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3UC

 Series Programmable Controllers

Programming Manual - Basic & Applied Instruction Edition

35 SFC Program and Step Ladder

• A state relay located at the head of an SFC program is the initial state relay. Only state relays S0 to S9 are 

available.

• The initial state relay is driven by way of another state relay (S24 in the example shown below).  But it is necessary 

to drive the initial state relay in advance by another measure at the start of operation.

• In the example shown below, the initial state relay is driven by the special auxiliary relay M8002 which turns ON and 

remains ON only momentarily when the PLC mode is changed from STOP to RUN.

• General state relays other than initial state relays should be driven by another state relay.  They cannot be driven 

by any other device.

• The state relay which may be driven by a contact other than the STL instruction is there by defined as the initial 

state relay, and should be described at the top of the flow.

• In inverse conversion from an instruction list into an SFC program, it is necessary to identify the top of the flow.  For 

this purpose, use the initial state relay S0 to S9.
If any other state relay number is used, inverse conversion is disabled.

• Program the STL instruction for the initial state relay before the STL instructions for subsequment state relays.  

Program  the RET instruction at the end.
By this programming method, if there are two or more independent flows, they the separated from each other.

• In the above example, even if the start button is pressed while the state relay S24 is ON, the command is invalid 

(S0 does not turn ON).
As a result, double start is prevented.

Y023

Y021

Ladder block

SFC block

M8002

SET

S0

The initial state relay is

driven.

0

20

0

1

4

24

5

S0

RET

END

X000

TRAN

X001

TRAN

Y021

Y023

X003

TRAN

X002

TRAN

Initial pulse

Initial state relay