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FX3G/FX3U/FX3UC Series Programmable Controllers
Programming Manual - Basic & Applied Instruction Edition
35 SFC Program and Step Ladder
35.1 SFC Program
35.1.4 Handling and role of initial state relay
Handling of the initial state relay
• 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.
Role of the initial state relay
1. Used as an identification device for inverse conversion
• 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.
2. Used to prevent double start
• 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