Step Ladder; Outline; Explanation Of Function And Operation - Mitsubishi Electric MELSEC FX3U Programming Manual

FX /FX /FX Series Programmable Controllers
3G 3U 3UC
Programming Manual - Basic & Applied Instruction Edition
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 each other if they are programmed in the same
rules respectively.
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
condition and output control are programmed in a state relay.
Because the preceding process is not performed any more 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 instruction to drive a coil, and use STL instruction for a contact.
• When a state relay turns ON, a connected circuit (internal circuit) is activated by way of an STL contact.
When a state relay turns OFF, a connected internal circuit is deactivated by way of an STL contact.
After one operation cycle, non-driving of an instruction (jump status) is not available.
• 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 so far turns OFF (transfer operation).
In the state relay ON status transfer process, the both state relays are ON only instantaneously (during one
operation cycle).
In the next operation cycle after the ON status was transferred, the former state is reset to OFF.
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.
• One state relay number can be used only once.
X001
X002
*1. Output coils can be used again in different state relays.
834
STL S31
*1
Y030
Process of
S31
SET Y31
When X001 turns ON,
S32 turns ON and S31
is automatically reset.
SET S32
STL S32
*1
Y030
Process of
S32
Y032
SET S33
35 SFC Program and Step Ladder
STL S31
Y030
SET Y31
X001
SET S32
STL S32
Y030
Y032
X002
SET S33
35.2 Step Ladder
Y31 programmed in
SET instruction remains
ON even if S31 is reset.
Activated