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

[Martin White]

I've had this on a Missile Command before. There's a LOT of various things that all hit on the data bus. If the wrong thing's getting enabled at the same time as you're trying to do a ROM read or check then you can see trackball data and allsorts in the results.

Pain as it was to do, the way I got around it was to snip the enable line of every single buffer that was directly connected to the bus until the phantom data went away, at which point I'd found the source of the problem.

Not desperately scientific, but it got the job done.

Personally I find Missile Command's to be an absolute nuisance!!

Martin.

On 6 Nov 2011, at 23:22, David Shoemaker wrote:


> There is something definitely going on odd with the data bus.

> 

> I pulled all the roms and verified the pull-up resistors were good.

> 

> I used the fluke to read address x7000 which should read FF as there is no

> rom there.  

> 

> I get a 00 read.  If I hit repeat I get something else (like 3F), if I then

> hit loop it shows me some changing values then settles on FF.

> 

> So there is something somewhere hitting the data bus at the same time.  But

> using the probe on Address sync I am not finding any LS244's that are

> selecting at the same time.  Going to go through them again on data sync.

> 

> I may actually need to pull the logic analyzer out for this.

> 

> Any suggestions on debugging this would be welcomed of course :)

> 

> Thanks,

> David

> 

> -----Original Message-----

> From: techtoolslist-bounces at flippers.com

> [mailto:techtoolslist-bounces at flippers.com] On Behalf Of Andrew Welburn

> Sent: Sunday, November 06, 2011 12:59 AM

> To: Technical Tools Mail List

> Subject: Re: [Techtoolslist] 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 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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