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Output Assign Module

The output assign modules uses a 16V8 GAL for address decoding and enable signal generation. Figure 44 shows the pin assignments for the GAL used in the output assign module. The VHDL code used to generate the GALs is included in the appendices, on page

**Figure 44:** Output assign module, address decoding GAL.
$\begin{figure} \psfig{file=schem/oa-addx.eps,width=6in,height=7in} \end{figure}$

The status of 18 pushbutton switches need to be sensed by the microcontroller. Many methods exist for connecting switches to a microcontroller bus. To keep parts count and CPU use down, simple latches are used to latch the status of eight switches at a time. Firmware debouncing routines shall take the place of hardware debouncing techniques. The lack of a multiplexing scheme means less CPU time is wasted in reading in the values of the switches, while the firmware debouncing routines mean fewer components are used on the board. Figure 45 shows the circuit used to achieve these goals. Three of these circuits are present, providing a total of 24 pushbutton inputs. Six of these inputs are used for pushbuttons in the universal user interface and cue stack sections of the control board.

**Figure 45:** Output assign module, momentary pushbutton decoding schematic. The output assign module uses three of these circuits, for a total decoding capability of 18 on-board and 6 off-board buttons.
$\begin{figure} \psfig{file=schem/oa-btn.eps,width=6in,height=7in} \end{figure}$

The output assign module contains several LED indicators that need to be controlled by the microcontroller. A simple current-sinking scheme involving 74LS373 octal latches is used, identical to that of the fader module. Figure 46 shows the circuit.

**Figure 46:** Output assign module, LED driver schematic. This module uses two such circuits, for a total of 16 LEDs.
$\begin{figure} \psfig{file=schem/oa-led.eps,width=6in,height=7in} \end{figure}$

The output assign module contains a pair of 7-segment LED displays, controlled by the microcontroller. While it certainly would have been possible to drive the displays in a similar fashion to the bare LEDs present on the same board, this was decided against due to the extra Pbus ports needed. Instead, BCD-to-7-segment decoders were used, with their inputs driven from a 74LS373 latch. Figure 47 shows the circuit used to drive the 7-segment displays. Note the use of the ripple zero-blanking feature of the 74LS247 BCD-to-7-segment chips. This feature will blank both displays when the a double-zero is displayed, and blank the leftmost display when its value is zero.

**Figure 47:** Output assign module, 7-segment display decoder and driver schematic.
$\begin{figure} \psfig{file=schem/oa-7seg.eps,width=6in,height=7in} \end{figure}$

Next: Transport Control Module Up: Control Board Previous: Fader Module Contents

Steve Richardson 2000-07-06

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