19-11-2021, 07:19 AM
From your text and photos, it appears that your 6545 CRTC is at least partially working.
When MWBASIC boots, it firstly initialises the Z80 PIO, then the CRTC registers, fills the screen RAM with spaces and loads the PCG with INVERSE characters.
So, if you're getting a stable picture of any kind then the 6545 CRTC is at least partially working. Seeing a flashing cursor is also a good sign; even better if it moves right and left with keystrokes..
If there's junk characters on your screen then there's and issue with char RAM access.
The boot beep come later immediately prior to the opening cold boot MWBASIC caption displayed.
Having an EPROM Emulator, a stack of EPROMs (or NVRAMS) with programmer is most practical way to fix bees as the enables one to write and use small diagnostic programs the simplest containing just one byte 0x76 - The HALT instruction.
When MWBASIC boots, it firstly initialises the Z80 PIO, then the CRTC registers, fills the screen RAM with spaces and loads the PCG with INVERSE characters.
So, if you're getting a stable picture of any kind then the 6545 CRTC is at least partially working. Seeing a flashing cursor is also a good sign; even better if it moves right and left with keystrokes..
If there's junk characters on your screen then there's and issue with char RAM access.
The boot beep come later immediately prior to the opening cold boot MWBASIC caption displayed.
Having an EPROM Emulator, a stack of EPROMs (or NVRAMS) with programmer is most practical way to fix bees as the enables one to write and use small diagnostic programs the simplest containing just one byte 0x76 - The HALT instruction.