So far the project has been going quite well. Yesterday I finished the first driver board, of which we’ll need one per display. The board has 16 relays, 16 transistors, 16 LEDs, and two octal transparent latches. With a 16 pin header this board will connect to a breakout board that performs address decoding for the latch enable signals on each latch.
Jon is working on assembling the breakout/decoding board, which is responsible for isolating the parallel port with closed-collector transistors on each of the twelve TTL logic signals we’re using, and decoding the four control lines into latch enables for all the display latches.
The interface design is fairly simple… the eight data lines are routed to each of the octal latch’s D inputs, and the four control signals are decoded into 16 active high signals, each routed to a latch enable input on a particular octal latch. The software is responsible for assigning the control bits to select a latch, then outputting data to be stored. Consequently the relays will open or close depending on the data bits set. This turns on or off individual segments in a given display.
To date I’ve written some software to test and debug the decoding circuit but nothing to really drive the display. Once that is done I’ll post the code. Hooray for Thanksgiving break!
Stay tuned for more updates and schematics/code.
