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

On 9/6/21 8:57 PM, Francesco Messineo wrote:
> Hi all,
> I'm sorry for the off-topic, if anybody has a better idea on where to 
> post such a question, please suggest it!
> A long story short: I'm repairing a few Atari Asteroids arcade PCBs. 
> This game uses a 6502A clocked at slightly more than 1.5 MHz.
> Two of the boards I have, were populated with a SY6502A and worked fine.
> One board was populated with a R6502 (non-A) and anyway worked fine too. 
> However
> I thought I'd use the correct speed-grade CPU, so I put in it an 
> UM6502A. While the game never
> crashes, with this CPU the vectors are randomly plotted in the bad 
> position. For example, some of the rocks have a side "glitching" between 
> the right and a slightly offset position as it moves.
> Some of the letters drawn on the screen are also "shaking" in the same way.
> I've tried a "chinese remarked" 65C02 that I'm not sure what it was as 
> the old markings have been sanded away, but it's some sort of 65C02 
> because the NMOS unconnected pins are in this chip
> actually connected to something and the chip has protection diodes to 
> both supply rails (unlike NMOS 6502s). However the unknown 65C02 also 
> works fine.
> A genuine MOS6502 also works fine.
> Another "chinese remarked" chip shows the same glitching/shaking 
> behaviour on random vectors, this one is some kind of NMOS 6502 but also 
> this one has had every original marking sanded away. Maybe another UM6502?
> Since I have two working Asteroids board (one is -04 revision and the 
> other is -02), I tried also the same CPUs on the other board and they 
> produce similar
> glitches but with different amount of shacky vectors. Board -04 show 
> much more
> glitching vectors than the -02, however, all the other 6502s work fine 
> on both.
> Is there any known difference of the UM6502? Any different drive 
> capability on data/address maybe?

None that I know of. I have a 1541-II clone, the Oceanic OC-168 and the 
CPU used is an UM6502 (and two UM6522 for the VIAs). The drive runs with 
an original ROM from a 1541-II and behaves normally.

If it's a load or level issue, you should be able to see differences on 
a scope. If the game never crashes, then you should take a closer look 
at the circuit between CPU and the beam control. Maybe some IC there is 
a bit more sensitive than it should be. Like 74HCxxx instead of 74HCTxxx.

  Gerrit