Re: PET 2001/2001N can "write" to ROMs

On 4/23/19 10:24 AM, Francesco Messineo wrote:
> On Tue, Apr 23, 2019 at 6:23 AM Gerrit Heitsch
> <gerrit_at_laosinh.s.bawue.de> wrote:
>>
>> On 4/22/19 10:14 PM, Francesco Messineo wrote:
>>> Hi all,
>>> Ok, the subject is a pun, but write is in double quotes anyway. I was
>>> wondering why the PET (at least the 2001 and 2001N, the ones I have),
>>> leave the ROMs selected during a write cycle on ROM space. This of
>>> course makes a data bus contention because the selected ROM will drive
>>> the data bus and the 6502 will drive the data bus too since it's on a
>>> write cycle.
>>> I wonder if this can be a factor in the high failure rate of the old
>>> 6540s and the newer 2332/2316.
>>
>> No, NMOS outputs cannot be damaged if they work against each other.
> 
> Ok, what about a 27Cxxx EPROM or 28Cxx EEPROM against the 6502 data
> bus? 

There it depends on the direction... If the 6502 pulls HIGH and the 
EPROM LOW nothing bad will happen. The other way round might not be a 
good idea, but the question is whether those very short shorts (less 
than 1 microsecond) do damage to the drivers.

  Gerrit