 Intercative mode of work；function available on the main program for didactic purposes,the use gives the signal to execute the step.

 LCD display,which shows the status of the system,values of inputs,outputs,timer and statistics of the sequence execution.

 Beep to give important alerts,stop ,start and emergency.

 Led to show power on and others to show the state of iputs and outputs.

4.2. security 

To make the final application works property ,a correct configuration to execute the steps in the right way is needed,but more then that it must offer solutions in case of bad functioning or problems in the execution of the sequence.The controller offers the possibility to configure two internalvirtual circuits that work in parallel to the principal.These two circuits can be used as emergency or reset buttons and can return the system to a certain state at any time.There are two inputs that work with interruption to get an immediate access to these functions.It is possible to configure the position,the buttons and the value of time-out of the system.

4.3. User interface

The sequence of strings can be programmed using the interface elements os the controller.A computer interface can be used to generate the user program easily.With a good documentation the final user can use the interface to configure the strings of bytes that define the steps of the sequence.But it is possible to create a program with visual resources that works as a translator to the user, it changes his work to the values that the controller understands.

To implement the communication between the computer interface and the controller a simple protocol with check sum and number of bytes is the minimum requirementas to guarantee the integrity of the data.

4.4.Firmware

The main loop works by reading the strings of the steps from the EEPROM memory that has all the information about the steps.In each step,the status of the system is saved on the memory and it is shown on the display too.Depending of the user configuration,it can use the interruption to work with the emergency circuit or time-out to keep the system safety .A block diagram of micro controller main program is presented.

5.Example of electro-pneumatic system

    The system is not a representation of a specific machine,but it is made with some common novements and components found in a real one.The system is composed of four actuators.The actuators A,B and C are double acting and D-single acting.Actuator A advances and stays in specified position till the end of the cycle,it could work fixing an object to the next action for example,it is the first step.When a reaches the end position,actuator C starts his work together with B,making as many cycles as possible during the advancing of B.It depends on how fast actuator B is advancing;the speed is regulated by a flowing control valve.It was the second step.B and C are examples of actuators working together,while B pushes an object slowly,C repeats ins work for some time 

When B reaches the final position,C stops immediately its cycle and comes back to the initial position.The actuator D is a single acting one with spring return and works together with the back of C,it is the third  step.D works making very fast forward and backward movement,just one time.Its backward movement is the fourth step.D could be a tool to make a hole on the object.

When Dreaches the initial position,A and B return too,it is the fifth step.Fig.6 show the first part of the designing process where all the movements of each step should be defined[2]. (A+)means that the actuator A moves to the advanced position and (A?) to the initial position. The movements that happen at the same time are joined 

together in the same step. Th system has five steps.These two representations of the system. 电气系统的可编程序控制器英文文献和中文翻译(3):http://www.youerw.com/fanyi/lunwen_54816.html