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Devices in Detail
4.2.1
Special relays
In addition to the relays that you can switch on and off with the PLC program there is also
another class of relays known as special or diagnostic relays. These relays use the address
range starting with M8000. Some contain information on system status and others can be used
to influence program execution. The following table shows a few examples of the many special
relays available.
Special
relay
M8000
M8001
M8002
M8004
M8005
M8013
M8031
M8034
FX Beginners Manual
Function
When the PLC is in RUN mode this relay is always set to "1".
When the PLC is in Run mode this relay is always set to "0".
Initialisation pulse (following activation of RUN mode this relay is set
to "1" for the duration of one program cycle.
PLC error
Low battery voltage
Clock signal pulse: 1 second
Clear all devices (except data registers D) that are not registered as
battery-latched.
Disable outputs – the PLC outputs remain off but program execution
continues.
Relays
Program processing
options
Poll signal state
Poll signal state
Set signal state.
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