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AN ELECTRONIC LITESHOW DISPLAY

Part 1ýThe Electronics Behind the Lights
by James Antonakos

Start ý The Big Picture ý Inside a Panel ý The Driver Board ý The 64-Bit Shift Register Board ý Interfacing with the PC ý A Little Math ý The Real Thing ý Sources and PDF

THE 64-BIT SHIFT REGISTER BOARD

The 64-bit shift register used to control all the lamps in a panel is formed by cascading eight 74HC595 8-bit shift registers. Typically, a latch is required on the output of the shift register to prevent unwanted flashing of the lamps when a new pattern is being loaded. Fortunately, the 74HC595 contains an 8-bit shift register as well as an 8-bit latch in the same package. So, after loading a new 64-bit pattern into the board, a single latch pulse is used to transfer the new pattern to the shift register outputs, updating all the lamps on the panel. Figure 6 shows the schematic of the shift register board. Remember that the panel is driven with a serial ý12-V datastream to allow for long cables between the panels and between panel one and the PC.

As a nod to history, the 1488 and 1489 line driver and line receiver chips are used to convert from TTL to ý12-V levels, and vice versa. These old chips were used instead of a device like the MAX232 because of their ability to handle larger drive currents. Unfortunately, their long rise and fall times (several hundred nanoseconds) led to an interesting problem during the initial software testing of the LiteShow display, when the last two panels did not update properly. Luckily, the problem was easily fixed by adding some delay between each bit clocked out to the display.

The beauty of the shift register design is that additional panels can be added to the LiteShow display without having to change the software or hardware. One line in a configuration file read by the software is used to indicate the number of panels in the display. The PC simply outputs additional groups of 64-bits when new panels are added.

Our department technician, Don McCarty, designed the artwork for the shift register board, which was sent to the same fabrication house that made the driver boards. Spare shift register and driver boards were ordered to help with any repairs needed in the future.

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