Re: Strange 8255 behavior

From: Gerrit Heitsch <*gerrit_at_laosinh.s.bawue.de*> Date: Sun, 10 Jun 2018 14:33:41 +0200 Message-ID: <19c85f38-d630-ca47-cbcd-82cf1c49ac19@laosinh.s.bawue.de> · gerrit_at_laosinh.s.bawue.de

On 06/10/2018 01:59 PM, Michał Pleban wrote:
> Hello!
> 
> smf wrote:
> 
>> I assume for cost purposes they don't keep the last output value when in
>> read mode, so when you switch back to output mode then zeros is probably
>> the lesser of two evils.
> 
> That might be the case when you switch from input to output, but when
> you "switch" from output to output (i.e. you leave the direction of the
> port unchanged, only modifying some other port) it's just ridiculous.
> 
>> Trying to replace a mos chip with an intel chip is blasphemy :-)
> 
> Guilty as charged :-) But it's for a good reason: there are only very
> little 6525's laying around, and if we want to make the 8088 card
> accessible to everyone, we had to change to a chip that's readily available.

How about two 6522 instead? With some clever logic and mapping of the 
address bits, you might be able to come up with a register map that 
comes close to what the 6525 has, limiting the amount of change in the code.

  Gerrit