Mitsubishi FX series PLC stepping instructions (STL/RET)

 FX series PLC basic logical instruction 20 or 27, step instructions 2, command more than 100 pieces (different series). FX2N this section, for example, described its basic logic instructions and step ladder instruction and its application.
FX series PLC stepping instructions
1. Step instructions (STL/RET)
Is designed for step instructions designed for sequential control instruction. Many control processes are available in the industrial control field controlled manner in order to achieve, using the step instruction sequence control is convenient and easy to read modify.
FX2N has two step instructions: STL (stepping contact instructions) and RET (stepping back).
STL and RET instructions only with the State s to get with step function. STL S200 for normally open contacts, called STL-contact, which in the ladder diagram symbols as it does not normally closed contact. We use for each State s record a step, STL S200 (ON), then enters the S200 represents a step (similar to the master switch), began implementation of this phase of the work of, and determine the conditions for entering the next meet. Once this signal is ON, then turn-off S200 to enter the next step, such as S201. Instruction is used to reset the STL RET instruction. After executing the RET will be back to the bus, exit the step.
2. State transition diagram
A sequence control process can be divided into several stages, also known as step or status, each State has a different action. When transitions between two neighboring States when conditions are met, it will be converted, that is performed by the previous state to the next State. We use state transition diagrams (the function diagram) describes this process of sequential control. As shown in Figure 1, each State s with a State record, x for the conditions. When X1 is ON, the system state S21 S20.

Each step in a State transition diagram contains three elements: the step-driven content, transfer conditions and instructions change objectives. S20 driving Y0 in Figure 1, when ON is X1, then the system state S21 S20, X1 conditions for conversion, convert to S21.
3. Step instructions instructions for use
1) STL-contact is normally open contacts connected to the bus bar to the left, an STL-contact switch on, the corresponding State is active;
2) and STL contact connected to the contacts application LD or LDI instruction, only executing RET return bus on the left side;
3) STL-contact can be driven directly or through other contact drive y, m, s, t, and other components of the coil;
4) due to the PLC only implementation step corresponds to the circuit, so the use of STL instructions allow double coil output (in different step sequencer can be driven with a coil several times);
5) STL-contact-driven circuit MC and MCR instruction cannot be used in a block, but can use the CJ instructions;
6) within the interrupt routine and subroutines, you cannot use the STL instructions.