Abstract:

This video details the ongoing reverse engineering and modernization of a vintage control display unit, focusing on replacing its original 6809 microprocessor board. The creator successfully designs and implements a new CPU board utilizing a PIC microcontroller to interface with the existing CRT display's data and address buses. The process involved desoldering the original EPROM and developing custom C code for the PIC to manage character and string display, including functions for setting character position, clearing the screen, and inverting text. The demonstration successfully shows custom text ("Le labo de Michel") being displayed on the old CRT, confirming the viability of the PIC-based replacement despite initial random characters and an unremovable hardware-defined line at the bottom of the display.

Summarizing the Control Display Unit Modernization:

• 0:00 Project Continuation: The video is a follow-up on efforts to reverse engineer and control a vintage display unit.
• 0:15 Legacy System Analysis: The original control processor board, featuring a 6809 microprocessor and EPROM, was analyzed. The 6809's instruction set is noted as complex.
• 0:35 New CPU Board Design: A new CPU board was designed and prototyped using a modern PIC processor, compatible with the existing display's connectors.
• 0:55 PIC Connectivity: The PIC processor is directly connected to the display control board's data and address buses. The new board also includes a debugger connector (ICD3), a trimmer for screen luminosity adjustment, and a 3.3V voltage regulator.
• 1:17 Initial Test (Blank PIC): An initial power-on test with an unprogrammed PIC resulted in random characters on the screen, indicating random values in the static memory.
• 1:47 Core Display Functions Developed: The creator began writing C code for the PIC, developing essential functions for display control:
  • 1:54 display character: Displays a specific character (0-127) at a given memory address (0-151), with an option for inversion. Special handling for bit 5 of the data bus was required due to a pin conflict with the reset line.
  • 3:31 display string: Iterates through a character string until a null terminator, displaying each character using the display character function.
  • 3:51 text locate: Calculates the memory address for text display based on given line and column, using a grid of 19 characters per line.
  • 4:19 clear display: Clears the entire display memory.
  • 4:33 inversion: Toggles the inversion mode for text display.
• 4:41 Successful Demonstration: The PIC-controlled board successfully displayed the string "Le labo de Michel" at a specified position, demonstrating that the new system works perfectly.
• 5:28 Hardware Limitation: A persistent line at the bottom of the screen was observed, which cannot be removed via software as it's a hardware-defined signal from the CRT controller. It's suggested it might be possible to physically cut the corresponding line on the schematics.