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FX3G/FX3U/FX3UC Series Programmable Controllers
Programming Manual - Basic & Applied Instruction Edition
35 SFC Program and Step Ladder
35.2 Step Ladder
35.2 Step Ladder
35.2.1 Outline
In programs using step ladder instructions, a state relay State S is assigned to each process based on machine
operations, and sequences of input condition and output control are programmed as circuits connected to contacts
(STL contacts) of state relays in the same way as SFC programs.
The concept of program creation and the types and operations of state relays are the same as for SFC programs.
However, because the contents can be expressed in the ladder format, step ladder programs can be handled as
familiar relay ladder charts even though the actual contents are the same as those of SFC programs.
In step ladder programs, the list format is also available.
SFC programs and step ladder programs can be converted inter changeably if they are programmed according to the
same rules.
This section explains the expressions and cautions of step ladder programs in comparison with SFC programs, and
the input order in the list format.
35.2.2 Explanation of function and operation
In a step ladder program, a state relay State S is regarded as one control process, and a sequence of input conditions
and output controls are programmed in a state relay.
Because the preceding process is stopped when the program execution proceeds to the next process, a machine can
be controlled using simple sequences for each process.
Operation of step ladder instructions
In a step ladder program, each process performed by the machine is expressed by a state relay.
A state relay consists of a drive coil and contact (STL contact) in the same way as other relays.
Use SET or OUT instructions to drive a coil, and use STL instructions for a contact.
Operations of internal circuits connected to state relays are classified into three types, execution in the
contact ON status, execution in the contact OFF status (for one operation cycle) and non-execution.
• Execution in the contact ON status:
When a state relay turns ON, a connected circuit (internal circuit) is activated with a STL contact.
• Execution in the contact OFF status (for one operation cycle):
When a condition (transfer condition) provided between state relays is satisfied, the next state relay turns ON, and
the state relay which has been ON before hand turns OFF (transfer operation). In the state relay ON status transfer
process, both state relays are ON only momentarily (for one operation cycle). In the next operation cycle after the
ON status is transferred to the next state relay, the former state relay is reset to OFF. A drive instruction connected
to the bus line of the reset state relay is executed in the contact OFF status in one operation cycle regardless of the
actual contact status before the drive instruction. When the transfer state relay S is used in a contact instruction,
however, the contact image is executed in the OFF status immediately after the transfer condition is satisfied.
• Non-execution:
An instruction is not executed in the contact OFF status after the operation cycle where the instruction was
executed in the contact OFF status (jump status).
• The figure below shows the timing chart of the state relay (internal circuit) activation status.
• One state relay number can only be used once.
Execution in the contact OFF status
in one operation cycle
Execution in the
contact ON status
Non-execution
(jump status)
Execution