Comment by zokier
> Yes, I remembered now, I had almost 200 really crappy PIC16F1782 chips, left over from an update for one of my clients
> First problem there are not enough GPIO pins on the 28-pin PIC to drive 128 LED’s, plus an address line, plus a serial receive pin, minimum GPIO’s for that is 24 (16×8 LED scan matrix)+4(address)+1(RXD) = 29. A simple solution is to add two 74HC595 shift registers to increase the outputs by 16 at the expense of 4 pins to control them.
Wouldn't the simple solution be just to split the matrix in half and throw second PIC in there? Or just make 1U panels instad of 2U?
Also the PIC is rated to sink/source 25 mA so you could have skipped the led driver transistors, making the boards even simpler.
Another note for simplification, you could use the builtin i2c peripheral and give each octet of leds its own address. You got 64 octets in total, which conveniently fits into 7 bit i2c address space. That should make the communication trivial to handle.
Also RPI has builtin i2c, so you don't need usb adapter for it.