
7. TROUBLESHOOTING
B–63505EN/02
288
920: Watch dog alarm or RAM parity in servo control circuit
has occurred in the 1st or 2nd axis
921: Above alarm has occurred in the 3rd or 4th axis
The timer used to monitor the operation of CPU is called the watch dog
timer. The CPU resets timer time every time a constant time has passed.
When an error occurs in CPU or peripheral device, timer is not reset but
the alarm is informed.
RAM parity error was detected in the servo control module.
(1)Main board is faulty
The main board includes RAM for servo of 1st/2nd axis, watch dog
timer circuit, etc.
Defectiveness of hardware, abnormality on malfunctioning of
defection circuit or the like is considered.
⇒ Change the main board.
NOTE
When the main board is replaced, all the data stored in
memory is lost. Refer to data input/ output item and set NC
data again.
(2)Servo interface module is faulty
The servo module includes RAM for servo of 3rd /4th axis, watch dog
timer circuit, etc.
Defectivenss of hardware, abnormality or malfunctioning of detection
circuit or the like is considered.
⇒ Change the servo module.
NOTE
See Subsec. 2.2.1 for the each specifications and locations
of modules.
(3)Malfunction by noise
The watch dog timer circuit or CPU may malfunction due to noise
intruded in the control unit.
⇒ A noise to the power supply or an inductive noise to a signal line
may be considered. Check operation of the other machine that is
connected to the same power line, relays and compressers which
may cause noise, then perform measures to supress noises. (See
Sec. 2.13).
7.31
ALARM 920 AND 921
(WATCH DOG OR
RAM PARITY)
points
D Watch dog timer alarm
D RAM parity error
Causes and Remedies