This module is responsible for activating up to 32 pixels. Therefor it recieves the current data of the 32 pixels with 38400 bps serial via a 10 mA Current Loop, which is galvanically isolated by an opto-coupler.
Then the microcontroller (PIC16F627A) is generating the signals with help of pulse width modulation (PWM). For that it puts 12800 times per second 32 bit into 4 hooked up shift registers (74HC4094), so that you get 32 parallel signals. These signals are boosted through Darlington- Transistor-Arrays (ULN2803) and causing the pixels to blink in proper greyscales.
The pulse width modulation simulates 128 greyscales by very fast on-off-switching. That means, for greyscale 0 the pixel stays off the whole time. For greyscale 1 it is on 1 time and then 127 times off. For greyscale 2 it is on 2 times and off 126 times. Finally it is always switched on for greyscale 128. Thats how you get a greyscale-picture after 128 output cycles. With 12800 output cycles per second it results in a quite pleasant regeneration frequency of 100 Hz. That is enough to delude the sluggish human eye into seeing greyscales just by really fast switching on and off.
The serial data for instance are coming from the TROIA distribution module.
6 of these modules were successfully used for the TROIA Prototype.