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

[David Shoemaker]

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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