Yugoslavian computer Galaksija
During my debugging I made a quick cheat sheet.
1. Screen without text and raster
→ 80A, 6116, 2732, 74LS04, 74LS593, 74LS74, 74LS123
2. Raster is present but no text
→ 2716, 74LS04, 74LS74, 74LS156
3. Image is heavily skewed
→ C3, C4, R12, R13
4. Certain characters are not decoded
→ 74LS156, 74LS251
5. Characters are displayed but some are missing, partially drawn, or deformed
→ Z80A, 2732, 6116, 74LS174
6. First and fifth lines display the same characters in parallel
→ 74LS174
7. All characters are displayed but some parts are missing
→ 74LS166
8. Letters are vertically split and lines appear on the right side of the screen (BRK)
→ 74LS174
9. Random characters appear in various parts of the screen
→ Z80A, 74LS32, 74LS156
10. Grainy raster across the entire screen
→ Z80A, 74LS156
11. Variable raster across the entire screen
→ Z80A, 74LS156
12. Diagonal raster with one vertical stripe
→ Z80A, 74LS156
13. Unresponsive text, does not accept commands
→ 74LS38, 74LS74
14. Vertical stripes without text
→ 74LS04, 74LS38
15. Dashes across the entire screen
→ 74LS38
16. Occasional random text appears alongside "READY"
→ 74LS38
17. "READY" appears on the right half of the screen, then characters appear all over
→ Z80A, 74LS74
18. Image moves vertically
→ 74LS04, 74LS123
19. Image is heavily skewed and moves vertically
→ C3, R12, 74LS123
20. Slanted dotted characters
→ 74LS123
21. A few white slanted lines
→ 74LS123
22. Vertically shifted or skewed characters
→ 74LS93, CD4040
23. Vertically moving image with no text
→ 74LS93
24. "PRINT MEM" shows less memory than expected
→ 6116, 74LS156Debugging tips taken from magazine Računari
T1, R4, R5, C6 and D1 are part of reset circuit (holds low level for 0,5s)
left 74LS156 is address decoder for RAM (enables OE on correct RAM or ROM chip)
74LS123 with of horizontal and vertical sync (horizontal sync R12 and C3, vertical sync R13 and C4)
74LS93 divides oscilator by 15 (6144000 Hz /15)
CD4040 divides number by 1024 (6144000 Hz /15/1024)
CD4017 divides by 10 (6144000 Hz /15/1024/10 = 50Hz) (refresh rate), one pin goes via 74LS04 (inverter) to INT on Z80, which triggers M1 and IORQ, which via 74LS74 activates WAIT
74LS174 tells the character ROM which line of 13 is currently being displayed. Bad connection on pin 2 and 3 gives a black square on right side
Decimal Binary
------- ------
0 0000
1 0001
2 0010
3 0011
4 0100
5 0101
6 0110
7 0111
8 1000
9 1001
10 1010
11 1011
12 1100
13 1101
right 74LS156 is keyboard decoder (active low) for columns
74LS251 is keyboard decoder (active low) for rows left 74LS156 is chip select for ROM and RAM (active low)
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