Re: Logic levels question 6502A (Missile command)

Bus contention. Look at the chip selects simultaneously on those roms, they may be being selected at the same time if they work in-circuit fine on their own. The first rom is always selected, and is only deselected when another rom is looked at, this is normal operation.

Andrew Welburn (mobile)
www.andys-arcade.com

On 6 Nov 2011, at 02:00, "David Shoemaker" <davids@oz.net> wrote:

> What really got me is I pulled all the roms, and put only the "bad" rom in.
> It passed the rom test on loop for 30 minutes no problem. I put the rom
> next to it back in and BANG right back to failures. But that second rom
> passes just fine.
>
> That is when I noticed the lowish address line values.
>
> -----Original Message-----
> From: techtoolslist-bounces@flippers.com
> [mailto:techtoolslist-bounces@flippers.com] On Behalf Of Rodger Boots
> Sent: Saturday, November 05, 2011 6:21 PM
> To: Technical Tools Mail List
> Subject: Re: [Techtoolslist] Logic levels question 6502A (Missile command)
>
> If you have it to one ROM it's probably the chip select line to that one
> part. And the signature changing would tend to indicate a timing problem
> such as a slow ROM or sloppy logic levels on that one line.
> On Nov 5, 2011 7:57 PM, "David Shoemaker" <davids@oz.net> wrote:
>
>> Well sounds like that theory is shot.
>>
>> I pulled all the roms then using the fluke I did a read on one of the
>> addresses looping. The fluke reports a series of values that builds
>> up to FF (there are pullup resistors on all the data lines so I
>> expected the value to be FF and stay there.
>>
>> I will check the voltage at the pullups.
>>
>> The fluke reports all ram tests ok (so I figure the 244's are ok).
>> And only one rom reports bad. I checked it in my burner and it is
>> fine there. I burnt a new one anyhow and it reported the same
>> failure.
>>
>> One thing about the rom test failing is the signature is not constant.
>>
>> Any other suggestions?
>>
>> Thanks,
>> David
>>
>>
>> -----Original Message-----
>> From: techtoolslist-bounces@flippers.com
>> [mailto:techtoolslist-bounces@flippers.com] On Behalf Of John
>> Robertson
>> Sent: Saturday, November 05, 2011 2:20 PM
>> To: Technical Tools Mail List
>> Subject: Re: [Techtoolslist] Logic levels question 6502A (Missile
>> command)
>>
>> David Shoemaker wrote:
>>> I am trying to trouble shoot a board. I am getting some odd
>>> responses reading the roms. I noticed that the address lines coming
>>> from the 6502 seem to be going high at 3.5 - 3.7 vdc. I suspect
>>> this is causing some
>> of
>>> the ttl to not latch high.
>>>
>>>
>>>
>>> This is checked through a meter using the fluke to read a particular
>> address
>>> on a loop and using my scope for the same and while running the game.
>>>
>>>
>>>
>>> Is this a normal value? I don't see any pull-ups on the address
>>> lines on the schematic.
>>>
>>>
>>>
>>> David
>>>
>>
>> Don't forget the Fluke pod has it's own address and data line buffering.
>> There are notes about watching out for that in the pod manuals...
>>
>> Input Low Voltage ..... 0.8V max.
>> Input High Voltage...... 2.0V min., +5.0V max.
>> Output High Voltage ... 2.4Vmin. with loh = -250ua
>>
>> etc. (Page 1-4, 6502 Instruction manual)
>>
>> John :-#)#
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Received on Sun Nov 6 03:21:44 2011