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When using the power supply, the user will usually not be concerned with its inner workings. Instead, he or she will be interested in what the power supply can do, or its capabilities. Also, the user should not need to be unnecessarily concerned with how to control the supply, therefore the user interface is kept simple. The user interface is designed around the some of the most common tasks, or use cases, that will be performed. Taken together, these three aspects constitute the high level specifications of the system.
The hardware interface has been designed such that a total of eight devices, each of which may have both a positive and negative half, may be addressed. Each half of each device may be controlled independently and is able to report its current and voltage.
The prototype to be implemented will consist of three supply modules:
Besides the physical power connectors (banana plugs), the user interface consists of a 20-key keypad for input and a 40 character by four-line LCD panel display for output.
Figure 1: Keypad Layout
The keypad at right in Figure 1 shows the planned layout of the keys. The keys in gray will not be functional in the prototype, but are reserved for controlling additional supplies and supply types. The keys will function as follows:
Figure 2 below shows the format of the display while the system is not in an error state. The top line and leftmost column indicate permanent labels. All the other text changes to indicate the current state of the supply.
Figure 2: Display Format
Although there are numerous use scenarios, all of which are specified in the software flow diagram (see Figure 8: Keypad ISR), considering three specific use cases for the system will give a good overview of its scope and abilities. The use cases to be examined include setting a voltage, stopping the system, and recovering from a short circuit fault. These use cases will be examined in the context of the three-supply prototype. More selectable supplies would complicate the use cases, but the underlying interface methodology would be unchanged.
In order to set a voltage on the +/-35 V supply, the user must first make sure that the system display indicates that voltage is being set. This is indicated by a "V" in a specific location on the display. If the display indicates "I" for current, the user must press the "I/V" button on the keypad to choose the voltage setting.
The user can then use the numeric and decimal point keys to set a voltage anywhere from 0.00 V to 35.00 V in 0.05 V increments. If a mistake is made while entering the voltage, the user can press the "clear" key to erase the entire entry, or the "backspace" key to erase just the last key pressed. When the desired voltage is on the display, the user presses the "enter" key.
After "enter" is pressed, the processor will adjust both the positive and negative half of the variable supply to obtain the desired voltage.
When the user presses the "stop" button, all supplies are disconnected from power. This allows the user to safely work on their circuit. All state information is maintained so that the user can restart at any time without reentering his or her desired settings.
When a short circuit or near short circuit occurs, the processor will attempt to compensate by continually raising the voltage set point. When the set point on any supply module exceeds a predetermined maximum, the processor will detect it and physically disconnect all supplies.
The normal readout will be replaced by a full-screen error message informing the user of two choices: