Handling And Role Of Initial State Relay - Mitsubishi Electric MELSEC FX3U Programming Manual

FX /FX /FX Series Programmable Controllers
3G 3U 3UC
Programming Manual - Basic & Applied Instruction Edition

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.
Ladder block
SFC block
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.
Initial pulse
M8002
SET S0
Initial state relay
0
X000
0
Y023
20
X001
1
X003
4
Y021
24
X002
5
S0
RET END
35 SFC Program and Step Ladder
The initial state relay is
driven.
TRAN
Y021
TRAN
TRAN
Y023
TRAN
35.1 SFC Program
31
32
33
34
35
36
37
38
A
B
807