Mitsubishi Electric MELSEC Q Series User Manual page 219

(2) Reserving empty slot
When an empty slot is reserved for the purpose of mounting additional CPU modules in the future, set "PLC
(Empty)" on the "I/O assignment" tab screen in the "PLC parameter" dialog box.
For example, when setting "4" as "No. of CPUs" in use of High Performace model QCPU and reserving one of
them for future use, set "CPU (Empty)" to slot 3.
In the mounted CPU module of CPU No.1, errors occur caused by the following error factors (1) or (2).
(1) When the number of mounted CPU modules exceeds the number set with the No. of CPU settings
(a)When CPU No.1 is Basic model QCPU/Universal model QCPU
"CPU LAY ERROR (error code: 7030)" occurs.
(b)When CPU No.1 is High Performance model QCPU or Process CPU
"PARAMETER ERROR (error code: 3010)" occurs.
(2) When CPU modules of which numbers are set in the No. of CPU setting are not mounted in the CPU module mounting
slots
(a)When CPU No.1 is Basic model QCPU/Universal model QCPU
"CPU LAY ERROR (error code: 7031)" occurs.
(b)When CPU No.1 is High Performance model QCPU or Process CPU
"PARAMETER ERROR (error code: 3010)" occurs.
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Figure 6.2 Empty slot setting screen