Programming instructions

97
LESSON 14 – Diagnostic Devices
No programmer is perfect, and no PLC is going to last forever. Fortunately, the FX-
series PLC line has a number of dedicated relays and registers that store information,
including error codes about the operation of the PLC.
Lesson Objectives: At the conclusion of this lesson, you will be able to…
9 Identify those registers and relays that assist in troubleshooting.
9 Interpret the information shown by the registers and relays.
9 Write small programs that can provide diagnostic functions
9 Use GX-Developer diagnostics to troubleshoot errors
Materials: FX-Series PLC Training Manual
FX-series Demo Kit
14.1 Special M Relays
The addresses M8000 and above are reserved for system use. In the
FX1S/1N/2N/2NC, the range is M8000-M8255. In the FX3U, the range is
M8000-M8511. These bits have various system functions. The complete list of
special M contacts is found in the FX3U Programming Manual in Chapter 36. It
can also be found in the GX-Developer software in the help menu.
These relays can be useful in your ladder program and troubleshooting. Some
of the more commonly used contacts are …
M8004 Indicates that an error has occurred
M8006 Low battery
M8064 ~ M8068 Latch on if various types of errors occur
M8064 – Parameter error
M8065 – Syntax error
M8066 – Program error
M8067, 68 – Operation error – typical example is dividing by 0.
These M806# bits have a companion D806# register which hold the error
number. These will be covered in the next section.
The following relays are not diagnostic relays per se, but are very useful in
writing programs:
M8000 Always on
M8001 Always off
M8002 On only for the first scan
M8003 Off only for the first scan
M8011 10ms clock pulse
M8012 100ms clock pulse
M8013 1 second clock pulse
M8014 1 minute clock pulse