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Fader Module

The fader module uses a standard block of Pbus interface logic. A 16V8 GAL (gate array logic) chip shall provide address decoding functionality. Figure 41 shows the pin assignments of this piece of programmable logic. While other means of address decoding are certainly possible, a GAL was used to reduce chip count, and provide enhanced functionality. The fact that GALs are completely programmable allows any address to be decoded. Thus, different GALs with different addressing may be used for duplicate boards in a system. The VHDL code used to generate the GALs is included in the appendices, on page [*].

Figure 41: Fader module, address decoding GAL pin layout.
\begin{figure}
\psfig{file=schem/fade-addx.eps,width=6in,height=7in}
\end{figure}

To reduce the number of A/D ports required, the eight slide potentiometers are connected to a 4051 8-to-1 analog multiplexer. The schematic shown in figure 42 shows how these potentiometers are connected. A single enable line generated by the GAL enables the address lines of the 4051.

Figure 42: Fader module, analog multiplexer circuit for potentiometers.
\begin{figure}
\psfig{file=schem/fade-pot.eps,width=6in,height=7in}
\end{figure}

Sixteen individually addressable status LEDs are driven with a pair of 74LS373 octal latches. The LEDs are arranged such that the '373 is sinking the current when the LED is on, thus the status of the LEDs will appear inverted. Note that other schemes could have been used to drive the LEDs, but the latched method reduces CPU load considerably, as there is no refresh cycle to chew up CPU time, as in a multiplexed design. A pair of enable lines, generated by the address decode GAL, activate each latch. Figure 43 shows one of two of the LED driver circuits needed for the fader module.

Figure 43: Fader module, LED driver schematic. The module needs two of these circuits, for a total of 16 LEDs.
\begin{figure}
\psfig{file=schem/fade-led.eps,width=6in,height=7in}
\end{figure}


next up previous contents
Next: Output Assign Module Up: Control Board Previous: Control Board   Contents
Steve Richardson 2000-07-06
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