Star Tech Journal

October 1 995
STAR*TECH Journal
......................................... .... ......................
CID
THE REruRN OF THE
NOP
(''NO-OP'')
HAPP CONTROLS
MANUFACTURER OF ELECTRONIC CONTROLS
TROUBLESHOOTER
TEAM SEGA
Ted K.ilpin
Eric Winston
Joe Blackwell
Jay Alfer (artwork)
Sega Pinball
Melrose Park, Illinois
momentary contact
pushbutton switches
• Standard and custom printing
• Numerous colors
• Protective front cover boots
SUBJECT
A handy little CPU troubleshooting gimmick that
has been used many times to repair those bow-wow
boards.
The "NO-OP" (hereafter NOP) socket isolates the
data pins of a given CPU and forces the NOP in-
struction onto them.
When a microprocessor comes out of reset, it looks
for start-up instructions at a specific ROM address
(the same address every time it comes out of re-
set). If the micro "sees" the NOP code, (instead of
what the board or game or whatever has been pro-
grammed at that location), the micro simply does
nothing for that instruction cycle, then MOVES ON
to the NEXTlocation on the address bus.
Again, the micro sees the NOP code since this code
comes from the Diagnostic socket and not the
board, and it MOVES ON to the next address,
and so on and so forth.
The net effect of all this is a continuously cycling
address bus (in an orderly pattern) as the micro
repeatedly tries EVERY ADDRESS looking for an
instruction.
This means that all CHIP selects and other
address-bus derived signals regularly become ac-
tive, and are now VERY EASY to check with a
'scope.
• Durable Joysticks
designed to fit
any application
• Digital & Analog Output
SPECIALTY CONTR
• Trackballs,
Coin Doors,
Beacon Lights,
Steering Wheels, Power
Supplies, Pedals, Meters, Shifters
HAPP CONTROLS, inc.
(708) 593-6130
(800) 593-HAPP (4277)
(708) 593-6137
Phone:
Toll-Free Fax within USA:
Fax:
106 Garlisch Drive • Elk Grove, IL 60007
contact us via our e-mail address: info@happmis.com
QUALITY • SERVICE • VALUE • INNOVATION