Diagnostic Devices 6
6-1
6. Diagnostic Devices
The followin
special devices are used b
the PLC to hi
hli
ht the current operational status
and identif
an
faults or errors that ma
be occurrin
. There are some variations in the
application of these devices to members of the FX PLC famil
, these are noted where
appropriate.
The Internal dia
nostic devices consist of both auxiliar
(M) coils and data (D) re
isters.
Often there is a correlation between both M and D dia
nostic devices for example M8039
identifies that the PLC is in constant scan mode but D8039 contains the value or len
th of the
set constant scan.
Devices unable to be set by user:
An
device of t
pe M or D that is marked with a “( )” cannot be set b
a users pro
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
ister b
a user BUT the
re
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
the followin
s
mbol “( )”.
Symbol summary:
• not able to be set b
user
•
automaticall
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
