This an Arduino support library for a strange and nearly useless type of input device consisting of one or more mechanical seven-segment display modules each of whose content the user sets by operating the module's three sliders to form the digits the user wishes to input. After setting the digits for the desired input value in this way, the value of the digits may be read using the library. (How the user signals when it's time to read the device is not a part of this library.)

A display module looks like this:

On a given module, the left slider controls the vertical segments on the left side of the digit display. All the way in, it shows no segments. Pull it out one click and you see the top segment. Another click and both segments are displayed. Pulling it out all the way (the next click) displays only the lower segment. The right slider works the same way, but for the vertical segments on the right side of the display.

The center slider controls the display of the horizontal segments. All the way in: no segments are displayed. One click out: Top only. Two clicks: top and middle. Three: top middle and bottom. Four: middle and bottom. Five: top and bottom. Six: middle only. Seven: bottom only.

Here's a drawing shows the sliders inside the device displaying '0'.

In the photo and the drawing, the left and right sliders are pulled out two clicks and the center one is out five clicks, causing the module to display the digit '0'.

The backs of the sliders have 4mm disc magnets strategically embedded in them. As the sliders are moved, the embedded magnets move over a set of A3144 Hall effect sensors below them, two sensors each for the left and right sliders, three for the center slider.

The drawing above is the view from the rear of the module. It shows the location of the magnets on the sliders (the small gray discs) and the location of the sensors (the small semitransparent brown patches labeled 'A' through 'G'. The seven sensors are connected to the parallel-load pins of an HC165 8-bit parallel/serial load, serial-readout shift register.

The HC165 is connected to a serial bus consisting of power, ground and four signal lines: clock (clk), parallel-load/serial-load (plBar), serial-in (sIn) and serial-out (sOut). These daisy-chain from one module to the next. The incoming power, ground, clk and plBar lines connect to the Vcc, Gnd, clk and plBar pins of the HC165 and then pass through to the next module. The sOut line from the previous module goes to sIn on the HC165 and the HC165 sOut goes to the sIn on the next module. The bus's sOut line just passes through from module to module without connecting to anything. After the last module there's a bus terminator that connects sIn on the bus to sOut and has no connection to the other lines. The Arduino that reads the user's input values is connected to the bus input of the first module in the string using the +5V (or +3.3V depending on the Arduino) and Gnd pins, and four digital IO pins.

Here's a photo of a module's sensor circuitry taken from the rear of a module with the back cover removed. As you can see, it's built dead-bug style:

The photo also shows the incoming bus connector (the black rectangular plug at the top right) a and the bus terminator (the black plug with the yellow sIn-to-sOut wire at the bottom right). The terminator is plugged into the modules outgoing bus socket. The modules are designed to plug together to create a multi-digit unit that logically behaves as a long parallel-load, serial-readout shift register of sensor values that the Arduino plugged into the right-most module controls.

Here's the schematic for a module:

When it's time to read the display, plBar on the bus, which is normally high, is taken low and clk is strobed from its usual low state to high and then back. This loads the state of the sensors in each module into that module's HC165. The plBar line is then taken low, and the clk line is cycled once for each sensor in the entire display. With each rising clk signal, the state of next sensor appears on the bus's sOut line.

Obviously, eight clk cycles are needed for each module. But how many modules are there in the device? We can figure that out at initialization. First we set the bus sIn line low and then cycle clk enough times to clear out the combined shift register of the biggest supported display (one with INSL_MAX_DIGITS modules). Then set sIn high and cycle clk until a high appears on sOut. The number of cycles that took is the length of the shift register in bits. Divide by 8 and we know how many modules there are.

Generally, the flow is to instantiate the device as a static global variable, call the begin() member function in setup(); and then, when it is time to read the digits the user has set in the device, call the update() member function to capture the current data. Finally, call getDigitValue(dNo) for each of the device's digits to retrieve their values.

The library recognizes digit patterns for the hexadecimal digits, 0 ... F. How the program interprets them is up to it. Decide the input is hex and allow all the digit values. Decide it's decimal and declare the digits A through F are invalid. Or octal and only take 0 through 7. Or binary even. The library doesn't support conventional duodecimal, though, because the digits ↊ and ↋ ("dek" and "el" respectively) are ambiguous in a seven-segment display, dek looking like '5' and el like 'E'. It is possible, as some infidels are reported to have done, to use A for dek and B for el. But such practice, obviously, cannot be countenanced.

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