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

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Series Programmable Controllers

Programming Manual - Basic & Applied Instruction Edition

34.1.4 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, and state relays S0 to S9 are

• 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 measures 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 instantaneously 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 in this way is called 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 STL instruction for the initial state relay before STL instructions for the subsequment state relays.

Program RET instruction at the end.

By this programming, if there are two or more independent flows, they are 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, such pressing is

invalid (because S0 does not turn ON).

As a result, double start is prevented.

Initial pulse

Ladder block

The initial state relay is

Initial state relay

34 SFC Program and Step Ladder

34.1 SFC Program

Fx3uc series Melsec-f