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APPX
Appendix 4  List of Special Relay Areas
Ethernet function
The following is the list of special relay areas relating to the Ethernet function.
*1 This is the memory used by the system when the CPU module is executing functions.
*2 Ethernet function initial processing involves updating parameters set for data communication to Ethernet-equipped modules to enable 
communication with external devices. Initial processing of Ethernet-equipped modules is performed by setting Ethernet parameters, 
writing them to the CPU module, and then powering off and on or resetting the CPU module. Note that if Ethernet parameters are not 
set, initial processing of Ethernet-equipped modules will be performed based on default parameters. (SM1524 (initial processing 
successful completion status) turns on the moment initial processing is complete, and communication with external devices becomes 
possible.)
CC-Link IE Field Network Basic function
The following is the list of special relay areas relating to the CC-Link IE Field Network Basic function.
*1 The CPU module where this function can be used supports these special relay areas.
No. Name Data stored Details Set by (setting timing) CPU
SM1520 IP address storage area 
write request
OffOn:Write request exists
OnOff:Writing completed
• When this relay is changed from off to on, 
IP address settings stored in SD1520 to 
SD1525 are written to the IP address 
storage area (system memory
*1
) of the 
CPU module.
• Once write to the IP address storage area 
(system memory
*1
) is completed 
(regardless of whether it was successful 
or not), this relay is turned off.
S (Status change)/U ALL
SM1521 IP address storage area 
write error
Off: No error
On: Error
This relay is turned to on when write to the 
IP address storage area (system memory
*1
) 
failed, and off when it was successful.
S (Status change) ALL
SM1522 IP address storage area 
clear request
OffOn:Clear request exists
OnOff:Storage area 
cleared
When this relay is changed from off to on, 
the IP address storage area (system 
memory
*1
) is cleared.
Once clear of the IP address storage area 
(system memory
*1
) is completed 
(regardless of whether it was successful or 
not), this relay is turned off.
S (Status change)/U ALL
SM1523 IP address storage area 
clear request
Off: No error
On: Error
This relay is turned to on when clear of the 
IP address storage area (system memory
*1
) 
failed, and off when it was successful.
S (Status change) ALL
SM1524 Initial processing successful 
completion state
Off: Completed with an 
error
On: Completed normally
This relay is turned on when the initial 
processing for the Ethernet function was 
completed successfully. 
*2
When the initial 
processing was completed with an error, 
SM1525 is turned on and SM1524 stays off.
S (Status change) ALL
SM1525 Initial processing error 
completion state
Off: Completed normally
On: Completed with an 
error
This relay is turned on when the initial 
processing for the Ethernet function was 
completed with an error. 
*2
When the initial 
processing was completed successfully, 
SM1524 is turned on and SM1525 stays off.
S (Status change) ALL
No. Name Data stored Details Set by (setting timing) CPU
SM1536 Cyclic transmission status Off: Not performed
On: Being performed
• This relay turns on when the cyclic 
transmission starts.
• This relay is turned off when the cyclic 
transmission stops.
S (Status change) Rn
*1
SM1540 Data link status Off: All stations normal
On: One or more faulty 
stations
• This relay turns on when an error exists 
even in one slave station.
• The status of each slave station can be 
checked in SD1540.
S (Status change) Rn
*1