mastercello
Member
Have a nice and clean but dead Toaplan Fire Shark on my test bench.
The game does nothing besides a few static humms sometimes.
What i found out till now:
CPU
- Clock is stable and present on the cpu
- Motorola MC68000 cpu is not on HALT nor RESET (both signals are logical high)
- ENABLE signal is pulsing every 10 clock pulses
Program (EP)ROMs:
- CE is LOW
- OE is HIGH (should be LOW to to read out the program code to the cpu)
So there we might have the first problem, right?
The OE is coming from an output of a 74LS138 decoder.
The input are matching the output signal of it. So my guess is, that the LS138 is working fine as the truth table (and the 8 outputs) is as expected with the input signals i find.
All outputs are as the input signals demand (ABC=LLH), so the Y0 which goes to OE of the program roms is high.
The strange part here is, that the 3 inputs of the LS138 (ABC) are adress line A18,A19,A20 of the 68000 cpu (A20 being the pin which should be low but is high)
I guess they are used to access the different program blocks.
I changed already the HC68000 and the 74LS138 with good ones (also checked the program roms and they are fine).
I cannot find any other device connected there.
So the big cuestion now is, what am i missing?
The game does nothing besides a few static humms sometimes.
What i found out till now:
CPU
- Clock is stable and present on the cpu
- Motorola MC68000 cpu is not on HALT nor RESET (both signals are logical high)
- ENABLE signal is pulsing every 10 clock pulses
Program (EP)ROMs:
- CE is LOW
- OE is HIGH (should be LOW to to read out the program code to the cpu)
So there we might have the first problem, right?
The OE is coming from an output of a 74LS138 decoder.
The input are matching the output signal of it. So my guess is, that the LS138 is working fine as the truth table (and the 8 outputs) is as expected with the input signals i find.
All outputs are as the input signals demand (ABC=LLH), so the Y0 which goes to OE of the program roms is high.
The strange part here is, that the 3 inputs of the LS138 (ABC) are adress line A18,A19,A20 of the 68000 cpu (A20 being the pin which should be low but is high)
I guess they are used to access the different program blocks.
I changed already the HC68000 and the 74LS138 with good ones (also checked the program roms and they are fine).
I cannot find any other device connected there.
So the big cuestion now is, what am i missing?
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