Need help repairing Philips MSX2 NMS 8250

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By Omega

Master (210)

Omega's picture

09-10-2019, 14:55

If your Z80 is still generating IORQ pulses when it should only be executing DI and JP 0, your CPU is not actually executing DI and JP 0. The Z80 is, normally, the only device on the bus that can generate IORQ. It would indicate towards address or databus issues which causes the CPU to read garbage instructions. Only the IN or OUT instructions on the Z80 should generate a /IORQ pulse.
It may be helpful to hook up a logic analyzer to address and databus, so at least 24 channels, so step by step the boot process can be followed..
Btw. If you give up I'd like to have a go at this machine, I like a challenge Smile

By Omega

Master (210)

Omega's picture

09-10-2019, 14:58

Have you checked continuity from Z80 databus pins to ROM data pins?

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