Re: OT: UM6502A can it be different in any way?

On 9/12/21 9:27 AM, Francesco Messineo wrote:
> 
> 
> On Sat, Sep 11, 2021 at 3:47 PM smf <smf_at_null.net <mailto:smf@null.net>> 
> wrote:
> 
>     I know people through ukvac.com <http://ukvac.com> who repair arcade
>     boards, for asteroids
>     the person who springs to mind is andy_at_andysarcade.net
>     <mailto:andy_at_andysarcade.net>
> 
> 
> I've also subscribed there. I have repaired quite a few Arcade boards 
> myself and also I've written some diagnostic code for the Asteroids 
> boards too,
> when I was suspecting my problems were due to some subtle vector RAM error.
> I've have now restored three different Asteroids board.
> 
> 
> 
>     However I don't have any idea whether you'll get a better answer than to
>     find a 6502A that works without glitches.
> 
> 
> I've put the UM6502A and the suspect UM (chinese sanded and rebadged 
> chip) on other Asteroids board and the "problem" follows these two CPU.
> R/W and phi2 signals just go directly to a 74LS244 input, so the CPU 
> doesn't need to provide fanout for all the inputs.
> My board Vcc is 4.9V at the 6502 pins, so it shouldn't be an issue.
> With the oscilloscope I've seen the rising edge of phi2 is taking a bit 
> more than 10nS. The UM6502 have the slowest rising edge of all CPUs I could
> test. Of course the 65C02 has a nearly vertical rising edge (and works 
> fine on Asteroids anyway).
> MOS/SY/R 6502 have a better phi2 rising edge than the UM, but the 
> differences are marginal, maybe 2-5ns as far as my scope can tell. Also 
> the "1" level
> on the MOS/SY/R are around 4.5V, while the UM goes to 4V (which 
> shouldn't matter on TTL logic anyway).

Have you tried to add a 3.3kohm or 4.7kohm pullup to PHI2 and maybe R/W 
on the UM6502A? That would make the signal rise faster and get you a 
higher HIGH.

  Gerrit