Re: high level on different 6502

From: Francesco Messineo <>
Date: Sat, 5 Nov 2016 22:01:31 +0100
Message-ID: <>
On Sat, Nov 5, 2016 at 9:56 PM, didier derny <> wrote:
> Hi
> the 65c02 high level was near VCC
> I had 2 Synertek 6502 both were at 2.5v for the high level...
> 2 differents machines   SYM1 and  CBM8032, same results...
>      looks like the problem is linked to the synertek

the High voltage at 2.5V isn't  a problem in itself (if it keeps the
level when loaded). TTL logic real "bad" noise margin is on the low
level (0.5V max output vs 0.8V max input).
So in theory 2.5V High should work as it has a still better noise
margin than the correct Low levels. Look also at the actual Low level
voltages on these parts however.
Then, one might wonder if Synertek parts were always marginal or they
didn't age well, but I can't surely tell :)

> On 05/11/2016 21:50, Francesco Messineo wrote:
>> On Sat, Nov 5, 2016 at 9:37 PM, didier derny <> wrote:
>>> while checking some signals on the 6502 of my 8032 motherboard
>>> I found the 'high' level rather low 2.5v  (I expected around 4v...)
>>> so I took my SYM1 board and tested several 6502
>>> MOS         6502       around  4v
>>> Rockwell  6502       around  4v
>>> Rockwell  65c02     around  5v
>>> Synertek  6502       around  2.5v
>>>                   CM630   around  4v      (bulgarian clone)
>>> I tried several 6502 of each type...
>> 2.5V is really a bit outside the acceptable output High level for TTL
>> logic.
>> 2.7V is the official minimum level for the High state. However, TTL
>> inputs should accept as High any voltage => 2V.
>> If the current capability of the outputs isn't degraded (i.e. if 2.5V
>> is maintained at all the inputs when loaded at the max fan-out), I'd
>> say it should still work.
>> CMOS outputs, on the other hand, should go to the power rails, if they
>> aren't broken.
>> Do you have many Synertek and they all have the same output levels?
>> Frank
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Received on 2016-11-05 22:00:02

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