【正文】 The most important characteristic for the PLC is reliability. If misoperation and signal disturbance can not be managed on time, it will mortal to the whole production. And the relative management incarnate from the I/O interface. There need several modules in the hardware frame of embedded PLC. They are calculation module, munication module, antiinterference module, storage module and I/O module. Moreover, there need some expedition module and arranged interfaces on the practical condition. The detailed frame is figure 1.Figure 1. System Hardware FrameFrom the former figure we can clearly know the main controller is the central part of the whole hardware system. Add the relative circuits such as storage, power supply and clock circuit and so on. It can connect with local extension I/O module through self integrated universal serial bus. The fieldbus CAN interface connect with long distance extension I/O interface or other equipments. Ethernet interface connect with network, I/O interface connect with sensors, RS232 interface connect with PC or the third party equipments and RS485 interface connect with field devices or instruments. All these will achieve the control of local equipments or long distance devices, in order to increase the flexibility and expansibility of this system.The functions of embedded PLC reflected in the CPU performance. So it is very important to chose the CPU while selecting the PLC. Here we introduce the ARM7 chip LPC 2292 of PHILIPS as the core centre of the system. The development system in this article is aiming at the pony mercial installations. And this kind of chip can pletely satisfy all the functions. The most important thing is to provide correct power design before designing the hardware system. LPC 2000 series emulate power supply isolate to the digital power supply in order to reduce noise disturb. The outside power supply is , the chip output power is bigger and the voltage accuracy is high. The power circuit is showed as figure 2.Figure 2. Power CircuitThere have two types of FLASH storage in the hardware system integration. They are NOR FLASH and NAND FLASH. The former can directly read the storage information very fast but the price is high. And the latter is reading information from piece to piece. Reuse the data line and address line with low price, but the speed is slow. According to reach the highest profit, it can choose the perfect storage on basis of the practical requirements. SYSTEM DESIGN OF EMBEDDED PLC After hardware frame is ready, the design analysis of software should be started. The software is development system and operation system. The development system is aiming at the program design and write. And the operation system is to manage the whole system to municate with the outside and save the data. General frame is figure 3.Figure 3. Software System FrameDevelopment environment: the primary mission is to write the function program of controlled member. And write these programs in the PLC. During the development of embedded PLC, there are two types of write languages: ladder diagram and instruction list. For the further systemupgrade and maintenances, the ladder diagram and instruction list can change into C language to increase the legibility and portability of the program.Operation system: imitate the traditional PLC functions through PLC virtual machine in order to achieve the functions of storage, control, calculation and munication. The operation of users is not operating the hardware directly, but through the virtual machine. There is no need to change the controller while outside equipments are changing. Just need to alter the configuration of virtual machine. The advantage is to increase the antiinterference of the software, short the construction cycle, develop the exploit efficiency and lay a basis for the latter system upgrade.The present types of embedded operation systems are various. The famous are Windows CE of Microsoft, eCos of RedHat, Vxworks of Wind River System and some other operation system of open source code such as UC/OS , CLinux system and so on. Each operation system has its originality. Here we use CLinux system. The basic construction is figure 4.Figure 4. Basic Construction of_CLinuxVirtual machine has important meaning in the system. It is a middleware and municates between hardware and application program. The design is bines with munication manager, IO map data storage area, plier, system manager and ponent library. The relation is as figure 5 shows.Figure 5. PLC Virtual Machine Construction FrameIV. NETWORK COMMUNICATION INTERFACE DESIGN OF EMBEDDED PLCThe present use munication interface is RS232 interface. And the characteristics are low cost, less line and easy operation. On the hardware frame design, is clearly show there has 2 RS232 interfaces in the embedded PLC and the codes are written into the PLC through this interface. On the same time, it can municate with configuration software through this interface and monitor the working condition of data and equipments. The other interface is to reserve for the further extension. The main interface circuit is figure 6.Figure 6. Serial Communication InterfaceIn the system, there need the SPI of full duplex to municate between controller and controlled devices. And the characteristic of this interface is high speed and easy protocol. And the SPI bus can collect the wireless data and expand the I/O module.Aim at the characteristic of full duplex, the embedded PLC can be designed in the masterslave mode. One main device can connect with different slave. It can solve by adding connections when munication increase. The detailed connection can operate under the specification.Data frames format:R/W is readwrite bit zone. The value should be 1 or 0. 1 is writing data into the output module. And 0 is writ