The 8085 doesn't halt unless it executes a 166 octal instruction.
000 octal is one of the many NOP instructions, and 377 is a RST7
instruction, which calls the 7th interrupt vector location,
and starts executing... so typically, it will just maraud through
memory forever, never finding a HLT instruction.
Those of us that hoped for better behavior learned to salt unused
EPROM and RAM space with 166's. We also typically stuck a HLT
instruction in the 7th interrupt vector location, so executing a
RST7 would stop the marauding processor cold in its tracks..
Note, the 8080 is an octal machine. Its structure makes sense
in octal, and nonsense in hex.
The boot sequence is pretty simple, it loads the program counter
with zero, and starts to execute whatever it finds there.
-Chuck Harris
John wrote:
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Roget thanks.
Yes, I was using an analogue scope and watching for any flicker no matter how slight. Might be time to break out the Rigol and try single shot mode. On the other hand without any boot code to read from the ROM the processor would just simply halt so which would presumably happen very quickly. It have been trying to find a description of the exact sequence of events the processor follows at power up, i.e. the boot-up sequence if you like, but have yet to find something on that subject. I am wondering whether your book contains such information?
I tracked the GPIB interrupt input to a connector on the memory board, but couldn't fathom where it went from there. I agree with your conclusion. BTW, I realized later that the buffer in non-inverting. Pins 1 and 19 were low so the buffer should be operating. I didn't expect that 1.5V on the input would be sufficient to cause a HIGH on the output side, but as you say, leaving it open is not good practice. Perhaps there needs to be a pull-down resistor to GND, but it might work as you suggest, that is the detection fails (as the GPIB board is not present) and the interrupt is masked off.
I just tried the procedure on page 28 bearing in mind the comment by "Tom" posted by Daveolla. The scope powers up without the fan running but I do get a trace, however, only CH2 seems to be displaying regardless of the input switch position so there seems to be no chopping. CH1 does not appear and its position control does not seem to do anything. Only CH2 works. On the other hand this does at least demonstrate that the failure of the digital mode is preventing a trace from running in normal operation and that in principle at least the analogue side does work. It is not entirely unexpected that there may be problems to resolve once the digital side is fixed. I have restored it back to normal operation for now.
Thank you for getting the ROMs in the post. I should be able to sort out some adapters.
Really appreciate the comments from everyone as well.
