UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Nascom repair

julie_m · 23rd Oct 2017, 11:15 pm

How does the display generation work? I'm guessing it's character-mapped as opposed to bitmapped; but is the CPU driving the display directly and then running your program during the vertical flyback, or is there some dedicated logic?

If it's using a character generator ROM, that seems fine, because the characters on screen seem properly formed. Likewise anything downstream of the chargen, and its address generator (if the CPU generates the display addresses itself, it's blameless for now). How about the display RAM? If it is misbehaving, the screen display might well not make sense.

To test whether the CPU is running, write a simple program to toggle an I/O port pin on and off to a PROM, and probe that I/O line. Use a simple DJNZ loop for dwell time in each state, and choose an inititial value to load into B to set the frequency. You could use a single 2716 to replace a pair of 2708s, if you connected up the extra address line A10 to a suitable place on the board.

23rd Oct 2017, 11:15 pm	#3
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Nascom repair - Looking for clues please How does the display generation work? I'm guessing it's character-mapped as opposed to bitmapped; but is the CPU driving the display directly and then running your program during the vertical flyback, or is there some dedicated logic? If it's using a character generator ROM, that seems fine, because the characters on screen seem properly formed. Likewise anything downstream of the chargen, and its address generator (if the CPU generates the display addresses itself, it's blameless for now). How about the display RAM? If it is misbehaving, the screen display might well not make sense. To test whether the CPU is running, write a simple program to toggle an I/O port pin on and off to a PROM, and probe that I/O line. Use a simple DJNZ loop for dwell time in each state, and choose an inititial value to load into B to set the frequency. You could use a single 2716 to replace a pair of 2708s, if you connected up the extra address line A10 to a suitable place on the board. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.