FAULTY VIDEO DISPLAYS

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S_U_C
Posts: 59
Joined: Sun Feb 17, 2019 6:41 pm

FAULTY VIDEO DISPLAYS

Post by S_U_C »

I worked out a mechanism for the repeating screen patterns on the MZ80K and MZ80A.
The Z80 is in a endless loop reading RST instructions.
The MZ80A REPEATING VERTICAL LINE PROBLEM i,e.char 39 + blank

RST CODE ADDRESS PUSHED ON TO STACK
0000h c7 1100 0111> 0000 0001 + 0000 0000 char 1 + blank
0008h cf 1100 1111> 0000 1001 + 0000 0000 char 9 + blank
0010h d7 1101 0111> 0001 0001 + 0000 0000 char 17 + blank
0018h df 1101 1111> 0001 1001 + 0000 0000 char 25 + blank
0020h e7 1110 0111> 0010 0001 + 0000 0000 char 33 + blank
0028h ef 1110 1111> 0010 1001 + 0000 0000 char 41 + blank
0030h f7 1111 0111> 0011 0001 + 0000 0000 char 49 + blank
0038h ff 1111 1111> 0011 1001 + 0000 0000 char 57 + blank

Is caused by the Z80 not seeing the M-ROM but FFH (RST 0038h) at address 0000h and then FFH at 0038h................Filling the V-RAM/D-RAM.
Character 57,(39H) is a vertical line and it is being written correctly to memory.
As the M-ROM has been replaced then suppect the /CS0 or the M-ROM data buffer chip.

The MZ80K REPEATING 2 CHARACTER PROBLEM i.e. CHARACTERS 142 AND 42
Again the Z80 is miss reading the M-ROM and ADDRESS 0000h containes a JP intruction C3h 1100 0011 which is nealy a RST.
So if d2 is miss read then it becomes a RST. The Z80 jumps to the rst address and executes the code but quickly finds another JP instucrtion (RST) and loops.
The Z80 is looping on a RST but all RSTs have a blank + charactor, but the display does not match expected address values.
As the address that is pushed onto the stack is generated by the Z80 and not read from memory then there also a write error.
The MZ80K has 2 primary data line RD/WR buffers but I can not work out which d-bits are faulty.
So this may mean that the buffer chips on the /RD, /WR lines are failing and the Z80 is reading/ writting incorect data.

I hope this explaination, may help others fix the machines.
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