Design Precautions
Install a safety circuit external to the PLC that keeps the entire system
safe even when there are problems with the external power supply o
r
the PLC module. Otherwise, serious trouble could result from
erroneous output or erroneous operation.
- Outside the PLC, construct mechanical damage preventing interlock
circuits such as emergency stop, protective circuits, positioning uppe
r
and lower limits switches and interlocking forward/reverse operation.
When the PLC detects the following problems, it will stop calculation and
turn off all output in the case of watchdog timer error, module interface
error, or other hardware errors.
However, one or more outputs could be turned on when there are
problems that the PLC CPU cannot detect, such as malfunction of output
device (relay, transistor, etc.) itself or I/O controller. Build a fail safe
circuit exterior to the PLC that will make sure the equipment operates
safely at such times. Also, build an external monitoring circuit that will
monitor any single outputs that could cause serious trouble.
Make sure all external load connected to output does NOT exceed the
rating of output module.
Overcurrent exceeding the rating of output module could cause fire, damage
or erroneous operation.
Build a circuit that turns on the external power supply when the PLC
main module power is turned on.
If the external power supply is turned on first, it could result in erroneous
output or erroneous operation.
Warnin
g