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Diagnostic Devices 6
6-1
6. Diagnostic Devices
The followin
g
special devices are used b
y
the PLC to hi
g
hli
g
ht the current operational status
and identif
y
an
y
faults or errors that ma
y
be occurrin
g
. There are some variations in the
application of these devices to members of the FX PLC famil
y
, these are noted where
appropriate.
The Internal dia
g
nostic devices consist of both auxiliar
y
(M) coils and data (D) re
g
isters.
Often there is a correlation between both M and D dia
g
nostic devices for example M8039
identifies that the PLC is in constant scan mode but D8039 contains the value or len
g
th of the
set constant scan.
Devices unable to be set by user:
An
y
device of t
y
pe M or D that is marked with a “( )” cannot be set b
y
a users pro
g
ram. In the
case of M devices this means the associated coil cannot be driven BUT all contacts can be
read. And for data devices (D) new values cannot be written to the re
g
ister b
y
a user BUT the
re
g
ister contents can be used in a data comparison.
Default Resetting Devices:
• Certain devices reset to their default status when the PLC is turned from OFF to ON.
These are identified b
y
the followin
g
s
y
mbol “( )”.
Symbol summary:
• not able to be set b
y
user
•
automaticall
y
reset to default at power ON.
•
R Also reset to default when CPU is switched to RUN.
•
S Also reset to default when CPU is switched to STOP.
FX Series Programmable Controller