad and RPG commands that are transferred to the power supply output via the serial I/O port to the primary interface circuits on the A2 interface board. The front panel microprocessor circuits also process power supply measurement and status data received on the serial I/O port. This data is displayed on the LCD.
A2 Interface Circuits
The circuits on the A2 interface board provide the interface between the GPIB interface, RS-232 interface, and front panel interface and the dc power supply. Communication between the power supply and a GPIB controller is processed by the GPIB interface and the primary microprocessor circuits on the A2 board. The A2 Interface board is assembly-level replaceable; it contains no user-replaceable parts.
With the exception of the front panel microprocessor, all digital circuits, analog-to-digital converters (ADC) and digital-to-analog converters (DAC) in the dc power supply are located on the A2 Interface board. All control signals between the A2 interface board and the A1 main board are either analog or level signals.
Primary Interface
The primary microprocessor circuits (DSP, ROM, and RAM chips) decode and execute all instructions and control all data transfers between the controller and the secondary interface. The primary microprocessor circuits also processes measurement and status data received from the secondary interface.
A Dual Asynchronous Control chip on the A2 board converts the RS-232, RI/DFI, and front panel data into the primary microprocessor's 8-bit data format. The serial data is transferred between the primary interface and the secondary interface via a serial bus and optical isolator chips. These chips isolate the primary interface circuits (referenced to earth ground) from the secondary interface circuits.
Secondary Interface
The secondary interface circuits include a programmed logic array, EEPROM, boot-ROM, 8 and 12-bit DAC circuits, and 8 and 16-bit ADC circuits. The programmed logic array translates the serial data received from the primary interface into a corresponding digital signal for the appropriate DAC/ADC circuits. The logic array is also connected directly to four DAC/ADC circuits. Under control of the logic array, the selected DAC converts the data on the bus into an analog signal. Conversely, the selected ADC converts the analog signals from the A1 board into a digital signal.
The logic array also directly receives status information from the A1 main board via three level-sensitive signal lines, which inform the array of the following operating conditions: constant voltage mode (CV_Detect*), constant current mode (CC_Detect*), negative current mode (CCN_Detect*), and overvoltage (OV_Detect*). The PM_Inhibit control signal is used to shut down the bias voltage to the output stages and keep the power supply output off. The OV_SCR* control signal is used to fire the SCR and keep the power supply output off when an overvoltage condition has occurred.
