[Techtoolslist] Logic levels question 6502A (Missile command)

[Andrew Welburn]

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 at 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 at flippers.com

> [mailto:techtoolslist-bounces at 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 at 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 at flippers.com

>> [mailto:techtoolslist-bounces at 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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