5.2.1 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 with the device identifier "SM".
These relays contain information on system status or can be used to influence program execu
-
tion. The following table shows a few examples of the many special relays available.
NOTE For an overview of all special relays please refer to the Programming Manual for the A/Q se
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ries and the MELSEC System Q, art. no. 87431.
MELSEC System Q Beginners Manual 5 – 5
Devices in Detail Relays
Special
relay
Function
Program processing
options
SM0 PLC error
Poll signal state
SM51 Low battery voltage
SM400 When the PLC is in RUN mode this relay is always set to “1”.
SM401 When the PLC is in Run mode this relay is always set to “0”.
SM402
Initialisation pulse (following activation of RUN mode this relay is set
to “1” for the duration of one program cycle.)
SM411 Clock signal pulse: 0.2 second (0.1 s ON, 0.1s OFF)
SM412 Clock signal pulse: 1 second (0.5 s ON, 0.5 s OFF)
SM413 Clock signal pulse: 2 second (1 s ON, 1 s OFF)
SM414 Variable clock signal pulse
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