1802 problems

[Tony Duell]

Does anyone really know the RCA 1802 processor hardware?

I am still debugging the microwriter, and have not got very far. I have 
an RCA user manual on the processor which seems quite comprehensive, I am 
asusming it's accurate.

Now, according to that manual, the processor can be in one of 4 states 
(fetch, execute, interrupt responze, DMA), and which state it's in is 
output on 2 lines called SC0 and SC1. 

The INT/ line is high, both DMA IN/ and DMA OUT/ are also high. I can 
detect no gltiches on any of these signals. The clock is running, the bus 
looks active, etc.

The problem is that I am getting active-high pulses on SC1. This, 
according to the data sheet means it's doing a DMA or interrupt response 
cycle. But why the heck should it?

I thought it was a defective 1802 chip, but another one pulled from an 
old board shows identical behaviour (right down to locking up after 
typing 8 characters on the microwriter keyboard). I can't believe 2 have 
failed in exactly the same way. Unless of course this is a well-known 
problem.

Any thoughts?

-tony