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

[John Robertson]

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 :-#)#