【正文】 ine the condition to each change between the states. In the end of the design, the system is defined by a sequence that never chances and states that have the inputs and the outputs well defined. The inputs are the condition for the transition and the outputs are the result of the transition. All the configuration of those steps stays inside of the microcontroller and is executed the same way it was designed. The sequences of strings are programmed inside the controller with 5 bytes。 each string has the configuration of one step of the process. There are two bytes for the inputs, one byte for the outputs and two more for the other configurations and auxiliary functions of the step. After programming, this sequence of strings is saved inside of a nonvolatile memory of the microcontroller, so they can be read and executed. The controller task is not to work in the same way as a conventional PLC, but the purpose of it is to be an example of a versatile controller that is design for an specific area. A conventional PLC process the control of the system using a cycle where it makes an image of the inputs, execute all the conditions defined by the configuration programmed inside, and then update the state of the outputs. This controller works in a different way, where it read the configuration of the step, wait the condition of inputs to be satisfied, then update the state or the outputs and after that jump to the next step and start the process again. It can generate some limitations, as the fact that this controller cannot execute, inside the program, movements that must be repeated for some time, but this problem can be solved with some external logic ponents. Another limitation is that the controller cannot be applied on systems that have no sequence. These limitations are a characteristic of the system that must be analyzed for each application.4. Characteristics of the controller The controller is based on the MICROCHIP microcontroller PIC16F877 [6,7] with 40 pins, and it has all the resources needed for this project .It has enough pins for all the ponents, serial munication implemented in circuit, EEPROM memory to save all the configuration of the system and the sequence of steps. For the execution of the main program, it offers plete resources as timers and interruptions. The list of resources of the controller was created to explore all the capacity of the microcontroller to make it as plete as possible. During the step, the program chooses how to use the resources reading the configuration string of the step. This string has two bytes for digital inputs, one used as a mask and the other one used as a value expected. One byte is used to configure the outputs value. One bytes more is used for the internal timer , the analog input or timeout. The EEPROM memory inside is 256 bytes length that is enough to save the string of the steps, with this characteristic it is possible to save between 48 steps （Table 1）.The controller () has also a display and some buttons that are used with an interactive menu to program the sequence of steps and other configurations.. Interaction ponents For the real application the controller must have some elements to interact with the final user and to offer a plete monitoring of the system resources that are available to the designer while creating the logic control of the pneumatic system ():?Interactive mode of work。 function available on the main program for didactic purposes, the user 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. ? Leds to show power on and others to show the state of inputs and outputs. . 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 internal virtual 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 [2]. 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 timeout of the system. . User interface The sequence of strings can be programmed using the interface elements of the controller. A Computer interface can also 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 munication between the puter interface and the controller a simple protocol with check sum and number of bytes is the minimum requirements to guarantee the integrity of the data. . 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 timeout to keep the system safety. In ,a block diagram of micro controller main program is presented.5. Example of electropneumatic system The system is not a representation of a specific machine, but it is made with some mon movements and ponents found in a real one. The system is posed of four actuators. The actuators A, B and C are double acting and Dsingle 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 (Fig. 5) , 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 ad