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

[Rodger Boots]

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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