User manual

72
Safety Measures
Precautions regarding system design
In certain applications, malfunction may occur for the following
reasons:
• The timing difference between opening and closing of the PLC
power supply, the I/O modules and power equipment
• An operation time lag when a momentary power failure occurs
• Abnormality in the PLC, external power supply, or other devices
In order to prevent a malfunction resulting in system shutdown choose
the adequate safety measures listed in the following:
Interlock circuits on the outside of PLC
When a motor clockwise/counter-clockwise operation is controlled,
provide an interlock circuit that prevents clockwise and counter-
clockwise signals from inputting into the motor at the same time.
Emergency stop circuits on the outside of PLC
Install the emergency stop circuits outside the PLC to stop the power
supply of the output device.
Start PLC after other devices (Start up sequence)
The PLC should be operated after all of the I/O devices and the power
equipments are energized.
• Switch to RUN mode after the start of PLC.
• Use a timer circuit to delay the start of PLC.
Watchdog timer
The watchdog timer is a program error and hardware error detection
timer. It goes On when the scan time exceeds 640 ms.
When the watchdog timer is activated, at the same time the ALARM
LED lights, the ALARM contacts on the power supply module turn to
On, all output modules are turned Off and the module is put in halted
state. (The system is in a non-processing state that includes
communications with programming tools as well.)