Mitsubishi Electric FX2N-32CAN, Training Manual

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A Appendix
A.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 start-
ing with M8000. Some contain information on system status and others can be used to influence
program execution. Special relays cannot be used like other internal relays in a sequence pro-
gram. However, some of them can be set ON or OFF in order to control the CPU. Represented
here are some of the most commonly used devices.
Special relays can be divided in two groups:
– Special relays whose signal state can only be read by the program (for instance using a LD
or LDI instruction).
– Special relays whose signal state can be read and written (set or reset) by the program.
The following tables feature a "Read" and a "Write" column. If the symbol "
쎲
" is shown in one of
these columns, the corresponding action is possible. The symbol "—" means that the corre-
sponding action is not allowed.
There are also special registers for word information in a FX CPU. They are descriped in the next
section.
Appendix Special Relays
Training Manual GX IEC Developer A - 1