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Commodore 3016 scrambled screen

Hello All,

I am back again with new 3016 project ;)
CPU is OK. (Did NOP test)
replaced 2114 ic's <-- one of them was defect.
I have checked all the 4116 ic's and all seems ok. Tested with RetroChipTester.
Where should i look first now?
Pettester now give this output:
 

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The 4116 DRAM in the lower bank is returning a hexadecimal value of FE. This is wrong.

When you performed the NOP test, did you look at the activity on the DRAM control signals and address decoding?

Dave
 
Sorry, travelling today...

There is no point in retesting the DRAMs if you have already tested them.

I assume (now) they are in sockets?

If it is simple to retest them, who knows.

Check the /RAS and /CAS pins of each DRAM.

You should have /RAS and /CAS action on all DRAM devices.

The R/W pin should always be HIGH (reading).

You should have a 'block' of /RAS and /CAS action on the lower 16K followed by a block of /RAS and /CAS action on the upper block of 16K.

However you should always see action on the /RAS line as the refresh operates.

When the DRAM is being accessed, you should observe one memory access followed by one refresh cycle. When the DRAM is not being accessed, you should only observe a refresh cycle (/RAS only).

Dave
 
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