Mitsubishi L02CPU

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5.3.2 Special relay (SM)
The special relay (SM) is an internal relay of which details are specified inside the CPU module, and the CPU module
status data are stored in this special relay.
MELSEC-L CPU Module User's Manual (Hardware Design, Maintenance and Inspection)
5.3.3 Special register (SD)
The special register (SD) is an internal register of which details are specified inside the CPU module, and the CPU
module status data (such as failure diagnostics or system information) are stored in this special register.
MELSEC-L CPU Module User's Manual (Hardware Design, Maintenance and Inspection)